Leptospirosis Serodiagnosis by the Microscopic Agglutination Test

The microscopic agglutination test (MAT) is the gold standard for sero-diagnosis of leptospirosis because of its unsurpassed diagnostic specificity. It uses panels of live leptospires, ideally recent isolates, representing the circulating serovars from the area where the patient became infected. A dilution series of the patient's serum is mixed with a suspension of live leptospires in microtiter plates. After incubating for about 2 hr at 30°C, results are read under the dark-field microscope. The titer is the last dilution in which ≥ 50% of the leptospires have remained agglutinated. Seroconversion or ≥ 4-fold titer rise in paired sera is consistent with current leptospirosis. The significance of a titer in a single sample depends on the frequency of residual titers due to past infections and cross-reacting other diseases in the population. Full standardization of the MAT is not possible, but quality assurance can be achieved by participation in the international MAT proficiency testing scheme.