Leptospiral and brucellar titers in frozen bovine sera after 20 years of storage.

Generally good agreement was obtained between the rankings of sera titers when originally tested (T1), and those of the same sera 20 years later (T2). At the later time, the majority of samples (88%) were in, or within 1 dilution factor of their classification at T1. Test results for Brucella abortus and Leptospira interrogans var pomona antibodies demonstrated that original titers, in the upper-lower limits of 20 to 50, had decreased over a wide range (20% to 90%), and the later tests show that the greater the original titer, the greater the proportional decrease. Thus, titers of 20,000 at T1 had decreased by 49% to 99% and those of 200,000, by 95% to 99.8%. Variations in T2 test results were noted between some samples which had given identical results at T1. A decrease in titer over time has been demonstrated. An estimate of the expected half-lives of sera of differing initial Leptospira antibody titers is that for each unit increase in log10 of the initial titer, the half-life is reduced by about 50%. The heterogeneity of the immune response is postulated as the reason for this variation.