False-positive results of a rapid K39-based strip test and Chagas disease.
BACKGROUND
The definitive diagnosis of visceral leishmaniasis (VL) requires invasive procedures with demonstration of amastigotes in tissue or promastigotes in culture. Unfortunately, these approaches require laboratory materials not available in poor countries where the disease is endemic. The correct diagnosis of VL is important, and made more difficult by the fact that several common tropical diseases such as malaria, disseminated tuberculosis, and enteric fever share the same clinical presentation. Serological tests have been developed to replace parasitological diagnosis in the field. A commercially available K39-based strip test for VL has been developed for this purpose. The endemic area of leishmaniasis in Brazil overlaps the endemic area of Chagas disease, a disease that can cause false-positive serological test results. The aim of this study was to evaluate the incidence of false-positive exams using a rapid test for VL in patients with Chagas disease.


METHODS
A rapid test based on the recombinant K39 antigen of Leishmania was used in: (1) 30 patients with confirmed Chagas disease, (2) 30 patients with a serological diagnosis of Chagas disease by ELISA, indirect immunofluorescence, indirect hemagglutination, and chemiluminescence, (3) 30 healthy patients from a non-endemic area as the control group, (4) 30 patients with confirmed VL, and (5) 20 patients with proved cutaneous leishmaniasis.


RESULTS
The sensitivity and specificity of the rapid strip test were 100% when compared with healthy volunteers and those with confirmed Chagas disease. One false-positive result occurred in the group with Chagas disease diagnosed by serological tests (specificity of 96%).


CONCLUSIONS
The rapid test based on recombinant K39 is a useful diagnostic assay, and a false-positive result rarely occurs in patients with a serological diagnosis of Chagas disease.
Amato Neto
Vicente
V
Laboratory of Medical Investigation-Parasitology (LIM 46), Hospital das Clínicas, Medical School, University of Sao Paulo, Sao Paulo, Brazil.
Amato
Valdir Sabbaga
VS
Tuon
Felipe Francisco
FF
Gakiya
Erika
E
de Marchi
Claudia Regina
CR
de Souza
Regina Maia
RM
Furucho
Célia Regina
CR
D000818:Animals; D000913:Antibodies, Protozoan / Q000097:blood*; D000953:Antigens, Protozoan / Q000276:immunology*; D001938:Brazil / Q000453:epidemiology; D014355:Chagas Disease / Q000150:complications* / Q000453:epidemiology; D005189:False Positive Reactions; D006801:Humans; D007118:Immunoassay; D015994:Incidence; D007891:Leishmania / Q000276:immunology; D007898:Leishmaniasis, Visceral / Q000150:complications / Q000175:diagnosis* / Q000453:epidemiology; D015800:Protozoan Proteins / Q000276:immunology*; D011933:Reagent Kits, Diagnostic; D011934:Reagent Strips; D012680:Sensitivity and Specificity; D013997:Time Factors
D000913:Antibodies, Protozoan; D000953:Antigens, Protozoan; D015800:Protozoan Proteins; D011933:Reagent Kits, Diagnostic; D011934:Reagent Strips; C079253:K39 antigen, Leishmania