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  • Research
  • Open Access

Performance of three multi-species rapid diagnostic tests for diagnosis of Plasmodium falciparum and Plasmodium vivax malaria in Oromia Regional State, Ethiopia

  • 1, 2Email author,
  • 1,
  • 1,
  • 3,
  • 4,
  • 5,
  • 6 and
  • 2, 5
Malaria Journal20109:297

https://doi.org/10.1186/1475-2875-9-297

  • Received: 30 June 2010
  • Accepted: 27 October 2010
  • Published:

Abstract

Background

Malaria transmission in Ethiopia is unstable and variable, caused by both Plasmodium falciparum and Plasmodium vivax. The Federal Ministry of Health (FMoH) is scaling up parasitological diagnosis of malaria at all levels of the health system; at peripheral health facilities this will be through use of rapid diagnostic tests (RDTs). The present study compared three RDT products to provide the FMoH with evidence to guide appropriate product selection.

Methods

Performance of three multi-species (pf-HRP2/pan-pLDH and pf-HRP2/aldolase) RDTs (CareStart®, ParaScreen® and ICT Combo®) was compared with 'gold standard' microscopy at three health centres in Jimma zone, Oromia Regional State. Ease of RDT use by health extension workers was assessed at community health posts. RDT heat stability was tested in a controlled laboratory setting according to WHO procedures.

Results

A total of 2,383 patients with suspected malaria were enrolled between May and July 2009, 23.2% of whom were found to be infected with Plasmodium parasites by microscopy. All three RDTs were equally sensitive in detecting P. falciparum or mixed infection: 85.6% (95% confidence interval 81.2-89.4). RDT specificity was similar for detection of P. falciparum or mixed infection at around 92%. For detecting P. vivax infection, all three RDTs had similar sensitivity in the range of 82.5 to 85.0%. CareStart had higher specificity in detecting P. vivax (97.2%) than both ParaScreen and ICT Combo (p < 0.001 and p = 0.05, respectively). Health extension workers preferred CareStart and ParaScreen to ICT Combo due to the clear labelling of bands on the cassette, while the 'lab in a pack' style of CareStart was the preferred design. ParaScreen and CareStart passed all heat stability testing, while ICT Combo did not perform as well.

Conclusions

CareStart appeared to be the most appropriate option for use at health posts in Ethiopia, considering the combination of quantitative performance, ease of use and heat stability. When new products become available, the choice of multi-species RDT for Ethiopia should be regularly re-evaluated, as it would be desirable to identify a test with higher sensitivity than the ones evaluated here.

Keywords

  • Malaria
  • Parasite Density
  • Health Post
  • Artemisinin Combination Therapy
  • Vivax Infection

Background

Approximately 52 million people in Ethiopia are considered to be at risk of malaria [1]. The disease is reportedly the leading cause of morbidity, accounting for about 12% of the total outpatient visits and 9.9% of the total admissions in 2007 [2]. Between 4-6 million clinical malaria cases are reported annually across all health facilities in the country, however the actual number of malaria cases is estimated to be as high as 10-15 million [3]. The major Plasmodium species causing malaria in Ethiopia are Plasmodium falciparum and Plasmodium vivax, for which the national guidelines prescribe treatment with the artemisinin combination therapy (ACT) artemether-lumefantrine (CoArtem®) or with chloroquine, respectively; Plasmodium ovale and Plasmodium malariae are rare. To reduce costs and minimise selection for drug resistance in Plasmodium parasites there is considerable pressure to minimise ACT use. This can only be achieved once parasitological diagnosis of malaria is routinely provided at all levels of the health system. Parasitological diagnosis is particularly important in low transmission settings where a large proportion of febrile illness is due to causes other than malaria.

Rapid diagnostic tests (RDTs) for detection of P. falciparum (ParaCheck-Pf®, Orchid Biomedical Systems, Goa, India) have been used by health extension workers (HEWs) at health posts in Ethiopia since 2005 [37]. (Description of Ethiopian health system in Additional file 1). Currently, there is no capacity for parasitological diagnosis of non-falciparum malaria at health facilities without laboratory services. RDTs using Plasmodium lactate dehydrogenase (pLDH) to detect non-falciparum malaria were initially less reliable than histidine-rich protein 2 (HRP2)-based tests [811]. However, recent studies have shown sensitivity and specificity of pLDH-based RDTs approaching those of falciparum-only tests [1215], to the extent where they may be appropriate for routine use in case management at facilities where microscopy is not available.

