The sera of 473 individuals were examined for sheep cell agglutinins both by the slide test and the Paul-Bunnell method. In this group there were 46 patients with positive slide tests and 35 of these individuals also had a diagnostic serum dilution test for heterophile antibody. In 11 cases the slide test was positive but the Paul-Bunnell test gave very low serum dilution values. However, when the slide test was carried out at 37 C, it was negative in 9 of the 11 cases. In the remaining 2 instances, one patient had a Forssman type of antibody which gave a 1:64 titer in saline and the slide test was positive at 37 C. In the other case no studies were made on the effect of temperature and the nature of the agglutination reaction was unfortunately not determined.
Using human and bovine albumen, sheep serum and human AB serum absorbed with sheep cells as a diluent no evidence for blocking or hyperimmune antibody was discovered in the cases of infectious mononucleosis studied in this series. Moreover, of the 6 patients with negative serology but with strong clinical and hematological evidence for the disease, no blocking or hyperimmune antibody was disclosed by the slide test or by the use of absorbed human AB serum. The conclusion seems justified that blocking, incomplete or hyperimmune heterophile antibody must be rather uncommon in infectious mononucleosis.
In the use of the rapid slide test it has been pointed out that cold agglutinins, (which may be abolished by warming to 37 C) and Forssman antibodies (which may be absorbed by guinea pig kidney) can give positive results. However, diseases in which cold agglutinins are strong enough to give a positive slide test are relatively rare and the occurrence of Forssman antibodies of a strength likely to give a positive slide test would be decidedly uncommon. In any event unless further experience reveals more serious discrepancies, the rapid slide test as described in this paper seems to offer a practical screening test to detect clinically significant amounts of heterophile antibody in cases of infectious mononucleosis.