SHORT REPORT: FAILURE OF THE OPTIMAL RAPID MALARIA TEST AS A TOOL FOR THE DETECTION OF ASYMPTOMATIC MALARIA IN AN AREA OF THAILAND ENDEMIC FOR PLASMODIUM FALCIPARUM AND P. VIVAX

We evaluatedthe efficacy of the OptiMAL assay in a cross-sectional malaria survey in western Thailand from April to August 2001. Expert microscopy of Giemsa-stainedthick andt hin bloodfilms was usedas the gold standard. Positive control lines were evident in 99% (1,128 of 1,137) of the assays tested. However, 34% (384 of 1,128) of assays produced an aberrant result (a positive P. falciparum-specific line anda negative panmalarial line). False- positive panmalarial and Plasmodium falciparum-specific lines occurredin 25.9% (270 of 1,042) and60.3% (628 of 1,042) of microscopy-negative samples, respectively. Due to the preponderance of false-positive test results, it was necessary to develop subjective criteria for test positivity based on line intensity. For determination of assay performance during this study, we therefore considered all test lines that were scored as intermediate or strong as positive and lines that were faint as negative. Using these criteria, we determined that the sensitivity of the OptiMAL assay for P. falciparum was 25% with > 500 parasites/l and10.5% with > 100 parasites/ l, while for P. vivax, the sensitivity at the same parasite rates was 100% and41.7%, respectively. Further studies are requiredto determine whether the problems we id entifiedare limitedto this particular lot of OptiMAL assays.