The present study was conducted to determine the performance of three multi-species RDTs for diagnosis of P. falciparum and P. vivax malaria in Ethiopia. The aim of this study was to establish which multi-species RDTs can be recommended for routine use at health posts in Ethiopia.

Methods

Study setting

The study was conducted from April to August 2009 in three woredas; Omo Nada, Kersa and Tiro Afeta; in the Jimma zone of Oromia Regional State, Ethiopia (Figure 1). Within each woreda, one health centre and three health posts were purposively selected according to reported malaria cases. Jimma zone is 300 km south-west of Addis Ababa, and has an elevation of approximately 1,780 meters above sea level. Malaria transmission in Oromia is perennial, but peaks from April to May and from October to December, after the seasonal rains.
Figure 1
Figure 1

Location of Oromia Regional State (solid shading) and Jimma zone (vertical shading) within Ethiopia. Detail of Jimma zone indicates study health centres (filled squares) and health posts (filled circles).

Rapid Diagnostic Tests

Three RDT products were compared: i) CareStart® pf-HRP2/pan-pLDH (AccessBio, USA, catalogue number G0131SK), ii) ParaScreen® pf-HRP2/pan-pLDH (Zephyr Biomedicals, India, catalogue number 50310025), and iii) ICT Combo® pf-HRP2/pan-aldolase (ICT Diagnostics, South Africa, catalogue number ML02). All of the respective RDTs were from the same lot, with expiry date July 2010 (CareStart) or January 2011 (ParaScreen and ICT Combo). CareStart RDTs are individually packaged with swab, lancet, capillary tube, and buffer. ParaScreen cassettes and sample loop are individually packaged, with a single buffer bottle per box. ICT Combo cassettes are individually wrapped but buffer and all other components are provided separately in the box.

Health centre quantitative assessment

Procedures were developed in accordance with Standards for Reporting of Diagnostic Accuracy (STARD) guidelines [16]. Patients attending the outpatient department of the three health centres with symptoms of uncomplicated malaria (axillary temperature > 37.5°C or report of fever in the previous 48 hours) were recruited into the study. Patients under six months of age or with any life-threatening condition were excluded. RDTs were transported unrefrigerated by air to Addis Ababa, where they were stored at ambient conditions until transfer to field sites. Temperature was monitored (Tinytag, Gemini Data Loggers, UK) but not controlled while RDTs were transported by road to health centres and during storage at the health centres. Temperatures during transport reached a maximum 36°C, but at health facilities temperatures did not exceed 30°C.

Sample size was calculated to compare performance of each RDT to microscopy in detecting both P. falciparum and P. vivax at each site, assuming RDT sensitivity of ≥ 95% and specificity of ≥ 80%, with 80% power and a 5% significance level [17]. Assuming > 15% of febrile patients have microscopy-confirmed malaria infection, at an estimated 60:40 proportion of P. falciparum to P. vivax, a total of 811 febrile patients were estimated to be needed at each of the three sites.

Basic demographic information and clinical details were recorded from each enrolled patient. A single finger prick was performed by a laboratory technician or nurse, and used to prepare two slides, each with one thick and one thin film. The same finger prick blood sample was used to carry out all three RDTs in parallel, following manufacturer's instructions. Thin blood films were fixed in methanol after air-drying, then slides were stained in 10% Giemsa solution for 15 minutes. Thick films were read at the health centre by a laboratory technician and considered negative if no parasites were seen after examination of 200 fields at ×1,000 magnification. When positive for parasites, the number of asexual parasites per 200 white blood cells, or 500 white blood cells for low density infections, were used to calculate the number of asexual parasites per μl of blood, assuming a standard count of 8,000 white blood cells per μl of blood [18]. Species, number of parasites/μl (p/μl) and presence of gametocytes were recorded. RDT and microscopy results were read by different staff at the health centre, each blinded to the results of the other diagnostic technique.

