Rapid ‘Dipstick’ Assays for the Detection of Malaria

to the editor: I enjoyed the recent article “Case Studies in International Travelers,”¹ in which Dr. Kitchen discussed several clinical vignettes of illnesses, including malaria, in travelers returning to the United States. Microscopy of serial blood smears to detect the presence of the malaria parasites (plasmodium species) continues to be the “gold standard” for clinical diagnosis because it allows estimation of parasitemia, distinction between parasite growth stages and identification of the four different plasmodium species of malaria. However, the article did not address the recent advances in rapid “dipstick” assays to detect malaria. Several commercially available immunochromatographic antigen detection assays are available (see the accompanying table). These assays have been used in various clinical settings in North America and endemic regions.

The older “dipstick” tests (Parasight-F, ICT Malaria P.f and PATH Falciparum Malaria IC Strip) detect only Plasmodium falciparum infection. All three assays detect histidine-rich protein 2, an antigen expressed only by P. falciparum trophozoites.^2,3 However, the two newer tests (the OptiMAL assay, and the ICT Malaria P.f/P.v assay) have the advantage of detecting infection with P. falciparum or Plasmodium vivax. The OptiMAL assay detects parasite lactate dehydrogenase (pLDH) and can distinguish between P. falciparum and P. vivax because of antigenic differences between their pLDH isoenzymes.^4,5 Because only live parasites produce pLDH, this test can be of clinical relevance because it distinguishes between living (current infection) and dead parasites (recently treated infection). The ICT Malaria P.f/P.v test detects histidine-rich protein 2 found only in P. falciparum and panmalarial antigens found in P. vivax, again, allowing the two species to be differentiated.⁶

Although some of these assays are relatively expensive and are awaiting final labeling approval from the U.S. Food and Drug Administration, they can provide excellent diagnostic aids for family physicians caring for immigrants or travelers who have recently returned from malarious areas, as well as for physicians providing health care to persons in developing countries—especially if a trained microscopist is not readily available.

in reply : Dr. Thompson's points are well taken. At the time I wrote my article,¹ I was unaware of the newer multispecies “dipstick” malaria-detection tests he describes.