Latent class analysis permits unbiased estimates of the validity of DAT for the diagnosis of visceral leishmaniasis

BACKGROUND Substantial uncertainty surrounds the specificity of the Direct Agglutination Test (DAT) for visceral leishmaniasis (VL) in clinical suspec ts, since no good gold standard exists for unequivocally identifying diseas ed subjects. We explored the Latent Class Analysis (LCA) modelling techniqu e to circumvent this problem. PATIENTS AND METHODS Data on 149 clinical suspects recruited in 1993-96 dur ing a multicentre study in Sudan were re-examined. Clinical data, lymph nod e and bone marrow aspirate and DAT results were available. IFAT was perform ed in 1997 on stored filter paper blood of 80 individuals. Classical Validi ty Analysis (CVA) in a 2 x 2 contingency table with parasitology as a gold standard was compared with the parameter estimates produced by the best fit ting LCA model. RESULTS The sensitivity estimates of DAT produced by CVA (98% (89%-100%)) w ere almost exactly reproduced by LCA. The specificity estimates by LCA were substantially higher than those obtained in CVA. Specificity of DAT depend ed, however, on whether the subject was treated for VL before. In subjects without prior treatment, CVA estimated DAT specificity at 68% (56%-79%), wh ereas LCA estimated it at 85% (63%-100%). CONCLUSION LCA modelling proved a useful tool, as it gave consistent estima tes of test characteristics and allowed for control of confounding factors and interaction effects. Since VL is a life-threatening disease for which e xpensive but effective and safe treatment exists, a clinical suspect in an endemic area should be treated on the basis of a positive DAT result.