Serological reactions of leptospirosis-positive (MAR and CFT) bovine sera in ELISA

The collection of test sera for measuring ELISA results was composed of bovine sera with MAT titres of greater than or equal to 1:200 in the leptospirosis MAT and of greater than or equal to 1:5 in the CFT together with sera from a serologically negative and clinically non-suspicious cattle herd. To establish cut-off ODs, the geometric mean net-extinction of the negative serum collection plus 1, 2, and 3 standard deviations were calculated. By comparison of 3 different conjugates from rabbits, it was demonstrated that results from anti-total bovine Ig were superior to anti-IgG and anti-IgM conjugates. Considerations regarding sensitivity and specificity led to the recommendation to use a test serum dilution of 1:160, to apply anti-total bovine Ig conjugates, and to establish the cut-off OD at the geometric mean net-extinction of negative sera plus 3 standard deviations. Under such conditions, agreement between leptospirosis MAT/CFT positivity on the one side and ELISA positivity on the other was reached in 74%. This recommendation is made for cross-sectional studies but not for examinations of clinically suspicious cattle herds.