All blood films were re-read a second time by an experienced microscopist at a regional malaria reference laboratory, blinded to initial microscopy and RDT results. A third, blinded, reading was conducted on all slides with discrepant first and second readings: presence/absence of asexual parasites, difference in species, or > 50% difference in parasite count. Microscopy results and parasite counts were corrected according to the third reading.

Health post ease of use assessment

Each health post was staffed by two female HEWs or nurses, who received basic training on the use and interpretation of multi-species RDTs. Each carried out 50 tests with each RDT product during their normal duties both at the health post and during community outreach visits. RDTs were discarded after use. Any patient with suspected malaria was included in the sample.

Interviews were conducted with HEWs after completion of each set of 50 RDTs. Respondents were asked to grade eleven specific features of the test, such as the packaging, blood collection device, buffer and results interpretation (Additional file 2), followed by open-ended questions to probe for any further preferences. The various aspects of each RDT were graded on a scale of one (difficult) to five (very easy). After completion of all three RDT products, a final interview round was conducted to assess comparative preferences.

Heat stability assessment

Heat stability assessments were conducted between August and December 2009 at the Ethiopian Health and Nutrition Research Institute located in Addis Ababa, designated as a regional reference laboratory by the World Health Organization (WHO) and the Foundation for Innovative New Diagnostics (FIND).

Heat stability and lot testing of RDTs was conducted according to the standard protocol developed by WHO/FIND [19, 20]. RDTs were exposed to 35°C and 45°C for up to 90 days and to 60°C for up to 72 hours. At pre-determined periods, RDTs were removed from incubators and allowed to reach room temperature, then HRP2 and pLDH or aldolase bands were assessed individually with a negative control and using cultured P. falciparum and P. vivax at concentrations of 200 and 2,000 p/μl. For the low parasite density infection, two of each RDT were tested because of the expected lowered sensitivity, while only one of each RDT was tested at each time point with high parasite density samples.

Statistical analysis

Health centre data were entered and verified using Microsoft® Access 2007 (Microsoft Corporation, Seattle). Second and third blood film microscopy results were entered into a Microsoft® Excel 2007 spreadsheet (Microsoft Corporation, Seattle). All data were analyzed using STATA version 8.0 (STATA Corporation, College Station, TX, USA). RDT performance was assessed in three categories: negative, non-falciparum mono-infection, and either mixed infection with P. falciparum and a non-falciparum species, or P. falciparum mono-infection. The sensitivity, specificity, negative predictive value (NPV) and positive predictive value (PPV) were calculated with 95% confidence intervals (CI) for each RDT [21]. An RDT was categorized as a true positive for P. vivax infection if only the pan band showed, while an RDT was categorized as a true negative if the test itself did not show either of the two test bands and if the corresponding blood slide was found to be negative for all Plasmodium spp. by microscopy. Similarly, an RDT was categorized as a true positive for P. falciparum or mixed infection if it showed either the HRP2 and pan band, or HRP2 band only, while it was considered as a true negative if it showed no test bands and if no Plasmodium spp. infection was diagnosed by examination of the corresponding blood slide using microscopy. The Kappa coefficient (κ), representing the proportion of agreements beyond chance, was used to quantify the level of agreement between each RDT and 'gold standard' microscopy; κ ≥ 0.8 was considered to indicate high reliability [22]. No adjustments were made for multiple comparison testing. Each of these performance indicators were compared between two RDTs using McNemar's test [23] to account for the paired nature of results.

For each of the eleven specific features of the RDTs graded by HEWs the mean score was calculated and each test received an overall rating: RDTs were ranked according to their combined score. All responses to open-ended questions given during interviews were read and coded according to common features.

Ethical considerations

The study protocol was approved by the institutional review boards of the London School of Hygiene and Tropical Medicine (Application No. 5444) and the Ethiopian Science and Technology Agency. Patients (or guardians of children ≤ 16 years) recruited at health centres provided written informed consent prior to inclusion. Study participants with microscopy-confirmed malaria infection were treated according to national guidelines [24]. At health posts, patients provided verbal consent before testing with a multi-species RDT was conducted in addition to routine ParaCheck use.

