Comparison of the complement-fixation and agar gel immunodiffusion tests for diagnosis of subclinical bovine paratuberculosis.

The performance of the serum complement fixation (CF) test was compared with that of a serum agar gel immunodiffusion (AGID) test on 74 subclinically infected and 154 uninfected cattle in 6 commercial midwestern dairy herds with Mycobacterium paratuberculosis infection and on 30 cattle in a herd that was free of infection. Infection status of cattle within herds was established by performance of a series of 3 or more fecal cultures and of ileocecal lymph node cultures of culled cattle. In cattle with subclinical infection detected by culturing, the sensitivity estimates of the CF and AGID tests were 10.8% (3.6% SE) and 18.9% (4.5% SE), respectively. In the cattle classified as disease free, the specificity estimates of the CF and AGID tests were 97.4% (1.3% SE) and 99.4% (0.6% SE), respectively. Neither set of estimates was significantly different. Negative test results obtained with the use of either test in apparently normal cattle from suspect herds should be interpreted with caution because both tests suffer from low sensitivities in subclinically infected animals. However, the AGID test may be more useful in regulatory situations in which the CF test is currently used because the AGID test is easier to perform and to interpret.