This study found varying precision of CareStart™ RDT to detect P. falciparum and P. vivax in “active” and clinical settings of highland-fringe areas of south-central Ethiopia. A greater proportion of RDTs than slides gave positive result for Plasmodium. In the survey, the test showed low sensitivity and high specificity for P. falciparum, but vice versa for P. vivax. The test revealed lower PPV for P. falciparum and lower NPV for P. vivax. Overall, season substantially influences RDT precision with good sensitivity at health facilities compared to household survey. However, specificity was found to be low in both settings with comparable results. Interestingly, better NPV obtained unlike lowest PPV in both survey settings. RDT showed low sensitivity in children aged below 10 years. No strong evidence for contribution of low parasite density for missed Plasmodium detection by the test product.
This study has got some limitations. This study used light microscopy for comparison of performance of RDT, in spite of its shortcomings related to variable techniques in specimen preparation and slide reading, and mainly the level of expertise of the examining microscopist . Thus, PCR could be the ideal gold standard in RDT evaluation , as applied in recent studies [38, 39]. The use of PCR appears plausible in hypoendemic areas like the present study area . There might be a possibility of multiple enrollments of participants between the two survey settings. Some of the strengths of this study include generating large sample data in different seasons at highlands of low malaria endemicity unlike other studies recent studies. RDT evaluation was performed using well trained and competent health personnel at different settings. It is believed that the strengths of this study outweigh the limitations. Thus, conclusions drawn and recommendations suggested are considerably valuable both in improving malaria control efforts and future research endeavor.
The present finding of low sensitivity for P. falciparum (87.5%) and P. vivax (92.8%) using CareStart™ RDT is comparable with a study conducted in south-west (85.6% for P. falciparum, and 85.0% for P. vivax) and North-West (92.9% for P. falciparum, 90.9% for P. vivax) Ethiopian highland areas [21, 24]. But the present finding reported lower sensitivity compared to other studies in South (99.4% for P. falciparum and P. vivax); North-East (98.5% P. falciparum and 98.0% for P. vivax); and South-West (96.4% for P. falciparum, and 95.3% for P. vivax) Ethiopia [22, 23, 25]. The low PPV value obtained in this study is in line with a study in South-West Ethiopia , and in low endemicity areas elsewhere .
The varying RDT performance in different seasons in the present household survey is generally consistent with another study in hypoendemic highland zones in Kenya and Uganda . Moreover, the finding of low PPV and temporally varying correspondingly with prevalence using household survey is in agreement with other studies [26, 40]. However, the results with good sensitivity and specificity from health facility were consistent with other studies done at health facilities [22, 23, 25]. The present finding of the lowest PPV and NPV during two months, November and December 2009, end of malaria transmission season normally in most parts of Ethiopia. The present result is in line with a recent study . The finding of less variability in RDT performance in different age categories is consistent with an evidence that showed age has no effect on malaria RDT performance .
Various factors such as clinical and epidemiologic characteristics of the study populations, reference standards and products of different lots influence the results of RDT-based diagnosis [11, 13, 16]. Thus, comparison of RDT results difficult. Similarly, the present study used data generated from large population and varying transmission seasons. However, field evaluation of CareStart™ RDT precision used a short duration of peak malaria transmission.
The present finding of low sensitivity of P. vivax (health facilities) and P. falciparum (survey) disagrees with other studies that consistently found multi-species RDTs to better detect P. falciparum infection than non-falciparum infection, most probably P. vivax[12, 42]. The low sensitivity in health centres for P. vivax may be interpreted as due to low performance of multi-species RDTs for this species. The low sensitivity obtained in this study may be explained by several factors pertinent to the manufacturing process and environmental conditions [11, 16, 20]. The present study was performed in highland area with no extreme weather condition.
In addition, low-prevalence, or hypoendemic, malaria poses particular diagnostic challenges since low population prevalence reduces the PPV of tests . Moreover, test sensitivity suffers when parasite densities within individual infections are low . The hypoendemicity diminishes test sensitivity, as well as the PPV . Very low overall prevalence of malaria was found in the present study area , which is known to influence RDT performance as reported .
The varying performance of sensitivity for P. falciparum and P. vivax for survey and health facility might be interpreted as the difference in clinical condition of patients. Study participants visiting health facilities might be clinically more specific than otherwise. Review evidences showed that higher RDT sensitivity in studies involving patients seeking relief from moderate to severe disease in clinical settings while low RDT sensitivity in studies that enrolled patients by means of “active” case finding [11, 41].
The RDT had low NPV for P. vivax at both survey and health centres. A past study showed that less sensitive for non-P. falciparum than for P. falciparum. There is rich evidence that extremely low sensitivity for both HRP-2 and p-LDH tests and batch specific problems were suspected . This implies that RDT missed some malaria cases or false negatives were higher. Review evidence cited a study that occasional false negative results may be caused by: (i) deletion or mutation of the HRP-2 gene; (ii) anti-HRP-2 antibodies in humans may explain why some tests were negative despite significant parasitemia; and (iii) presence of an inhibitor in the patient’s blood preventing development of the control line . The present finding of low PPV in P. falciparum is likely due to sequestration . False positive RDT results occur in a few percent of tests, which can be due to cross-reactivity with rheumatoid factor in blood [16, 20]. Cross-reactivity with heterophile antibodies may also occur .