Evaluation of a standardized direct agglutination test (DAT) for the diagnosis of visceral leishmaniasis in Kenya

Abstract

A prototype test kit being developed, by the World Health Organization (WHO), for the diagnosis of visceral leishmaniasis (VL) was evaluated in the Baringo district of Rift Valley province in Kenya. The screening of approximately 10 000 individuals for the signs of VL produced 305 suspected cases. These cases and 304 controls matched for sex and age (± 2 years) were then tested with the kit, which is based on a direct agglutination test (DAT). The evaluation was a three-stage process. The first stage, the field screening, involved screening filter-paper samples of dried blood from the suspects and controls at a DAT titre of 1:500. The second stage, the laboratory titration, involved screening of the same individuals by testing freshly eluted filter-paper samples at 1:500 to 1:2000 dilution. In the third stage, the full-scale titration, all samples that had been positive at 1:2000 were titrated at 1:500–1:512 000. All the suspects giving DAT titres of 1:2000 or higher were considered positive for VL. This diagnosis was checked, whenever possible, by the examination of smears and/or cultures of splenic aspirates for leishmanial parasites. Those found to be parasitologically positive were put on a standard treatment regime of 20 mg sodium stibogluconate (Pentostam®)/kg.day.

Although 42 (13.8%) of the 305 clinical suspects investigated were DAT-positive (at 1:2000), it was only possible to take splenic aspirates from 32. Four (12.5%) of these 32 were apparently false-positives by DAT, as no parasites could be detected in their splenic aspirates. The others provided positive smears and cultures (27 cases) or a negative smear but a positive culture (one case). It was possible to re-examine two of the four serologically positive but parasitologically negative VL suspects at a 3-month follow-up: neither had a palpable spleen, one had seroconverted and the other had much lower DAT titre (1:32 000) than when investigated previously (1:128 000). All the parasitologically confirmed cases remained DAT-positive (1:2000) at this follow-up. The low cut-off titre (1:2000) and the simple procedure should make the kit suitable for use by health workers at all levels of primary-health care, including those with limited training and skills, for screening rural communities at risk of VL.