Dependence of malaria detection and species diagnosis by microscopy on parasite density

Giemsa-stained blood smears from each of 2,190 patients from Thai government-operated clinics on the Thailand-Myanmar border were independently examined by the on-duty microscopists at the clinics and by 2-3 research microscopists, each blinded to the clinics' and each other's reports. Using a strictly defined protocol, a consensus reference-standard blood smear interpretation for each sample was produced by the research microscopists. This result was compared with the clinic's diagnostic interpretation for the corresponding sample with respect to detection of parasitemia and diagnosis of infecting species. Reference-standard results reported parasitemia in 13.2% of the samples reported negative by the clinic. Reference-standard results were negative in 24.3% of the samples reported parasite-positive by the clinic. For samples in which both the reference-standard result and the clinic result reported parasitemia, species identification differed for 13.7% of the samples. The likelihood of parasite detection and correct diagnosis at the clinic varied in accordance with the reference-standard estimates of parasite density.