Results

Study population description

A total of 2,400 febrile patients were enrolled into the study between May 1 and July 31 2009, 800 at each of the three health centres. Data from 17 individuals were excluded due to missing RDT results or unreadable blood films, therefore full data are available from 2,383 individuals.

Microscopy results

Five hundred and fifty two (23.2%) patients were diagnosed with malaria by microscopy, of which (297) 53.8% and (246) 44.6% were infected with P. falciparum and P. vivax respectively. Nine (1.6%) confirmed cases had mixed infection of P. falciparum and P. vivax (Figure 2). Eight individuals had gametocytes (seven P. falciparum, one P. vivax) but no asexual parasites. All 2,383 blood slides were read twice. A total of 496 slides were read a third time to address discrepancies following the first and second microscopy readings (Figure 3).
Figure 2
Figure 2

Flowchart of microscopy and RDT results.

Figure 3
Figure 3

Flowchart of microscopy results discrepant between first and second readings.

One third (38.4%) of P. falciparum cases had parasite densities > 5,000 p/μl and one-fifth (20.6%) < 200 p/μl. About half (52.6%) of patients with P. vivax had parasite density > 5,000 p/μl, while only 3.5% had low density infection (< 200 p/μl). The geometric mean (95% CI) parasite count was 1,681 p/μl (1315-2148) for P. falciparum and 3,406 p/μl (2844-4080) for P. vivax.

RDT compared to 'gold standard' microscopy

RDT results had moderate agreement with microscopy in detecting P. falciparum or mixed infection (κ range 0.6910 to 0.6976) and moderate to good agreement with microscopy for detection of non-falciparum malaria (κ range 0.7531 to 0.8020) (Table 1).
Table 1

Comparative performance indicators of each RDT

 

CareStart

ParaScreen

ICT Combo

McNemar p value1

P. falciparum 2

n = 2137

n = 2137

n = 2137

CS-PS

CS-ICT

ICT-PS

Sensitivity (95% CI3)

85.6%

(81.2-89.4)

85.6%

(81.2-89.4)

85.6%

(81.2-89.4)

1.0

1.0

1.0

Specificity (95% CI)

92.4%

(91.1-93.6)

92.4%

(91.1-93.6)

92.7%

(91.4-93.8)

1.0

0.10

0.10

PPV (95% CI)

65.3%

(60.5-70.0)

65.3%

(60.5-70.0)

66.2%

(61.3-70.8)

1.0

0.76

0.76

NPV (95% CI)

97.5%

(96.6-98.2)

97.5%

(96.6-98.2)

97.5%

(96.6-98.2)

1.0

1.0

1.0

κ4

0.6910

0.6910

0.6976

1.0

0.70

0.70

Non-falciparum only 5

n = 2077

n = 2077

n = 2077

CS-PS

CS-ICT

ICT-PS

Sensitivity (95% CI)

85.0%

(79.9-89.2)

82.5%

(77.2-87.1)

85.0%

(79.9-89.2)

0.11

1.0

0.08

Specificity (95% CI)

97.2%

(96.4-97.9)

96.2%

(95.3-97.1)

96.7%

(95.8-97.5)

< 0.001

0.05

0.11

PPV (95% CI)

80.4%

(75.0-85.0)

74.7%

(69.0-79.7)

77.7%

(72.2-82.6)

0.10

0.39

0.43

NPV (95% CI)

98.0%

(97.2-98.6)

97.6%

(96.8-98.3)

98.0%

(97.2-98.6)

0.78

1.0

0.50

κ

0.8020

0.7531

0.7851

< 0.001

0.23

0.03

1 Significant relationships (p > 0.05) in bold type

2 Individuals with P. falciparum mono-infection or mixed infection with P. falciparum and P. vivax. P. vivax mono-infection excluded.

3 95% Confidence interval

4 Kappa (κ) is the proportion agreement between microscopy and RDT results, where 0.8 indicates good agreement.

5 Individuals with P. vivax mono-infection. Mixed infections and P. falciparum mono-infection excluded.

For some Plasmodium-infected patients the RDT result was positive but the species was incorrectly identified. The most common species misdiagnosis was where the RDT showed only the Pan band for P. falciparum mono-infections (26 CareStart, 27 ParaScreen and 25 ICT Combo). Only two of these infections were > 200 p/μl; with 12,800 p/μl and 48,000 p/μl. Additionally, RDTs occasionally indicated P. falciparum mono-infection for what was diagnosed by microscopy to be P. vivax mono-infection (three CareStart, two ParaScreen and two ICT Combo). Interestingly, all nine mixed infections resulted in HRP2 and pan band activation in CareStart and ICT Combo, while the ParaScreen pan band failed to show in one of the nine mixed infections.

