To the Editor: Light-microscopical examination of blood smearsis the cornerstone of the diagnosis of malaria. Recently, tworapid immunodiagnostic tests, the ParaSight F test (Becton Dickinson,Cockeysville, Md.)1 and the ICT Malaria Pf test (ICT Diagnostics,Balgowlah, Australia) ,2 were developed for the diagnosis ofPlasmodium falciparum infection. Both are immunochromatographictests based on the detection of circulating P. falciparum histidine-richprotein-2 antigen in whole blood.3 Recently, Laferl [not Laferi,as published] et al., in a letter to the editor,4 and Bartoloniet al.5 reported false positive results of ParaSight F testsin 67 percent and 68 percent of patients with rheumatoid factors,respectively. Since the production of rheumatoid factors occurscommonly in patients with a variety of infectious and noninfectiousdiseases, as well as in 5 percent of the healthy people, cautionwas urged in the use of the ParaSight F test.
With the aim of determining whether the ICT Malaria Pf testpresents the same limitation as the ParaSight F test, we performedboth tests on whole-blood samples obtained from 23 patientswith rheumatoid factors and thick blood films that were negativefor malaria. The ParaSight F test was positive in 19 patients(83 percent), whereas the ICT Malaria Pf test had no false positiveresults. The different performance of the two rapid tests, bothbased on the capture of histidine-rich protein-2 antigen, isprobably due to the fact that the antibody used in the ICT MalariaPf test is different from that used in the ParaSight F test.
Alessandro Bartoloni, M.D. Università di Firenze 50121Florence, Italy
Guido Sabatinelli, M.D. Istituto Superiore di Sanità 00161Rome, Italy
Maurizio Benucci, M.D. Azienda Sanitaria Firenze 50122 Florence,Italy
References
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Laferl H, Kandel K, Pichler H. False positive dipstick test for malaria. N Engl J Med 1997;337:1635-1636. [Free Full Text]
Bartoloni A, Strohmeyer M, Sabatinelli G, Benucci M, Serni U, Paradisi F. False-positive Para-Sight-F test in patients with rheumatoid factor. Trans R Soc Trop Med Hyg 1998;92:33-34. [Medline]
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