Comparison of three antigen detection methods for diagnosis and therapeutic monitoring of malaria: a field study from southern Vietnam

OBJECTIVES To compare the sensitivity, specificity and post-treatment persistence of three commonly used rapid antigen detection methods. METHOD We studied 252 Vietnamese patients aged from 4 to 60 years, 157 with falciparum and 95 with vivax malaria and 160 healthy volunteers. An initial blood sample was taken for microscopy, and OptiMAL(R), immunochromatographic test (ICT.) malaria P.f./P.v.(R) and Paracheck-Pf(R) tests. Patients with falciparum malaria were treated with an artesunate-based combination regimen and those with vivax malaria received chloroquine. Eighty-seven patients with falciparum malaria who were initially positive for one of the antigen tests and who remained blood smear-negative underwent follow-up testing over 28 days. RESULTS Paracheck-Pf(R) was the most sensitive test for Plasmodium falciparum (95.8% vs. 82.6% for ICT malaria P.f./P.v.(R) and 49.7% for Opti.MAL(R)). Specificities were all 100%. For vivax malaria, OptiMAL(R) performed better than ICT malaria P.f./P.v.(R)(sensitivities 73.7% and 20.0%, respectively), with 100% specificity in both cases. All tests had low sensitivities (less than or equal to 75.0%) at parasitaemias < 1000/mu l regardless of malaria species. During follow-up, Paracheck-Pf(R) remained positive in the greatest proportion of patients, especially at higher parasitaemias (> 10 000/mul). Residual OptiMAL(R) positivity occurred only in a relatively small proportion of patients (< 10%) with parasitaemias > 10 000/mul during the first 2 weeks after treatment. CONCLUSIONS Although microscopy remains the gold standard for malaria diagnosis, Paracheck-Pf(R) may prove a useful adjunctive test in uncomplicated falciparum malaria in southern Vietnam. OptiMAL(R) had the lowest sensitivity for P. falciparum but it might have a use in the diagnosis of vivax malaria and perhaps to monitor efficacy of treatment for falciparum malaria where microscopy is unavailable.