High Malaria Parasitemia Affects Rapid Diagnostic Tests

Rapid diagnostic tests (RDTs) for malaria are devices that detects malaria antigen in a small amount of blood. Monoclonal antibodies directed against the target parasite antigen are impregnated on a test strip.

A small amount of finger prick blood is put on the immunochromatographic strip and the result, usually a colored test line, is obtained in 5-20 minutes. RDTs require no capital investment or electricity, are simple to perform, and are easy to interpret.

Most malaria RDTs detect Plasmodium falciparum, and an antigen common to all four species. Plasmodium vivax-specific RDTs target P. vivax-specific parasite lactate dehydrogenase (Pv-pLDH). P. falciparum parasites cause the fatal form of malaria and patients may need hospitalization, while P. vivax and the other species are more benign.

RDTs are widely used in both the developed and developing world and they are replacing the traditional methods that require highly trained and skilled microscopists. In a recent study, nine P. vivax specific RDTs were tested with blood from 85 patients with P. falciparum whose parasite density was equal or greater than 2%. The RDTs included two-band (detecting Pv-pLDH), three-band (detecting P. falciparum-antigen and Pv-pLDH) and four-band RDTs (detecting P. falciparum, Pv-pLDH and pan-pLDH). False positive Pv-pLDH lines were observed in six of the nine RDTs, including two- three- and four-band RDTs. They occurred in the individual RDT brands at frequencies ranging from 8.2% to 29.1%. Over 22% of samples generated a false positive Pv-pLDH line at least two RDT brands. False positive lines were of medium or strong line intensity in 18% of the samples.

False positive Pv-pLDH lines in P. falciparum samples with high parasite density occurred in six of the nine P. vivax-specific RDTs. This is of concern, as both malarial parasites are cocirculating in many regions. The diagnosis of life-threatening P. falciparum malaria may be missed (two-band Pv-pLDH RDT), or the patient may be treated incorrectly with primaquine (three- or four-band RDTs) which may cause hemolysis in patients with glucose-6 phosphate dehydrogenase (G6PD) deficiency.

As batch lots of RDTs are variable in their specificity, the authors of the study did not give manufactures details, which were chosen from the World Health Organization's (WHO; Geneva, Switzerland) list of recommended kits. The results of the study were published in the July 2010 issue of the Malaria Journal.

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