COMPARISON OF 2 METHODS FOR ENUMERATING MALARIA PARASITES IN THICK BLOOD FILMS

Calculation of parasite densities is important for estimating herd imm unity to malaria, and for determining end points in field trials for i nterventions such as malaria vaccines, impregnated bed nets, and chemo suppression. Two methods of enumeration were compared: method 1, in wh ich 100 consecutive high-power fields (HPFs) are examined, and if they all contain at least one parasite, the number per field is then count ed in 10-100 of these fields according to density; and method 2, in wh ich the actual number of parasites present in 100 consecutive fields a re counted. The first method significantly underestimates parasite den sity in samples in which less than all high-power fields are parasite- positive. A correction of method 1 is suggested, which results in a pa rasite density, which is comparable with that obtained using method 2. The correction factor estimated was 2(- In(1 - p)), where p is the pr oportion of positive HPFs. The correction factor presented will allow accurate estimate of parasite densities per volume of blood even if on ly the proportion of parasite-positive high-power fields containing at least one parasite are counted.