The sensitivity of all RDTs in detecting P. falciparum infections was 85.6%. Specificity was statistically similar for all three RDTs at 92.4% to 92.7% (Table 1). The PPVs for the RDTs were comparable in detecting P. falciparum infection at around 65%. NPVs were the same for all RDTs at 97.5%. The false negative RDT results for P. falciparum had parasite density between 40 and 50,000 p/μl, with median density 80 p/μl. Species misdiagnoses were not considered as false negative results.

CareStart was the best performing RDT in detecting non-falciparum infections based on kappa (κ = 0.8020) (Table 1), with both CareStart and ICT Combo showing superior kappa values to that of ParaScreen (p < 0.001 and p = 0.03, respectively). The PPV and NPV of all three RDTs were comparable (p > 0.05 for all pairs), however specificity of CareStart was significantly higher than that of ParaScreen and ICT Combo (p < 0.001 and p = 0.05, respectively). False negative RDT results against P. vivax were generally low density infections: range 40 to 7080 p/μl with median 280 p/μl. Few of the false positive RDT results for P. vivax or P. falciparum were due to residual antigen from cleared infections, since only 11% of patients with false positive results had received anti-malaria treatment in the previous four weeks.

Sensitivity was calculated for P. falciparum infection and for P. vivax mono-infection at different parasite densities (< 200 p/μl of blood; 200-500 p/μl; 500-5,000 p/μl; > 5,000 p/μl) (Table 2). All three RDTs showed improved sensitivity with increasing P. falciparum density (p < 0.001), but sensitivity was generally comparable between RDTs within each parasite density category. No statistical association was seen between P. vivax density and RDT sensitivity, likely due to the small number of low density infections. However, CareStart and ICT Combo were seen to be more sensitive than ParaScreen in detection of P. vivax infection at > 5000 p/μl (p = 0.03).
Table 2

Sensitivities of RDTs at different parasite densities

  

CareStart

ParaScreen

ICT Combo

McNemar p value1

P. falciparum

       

Parasite density

N

Sensitivity: Pf or Pf+pan2 (95% CI3)

CS-PS

CS-ICT

ICT-PS

< 200 p/μl

63

66.7%

(53.7-78.1)

66.7%

(53.7-78.1)

66.7%

(53.7-78.1)

1.0

1.0

1.0

< 500 p/μl

90

70.0%

(59.4-79.2)

70.0%

(59.4-79.2)

70.0%

(59.4-79.2)

1.0

1.0

1.0

< 5000 p/μl

188

79.1%

(72.6-84.7)

79.1%

(72.6-84.7)

79.1%

(72.6-84.7)

1.0

1.0

1.0

> 5000 p/μl

118

95.8%

(90.4-98.6)

95.7%

(90.4-98.6)

95.7%

(90.4-98.6)

1.0

1.0

1.0

P. vivax

       

Parasite density

N

Sensitivity: pan band only4 (95% CI)

CS-PS

CS-ICT

ICT-PS

< 200 p/μl

9

77.8%

(40.0-97.2)

88.9%

(51.8-99.7)

66.7%

(29.9-92.5)

0.32

0.56

0.16

< 500 p/μl

28

82.1%

(63.1-93.9)

89.3%

(71.8-97.7)

85.7%

(67.3-96.0)

0.32

0.65

0.56

< 5000 p/μl

116

82.8%

(74.6-89.1)

82.8%

(74.6-89.1)

82.8%

(74.6-89.1)

1.0

1.0

1.0

> 5000 p/μl

128

86.7%

(79.6-92.1)

82.8%

(75.1-88.9)

86.7%

(79.6-92.1)

0.03

1.0

0.03

1 Significant relationships (p > 0.05) in bold type.

2 RDT result showed either P. falciparum HRP2 band only, or HRP2 band and pan pLDH band. Individuals with P. vivax infection only excluded.

