Evaluation of microscopic agglutination test as a diagnostic tool during acute stage of leptospirosis in high & low endemic areas.

BACKGROUND &
OBJECTIVES: Making a diagnosis on the results of a single microscopic agglutination test (MAT) is difficult because of the uncertainties about the cut-off titre. The present study was conducted to determine the significant titre for a single MAT in areas of high and low endemicity for leptospirosis.
METHODS: A total of 1944 serum samples were collected from healthy individuals and confirmed patients residing in areas of high and low endemicity. All the sera were screened by MAT using 10 live leptospiral strains as antigens. From the distribution of titres among healthy individuals and in patients, the sensitivity and specificity at different cut-off titres were calculated. Likelihood ratio positive (LR+), likelihood ratio negative (LR-), and LR+/LR- were calculated. Receiver operating characteristics (ROC) curves were plotted for the early and late stages of the disease in both the areas.
RESULTS: The ROC plot was totally below the no benefit line during the first week of illness in high endemic area. During the second to fourth weeks it showed better characteristics and the best cut-off titre was 1:200, where the sensitivity was 93.4 per cent and specificity 74.7 per cent LR+ LR- ratio was 41.82 indicating reasonable separation between the positive and negative test results. In the other states the ROC plot was above the no benefit line even during the first week, the best cut-off being 1:50 where the sensitivity was 56.7 per cent and specificity was 90.6 per cent. During the second to fourth weeks the test showed the best characteristics in the low endemicity regions with an ROC curve having the ideal shape. Best cut-off was at 1:100 where the sensitivity was 96.6 per cent and specificity 94.8 per cent LR+ LR- ratio was 523.25 indicating a wide separation between the positive and negative test results. INTERPRETATION &
CONCLUSION: MAT does not have any diagnostic value during the first week, particularly in high endemic areas. The best cut-off to be used will be 1:50 in low endemicity areas during the first week, 1:100 during the second to fourth week and 1:200 in high endemicity regions during the second to fourth weeks.