False Positive Dipstick Test for Malaria

Abstract

The ParaSight F antigen-capture assay (dipstick test) (Becton Dickinson Microbiology Systems, Sparks, Md.) is a new diagnostic test for Plasmodium falciparum malaria based on the detection of circulating histidine-rich protein-2 antigen. Several studies have shown that this dipstick test is a simple and accurate way to diagnose P. falciparum infection, both in areas where malaria is endemic and in travelers returning from such areas.^1,2 This test was used in conjunction with conventional Giemsa-stained blood films in a busy department of internal medicine that is also responsible for the management of imported tropical diseases. Over the one-year period from July 1, 1996, to June 30, 1997, the ParaSight F test was used in 173 patients presenting with fever after returning from a malaria-endemic area. As compared with microscopical examination, the test was true positive in 26 patients, false positive in 4, true negative in 142, and false negative in only 1. Of special interest were the four patients in whom the ParaSight F test gave repeatedly positive results although no malaria parasites were seen in thick blood films. Three of these patients had rheumatoid factor, and the fourth had low titers of antinuclear antibodies. This prompted us to perform the ParaSight F test on serum samples obtained from 12 patients with rheumatoid-factor–positive rheumatoid arthritis; the test gave a clear positive result in 8 of the 12 patients (67 percent). None of these 12 had ever been in a malaria-endemic area or received a blood transfusion within the past year. Rheumatoid factors may be present in patients with a variety of infectious and noninfectious diseases as well as in 5 percent of healthy people (up to 20 percent in people over 65 years of age).³ We believe that further investigations are required before the ParaSight F test can be recommended without restriction to replace microscopical examination as the diagnostic tool for malaria in the future.^4,5