3 95% Confidence Interval

4 RDT result showing pan band only. Individuals with any P. falciparum infection excluded.

Ease of use assessment

All HEWs involved in the ease of use assessment had previously used ParaCheck, and commented on the importance of using a multi-species RDT in their work at the health post and in the community. "This kind of multi-species RDT is important for our routine activities and should be immediately applied in our work."

When all scored components were averaged, CareStart scored highest (4.5) followed by ParaScreen (4.4) and ICT Como (4.2). A number of HEWs indicated that they preferred the packaging of CareStart in individual 'lab in a pack' sets with all components needed for a single test (cassette, swab, lancet, blood collection pipette, buffer, and instructions) included in each packet: "I liked having all materials within a single packet, it makes moving from house-to-house easier as I don't have to carry a large box, so is very comfortable for my work in the community."

The most salient factor influencing the ease of use of each RDT was the presence of clear markings on the cassette for interpretation of results. ParaScreen had the most detailed labelling with the band positions marked with 'C' at the control region, then 'Pan' and 'Pf' in the test region. CareStart has 'C' at the control band, with '1' for P. falciparum and '2' for Pan. ICT Combo has no separate markings for Pf and Pan bands, meaning that users must remember the band location when reading results.

Heat stability

CareStart and ParaScreen were shown to have superior stability to that of ICT Combo during the heat stability testing. When tested after more than four hours storage at 60°C, the HRP2 band of ICT Combo failed to detect infection. The ICT Combo HRP2 band showed only very weak positive results when tested after 30 days storage at 35°C and 45°C. However, both CareStart and ParaScreen HRP2 bands were stable after storage for 72 hours at 60°C, and 90 days at 35°C and 45°C, all giving positive results when tested with P. falciparum.

All RDTs had pan bands that were more heat-sensitive than HRP2, appearing faint throughout the testing with both high and low density P. vivax infections. Both CareStart and ParaScreen pLDH bands completed the testing procedure at 35°C, 45°C and 60°C. The ICT Combo aldolase band failed to detect P. vivax at 200 p/μl after 60 days stored at 35°C and after 60 days stored at 45°C. However, after 60 days storage at these temperatures, ICT Combo did detect P. vivax at the higher density of 2,000 p/μl. ICT Combo aldolase band failed to detect 200 p/μl P. vivax after 72 hours stored at 60°C.

Discussion

RDTs for malaria are being increasingly adopted across endemic countries to strengthen parasitological diagnosis and appropriate management of all fever cases [25]. A large number of products are now available, and while a detailed assessment of performance has been undertaken by WHO/FIND [19, 20] and a list of recommended products produced [26] it is important that the most appropriate products for each transmission setting and operational context are identified. An appropriate RDT for implementation across Ethiopia should be able to detect P. falciparum and P. vivax, be highly sensitive and specific, and also able to detect low parasite density infections. PPV and NPV give an indication of usefulness of the test in practice, but these indicators are influenced by the prevalence of malaria in the test population. To guide product selection by the FMoH, the present study set out to determine the performance of three multi-species RDTs for diagnosis of P. falciparum and P. vivax malaria in Ethiopia.

The sensitivity of the three RDTs in detecting P. falciparum was found to be similar to their sensitivity in detecting P. vivax, and there was little difference in RDT performance when compared to microscopy. Overall, ICT Combo and CareStart showed better performance than ParaScreen. It was unexpected that the RDTs would have similar sensitivity for detection of both P. falciparum and P. vivax infection, since other studies in co-endemic areas have consistently found multi-species RDTs to be better at detecting P. falciparum infection [8, 13, 27]. The present finding may be due to misdiagnosis and/or under diagnosis of species at low parasite density by microscopy. More low density P. falciparum than P. vivax infections were detected by microscopy, contrary to findings in other co-endemic areas where P. falciparum is usually presents with higher parasite density than P. vivax[9, 13]. However, a study in Columbia found similar P. falciparum and P. vivax infection densities, but amongst children under five years the P. vivax parasite density was higher than P. falciparum[10]. In the present study all slides were subject to rigorous quality control testing: read twice by qualified technicians, with any discrepancies corrected according to an expert microscopist, suggesting that the result is unlikely to be due to human error.

