Evaluation of darkground microscopy as a rapid diagnostic procedure in leptospirosis

Although darkground microscopy (DGM) is the method employed for examining the growth of leptospires in culture, it is not useful for diagnosis when used directly on body fluids. However, it continues to be used as a method for diagnosing leptospirosis in many centres in India. The present study was undertaken to evaluate the diagnostic accuracy of DGM by comparing it with established tests like culture, microscopic agglutination test (MAT), IgM ELISA and Lepto-Dipstick.

A total of 170 patients clinically suspected to have leptospirosis were included in the study. The gold standard for diagnosis was positive blood culture, or seroconversion, or a four-fold rise in titre in MAT. DGM was done on plasma after centrifugation at 1000 g and 3000 g and on serum. Indices of accuracy including sensitivity, specificity, predictive values and kappa value of agreement with the gold standard diagnostic criteria were calculated.

DGM had a sensitivity of 40.2 per cent, specificity of 61.5 per cent, a positive predictive value of 55.2 per cent and a negative predictive value of 46.6 per cent. It had agreement with the gold standard in only 50 per cent cases the kappa value being 0.017141.

DGM has low indices of accuracy. The results obtained in patients who met the gold standard criteria for diagnosis and those who did not were identical indicating that the test results are not determined by the presence of leptospiral infection in the patient. DGM, therefore, is not recommended as a sole diagnostic procedure for early diagnosis of leptospirosis.