The agglutination test when carried out so as to give the entire range of serum dilutions to the limit of clumping is a delicate test which reflects a variety of conditions involved in infection with Bacillus abortus. Among these conditions are its time relation to the act of abortion and the length of time the abortion bacilli live and multiply in the pregnant uterus. It is obvious that if a uterus be infected in the 8th month of pregnancy, the opportunity for agglutinins to accumulate are poorer than if the uterine infection lasts 3 or 4 months. The presence of Bacillus abortus in the udder determines in many cases the intensity of the reaction. No definite rules can therefore be formulated for the interpretation of the agglutination reaction quantitatively, since it is bound up with a complicated process varying from case to case. In the individual cow in general a titer of 1:40 or less may be regarded as indicating that the cow is not infected with Bacillus abortus at the time of the blood examination. It does not exclude former infections in the case of older cows, nor does it absolutely exclude very recent infection (Nos. 278 and 351).
The highest titers, 1:640 and above, generally indicate recent infection and in the absence of recent premature births infection of the udder. Even when abortion has just occurred, it may be due to other agencies and the high titer maintained by a chronic infection of the udder dating from an earlier uterine infection with Bacillus abortus. Intermediate titers may indicate a gradual rise or decline of agglutinins preceding or following abortion without infection of the udder. They may also stand for a relatively high resistance or partial immunity of the cow.
In any herd a uniformly low titer (1:40 or less) in all animals may be regarded as indicating the entire absence of Bacillus abortus. A high titer in any one cow serves to indicate quite definitely the presence of infection in the herd. To determine more accurately the character of the infection in any individual cow there is needed in addition to the quantitative agglutination test a bacteriological study of the milk and of any prematurely discharged calf or fetus.