The maximum recommended storage temperatures for CareStart and ParaScreen (30°C) was exceeded in the current study during transport of RDTs to the study sites (maximum temperature during transit 36°C). This is unlikely to have contributed to any reduction in performance since heat stability testing indicated that both CareStart and ParaScreen were able to detect infection after longer periods being stored at higher temperatures. Nevertheless, it is important to consider transport and storage conditions for RDTs during routine use in the health system.

The RDTs had high NPVs, meaning that they were reliable in ruling out malaria. However the lower PPV means that patients will occasionally be falsely diagnosed as positive for malaria and unnecessarily treated. False positives in this study were not due to residual antigen from previous infections, since only 11% of the total false positive results were from patients who had received any anti-malarial treatment in the previous four weeks. Alternative explanations include sequestration: erythrocytes containing mature parasites clump together in the microvasculature, therefore are not seen in the peripheral circulation and blood films, while antigen continues to be released [28]. It may also be possible that the parasite density was too low to be seen by microscopy, but there was sufficient parasite antigen to result in a positive RDT [29]. Whilst previous work has shown that some RDT false positives are due to patients with residual gametocytaemia [30], this association was not seen in the current study.

The WHO/FIND product testing of malaria RDTs [19, 20] is the most comprehensive assessment of malaria RDTs on the market to date. Contrary to the present results, ParaScreen performed poorly in the WHO/FIND testing. However CareStart performed slightly better in the WHO/FIND testing than in the current study, as did the HRP2 band of ICT Combo. The aldolase band of ICT Combo was comparatively better at detecting low density P. vivax in our findings than in the WHO/FIND tests. There are a number of possible reasons for this differential performance of the same RDTs in the two studies. Firstly, the WHO/FIND product testing was carried out by highly experienced technicians in a controlled laboratory setting, while this study involved staff engaged in routine activities at the health facility. Other influencing factors may have been variation between product lots, the impact of transportation and storage conditions, or the use of fresh finger-prick blood samples from patients rather than previously frozen samples. It is also possible that genetic variation of target antigens can lead to variation in performance of RDTs in different regions: Baker et al found significant genetic variation in HRP2 isolates from different countries which was associated with reduced sensitivity of RDTs at low density infections [31]. The prozone effect, whereby immunological tests are not activated by very high density infections, has been suggested as a reason for HRP2 false negative results in malaria RDTs [32]. However in the current study, none of the samples which elicited false-negative RDT results were hyperparasitaemic (> 5% red blood cells parasitised), therefore the prozone effect is unlikely to be contributing to the presence of false negative HRP2 results.

In the Ethiopian context, a number of studies have been carried out to assess performance of a variety of multi-species malaria RDTs, to guide the FMOH decision of a replacement for ParaCheck-Pf. An initial study found ParaScreen to have much lower sensitivity [27] than the present study, while a subsequent investigation during peak transmission season reported comparable performance [33]. A CareStart pf-HRP2/pv-pLDH combination RDT has also been evaluated at two locations in Ethiopia [4, 5]. The sensitivity of the test in detecting P. falciparum by HRP2 at Wondo Genet was 99.4% and 96.4% in Jimma, both very much higher than for CareStart pf/pan in the current study. The P. vivax-specific pLDH band of the pf/pv RDT also appeared to have much higher sensitivity than the Plasmodium-specific pLDH band of the pf/pan RDT tested in this study. Although P. ovale and P. malariae are believed to be responsible for < 1% of all malaria cases in Ethiopia, there remains a need to provide parasitological diagnosis for these species. It is likely that lower performance of pf/pan RDTs in this study compared to prior pf/pv RDT findings is a result of conducting the study under operational conditions. Furthermore, the WHO/FIND evaluation indicates that pf/pan and pf/pv RDTs have comparable performance [19, 20].

CareStart also manufactures a pf-pLDH/pan-pLDH combination test, which has been evaluated in Madagascar [12] and Myanmar [9]. The sensitivity of pan-pLDH in the current study falls between the values determined in Madagascar and Myanmar, indicating that our results for CareStart pan-pLDH are reliable.

