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 ag-glutination 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 ap-plied 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 qualita-tively both RKT and DAT demonstrated comparable reliability for VL detection (sen-sitivity = 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 posi-tive 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 (<1:8 and 1:16-1:32 >1:256) or DAT (< 1:25,600 and 1:51,200- 1:409,600 > 1:3,276,800) in patients with confirmed or uncon-firmed VL infection, exclusively strong positive intensities were obtained with RKT. Conclusion: For further optimizing diagnosis and simultaneously assessing magni-tude of immune response to L. donovani infection in Sudanese patients, the combined application of RKT and DAT is recommended.