This large population based survey was conducted during the late transmission season during a non-epidemic year for malaria in Ethiopia. The study used the conventionally accepted standard malaria diagnostic method, light microscopy of peripheral blood slides, and a new technique (ParaScreen rapid diagnostic test, RDT). Overall, more infections were detected by blood slide microscopy (4.1%) than by ParaScreen RDT (3.3%), although the difference was not statistically significant. However, by region, significantly more infections were detected by RDT in Oromia and significantly fewer by RDT in Amhara.
Despite little difference between the overall prevalence of malaria detected by the two tests, the overall sensitivity of ParaScreen that we observed was low (47.5%). This indicates that there was substantial non-overlap between the positives detected by each test. The positive predictive value was correspondingly low (56.8%). Surprisingly, SNNP region had a positive predictive value of only 36.1% (95% CI 27.5–45.4) which was lower than expected given that SNNP had the highest prevalence of malaria parasites based on microscopy as the gold standard.
Reports from elsewhere indicated that RDTs have shown a comparable level of accuracy to microscopy in clinical settings [19, 20]. The sensitivity of the RDTs observed in our study is much lower than predicted by previous studies [13–15]. This difference may arise for two reasons: firstly, it is possible that parasitaemias were very low in people not seeking treatment; and secondly, the RDTs were possibly defective or handled inappropriately causing them to lose sensitivity. Light microscopy can routinely detect parasitaemia levels as low as 40 parasites/μl, and experienced microscopists can detect as low as 5–10 parasites/μl of blood , whereas RDTs usually have a capacity to detect 100 parasites/μl of blood [11, 19]. The lack of sensitivity of RDTs at low parasitaemia compared to microscopy is one of the shortcomings noted elsewhere [11, 19, 20], which is possibly also reflected in our findings. Regarding the quality of RDTs, the diagnostic accuracy can be affected by several factors such as quality of the products, storage temperature and humidity, and end users' performance [11, 21]. The RDTs used in this study were manufactured in May and June 2006 with an expiry date of April 2008 suggesting that they should have been in a good condition during the survey field work (mid-December 2006 to mid February, 2007). However, the results suggest that an appropriate quality control scheme should accompany any effort to initiate the use of RDT at a population based scale, especially in remote settings.
In general, the prevalence of malaria showed significant variation among the regions surveyed, but the overall prevalence was low. As would be expected, two malaria parasite species, P. falciparum and P. vivax were identified by microscopy in this study. In Ethiopia, the two predominant malaria species recorded are P. falciparum (~60%) and P. vivax (~40%) [3, 22, 23]. However, this proportion can change during hot-dry seasons following the peak transmission period, when more relapse cases could be expected due to P. vivax infection, . The preponderance of one malaria species over the other at a particular period might vary from one area to another, not only depending on climatic and seasonal factors but also owing to variation in geographical localities .
Even though clinical history of the participants was not recorded in our study, evidence from other studies showed that RDT positive cases missed by microscopy might be individuals who had been treated but in whom antigenemia persists [20, 21]. ParaScreen RDT exhibited more sensitivity to P. falciparum or mixed infections than to non- P. falciparum (most likely P. vivax) infection. In addition to intrinsic lower sensitivity of the antigen used, this might be attributed to the longer persistence of P. falciparum antigen (HRP-2) after treatment or resolution than pLDH antigen [21, 26]. Other studies have also demonstrated that the HRP-2 assay showed more sensitivity compared to the pLDH antigen based assay due to quick clearance of the latter antigen after treatment .
In this study, quantitative parasite counts were not conducted; nonetheless, there were cases with high loads of malaria parasites with asexual stages (ranging ++ to ++++ per high power field) which turned out to be negative by RDT. This phenomenon suggests the possibility of under diagnosis of malaria parasites by RDT  and requires further investigations in the Ethiopian context. A similar observation was reported from other settings where RDT showed false negative results in the presence of high parasitaemia  implying the need for a continuous quality assurance system to be instituted at every step before disseminating the RDT products to the end users.