Ability of the end-user to correctly prepare and interpret the RDT is crucial to operational success. Ease of use has been shown to vary considerably with different RDT formats [34, 35] and reduce test accuracy if health workers do not receive sufficient training or support tools [36]. WHO acknowledges the importance of selecting an RDT appropriate to the level of training and supervision of the end-user [19]. The 'lab in a pack' format and clear labelling of the cassette bands were factors that HEWs valued, supporting adoption of CareStart for use at health posts in Ethiopia, but the current work did not investigate whether HEW preferences affect the use of RDT results to make case management decisions. In order for multi-species RDTs to be fully effective at health post level, it is necessary to confirm that results of RDTs are correctly transferred to treatment or referral procedures.

Results of heat stability assessment for CareStart were consistent between the WHO/FIND testing and the current study, with strong detection rates after storage at 35°C and 45°for low and high-density P. falciparum and P. vivax samples. ParaScreen was found to perform better in heat stability testing during the current study than in the WHO/FIND product testing, where failures occurred with low density P. falciparum and P. vivax infections. ICT Combo performed well in WHO/FIND testing, with the HRP2 band detecting all P. falciparum samples, but in our study ICT Combo failed to detect one high density P. falciparum sample and showed weak responses towards the end of the testing period. Test procedures were the same for both studies and the discrepancy in ParaScreen results may therefore be due to variation between product lots.

Conclusions

All three RDTs showed performance below WHO recommendations for sensitivity compared to 'gold standard microscopy'. Health extension workers were easily able to adapt to the use of multi-species RDTs after limited training. For use at health posts, ICT Combo seemed least suitable, as it did not provide reliable results after being stored at ambient temperatures. Considering a combination of reliability compared to microscopy, heat stability, and ease of use judged by health extension workers, CareStart pf-HRP2/pan-pLDH is found to be the most appropriate tool to allow for parasitological diagnosis at health post level in Ethiopia. However the selection of RDT should be regularly re-evaluated as new products are released, to ensure that the most appropriate test is used.

List of abbreviations

ACT: 

artemisinin-based combination therapy

FIND: 

Foundation for Innovative New Diagnostics

FMoH: 

Federal Ministry of Health

HEW: 

health extension worker

HRP2: 

histidine-rich protein-2

NPV: 

negative predictive value

pLDH: 

Plasmodium lactate dehydrogenase

PPV: 

positive predictive value

RDT: 

rapid diagnostic test

WHO: 

World Health Organization.

Declarations

Acknowledgements

This work was supported by the United States Agency for International Development (Cooperative Agreement 663-A-00-09-00404-00). BC is funded by the ACT Consortium which is supported by a grant from the Bill & Melinda Gates Foundation to the London School of Hygiene and Tropical Medicine. Manufacturers supplied CareStart and ParaScreen RDTs free of charge for this evaluation, and ICT Combo was provided at a reduced price. We would like to thank Oromia Regional Health Bureau for facilitating this study, and the Ethiopian Health and Nutrition Research Institute for conducting heat stability testing. Jimma zone health department provided assistance in coordinating the data collection. We would like to thank all staff and patients of the health facilities involved in this study. Afework Hailemariam made contributions to the development of this study, and Agonafer Tekalegne provided management oversight throughout. Joe Malone, Hiwot Teka and Michael Aidoo gave valuable comments on an earlier version of this paper. The opinions expressed in this paper are those of the authors and may not reflect the position of their employing organizations nor of their work's sources of funding.

Authors’ Affiliations

(1)
Malaria Consortium - Ethiopia Office, Ethio-China Road, PO Box 100224, Addis Ababa, Ethiopia
(2)
Malaria Consortium - Africa Regional Office, PO Box 8045, Plot 2, Sturrock Road, Kampala, Uganda
(3)
Malaria Consortium International, Development House 56-64, Leonard Street, London, EC2A, 4LT, UK
(4)
Oromia Regional Health Bureau, PO Box 24341, Addis Ababa, Ethiopia
(5)
London School of Hygiene & Tropical Medicine, Keppel Street, London, UK
(6)
U.S. Agency for International Development, Addis Ababa, Ethiopia

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© Ashton et al; licensee BioMed Central Ltd. 2010

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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