Qualitative and semi-quantitative comparison of an rK39 strip test and direct agglutination test for detection of anti-Leishmania donovani antibodies in the Sudan.

Background: Until now, the comparison of the rK39 strip test (RKT) and direct agglutination test (DAT) for detection of visceral leishmaniasis (VL) is exclusively based on either positive or negative qualification of the reaction outcome.

Objective: In this study, we compared the diagnostic performance of RKT and DAT for VL both qualitatively and semi-quantitatively.

Methods: For comparison based on semi-quantitative grounds, the execution of RKT and DAT was according to the standard procedures. For comparison on semi-qualitative grounds with DAT, the RKT was applied to aliquots from positive samples that were two-fold serially diluted in saline to determine, as for the DAT, the end-point reaction in RKT.

Results: While qualitatively both RKT and DAT demonstrated comparable reliability for VL detection (sensitivity = 96% and specificity = 98.7% or 99.3%), no significant correlation (r = 0.13) could be established between intensities of their positive reactions in 25 cases studied. A negative correlation was further determined in those 25 VL cases between the positive intensities of the RKT and antibody levels measured semi-quantitatively with the same procedure (r = -0.36) or the DAT (r = -0.30). Irrespective of the low, moderate or high antibody levels measured with RKT (< or = 1:8 and 1:16-1:32 > or = 1:256) or DAT (< or = 1:25,600 and 1:51,200- 1:409,600 > or = 1:3,276,800) in patients with confirmed or unconfirmed VL infection, exclusively strong positive intensities were obtained with RKT.

Conclusion: For further optimizing diagnosis and simultaneously assessing magnitude of immune response to L. donovani infection in Sudanese patients, the combined application of RKT and DAT is recommended.