The goal of this study was to compare the sensitivity of DNA-based malaria diagnostic assays using capillary and venous blood. In contrast to most of the previous studies comparing these two sample types [4, 10-13], we assessed the sensitivity of two PCR assays capable of detection multiple species of Plasmodium instead of determining the parasite density by microscopy. Our analysis found that differences in sensitivity of malaria detection (or lack thereof) between these two sample types varied based on the assay and particular Plasmodium target being detected.
We did not observe any significant differences in sensitivity of detection between capillary and venous blood for all targets (P. falciparum, P. vivax and universal malaria) using MMSR assay, and P. vivax/P. ovale targets using the GFP assay. These results are in line with several previous studies that also found no differences in parasite density or diagnostic sensitivity between capillary and venous samples [10, 12, 13]. However, our data have shown that the GFP assays has significantly higher sensitivity for detection of Plasmodium spp. (p<0.002) and P. falciparum (p<0.05) markers in venous samples. The greater sensitivity of the GFP assay for Plasmodium spp. (versus P. falciparum) is largely responsible for the overall higher sensitivity in venous blood by this assay (p<0.05).
The higher overall sensitivity of GFP assay is most likely the result of using nested PCR as a core detection technology of all FilmArray assays [15] as compared to simpler, single-step PCR chemistry used by MMSR. The differences in sensitivity can be also a consequence of using different diagnostic targets, the effect especially visible in case of the universal malaria targets.
We could not unambiguously assess the sensitivity differences in detecting P. vivax (MMSR) and P. vivax/P. ovale (GFP) due to their different intended specificities and low prevalence of P. vivax within West Africa [1, 21, 22]. The presence of small but significant numbers of samples positive for P. vivax/P. ovale in the GFP assay or Plasmodium spp. only in both assays provide further evidence for circulation of P. ovale and P. malariae in the tested population as previously reported [20].
Three studies have previously reported higher parasite density or improved malaria detection sensitivity in capillary blood [4, 11, 12]. However, we are not aware of any previous reports of higher rates of detection in venous samples. The hypothesis that capillary blood may contain higher concentrations of malaria parasites is based on the well-documented feature of malaria pathology, namely increased adherence of Plasmodium-infected erythrocytes to vascular endothelial cells, which leads to their sequestration in certain organs [9]. However, while it is known that malaria-infected erythrocytes can sequester in specific organs (e.g., brain, lungs), it is not clear if significant sequestration occurs in capillaries used for diagnostic sample collection. The results obtained in this study show that for in some circumstances and for certain Plasmodium markers, an opposite effect may be observed, resulting in greater diagnostic sensitivity in venous blood.
A notable difference between the current and previous studies is the population age distribution. Whereas most other studies emphasized testing of children or young teens [4, 10, 11, 13, 14], our study population was significantly older, with a median of 27 years and 25% of the tested subjects between 21 and 25 years of age. It is possible that age-related physiological differences may affect the parasite’s lifecycle, its clinical presentation, and potentially even its prevalence within various compartments [23-25]. An age-controlled study with a higher number of tested subjects and longer collection period would be needed to explore these variables.