A validation study of microscopy versus quantitative PCR for measuring Plasmodium falciparum parasitemia

Microscopy and 18S qPCR are the most common and field-friendly methods for quantifying malaria parasite density, and it is important that these methods can be interpreted as giving equivalent results. We compared results of quantitative measurement of Plasmodium falciparum parasitemia by microscopy and by 18S qPCR in a phase 2a study. Microscopy positive samples (n = 355; median 810 parasites/μL [IQR 40-10,471]) showed close agreement with 18S qPCR in mean log₁₀/mL transformed parasitemia values by paired t test (difference 0.04, 95%CI - 0.01-0.10, p = 0.088). Excellent intraclass correlation (0.97) and no evidence of systematic or proportional differences by Passing-Bablok regression were observed. 18S qPCR appears to give equivalent parasitemia values to microscopy, which indicates 18S qPCR is an appropriate alternative method to quantify parasitemia in clinical trials.