Leptospirosis in dogs can be diagnosed by detecting the organism by PCR or culture, or by demonstrating a fourfold rise in MAT titers. A single MAT titer of > 1:800 with compatible clinical signs and laboratory abnormalities is also considered diagnostic of leptospirosis. The purpose of this study was to determine the sensitivity and specificity of an initial MAT titer of > 1:400 or > 1:800 at different laboratories for diagnosing leptospirosis.

Initial MAT titers were measured at 1 to 5 laboratories in 10 dogs with leptospirosis confirmed via a fourfold rise in MAT titers or positive PCR, in 18 SPF dogs, and in 16 dogs with clinical signs and laboratory abnormalities consistent with leptospirosis in which an alternate diagnosis was confirmed. Sensitivity and specificity at titers of > 1:400 and > 1:800 for any serogroup were calculated for each laboratory.

Sensitivity of an initial titer of > 1:400 ranged from 50% to 67%. Sensitivity of an initial titer of > 1:800 ranged from 22% to 67%. Specificity at an initial titer of > 1:400 or > 1:800 was 100% at all laboratories for identifying SPF dogs. Specificity at a titer of > 1:400 ranged from 69% to 93% and at an initial titer of > 1:800 ranged from 69% to 100% in ill dogs without leptospirosis.

Sensitivity of an initial MAT titer varied between laboratories and at different cutoffs. False positives did not occur in SPF dogs but occurred in sick dogs that do not have leptospirosis.