False positive hemoagglutination by allantoic fluids of embryonated eggs inoculated with unfiltered throat washings.

The recognition of hemoagglutination in the presence of influenza viruses has greatly simplified virology. Since Hirst's (1941) observation, several other agents such as the viruses of vaccinia (Clark and Nagler, 1943) and mumps (Levens and Enders, 1945) and a pleuropneumonialike organism (Van Herick and Eaton, 1945) have been found to produce this as yet unexplained phenomenon. These reactions can be readily differentiated from one another since they are inhibited by specific antisera. Occasionally in this laboratory certain bacterially contaminated allantoic fluids, obtained from live chick embryos 48 hours after inoculation with unfiltered but penicillin-treated throat washings, have also been found to react with chicken cells. This hemoagglutination may be present even in dilutions of 1:80 and 1:160 and is grossly indistinguishable from that associated with PR-8 or Lee influenza viruses. Unlike those reactions, however, it is inhibited by very small amounts of "normal" rabbit or human sera. The inoculation of unfiltered throat washings into chick embryos is currently becoming a diagnostic procedure (Rickard et al., 1944; Hirst, 1945; Rose et al., 1945; Salk et al., 1945), and so the existence of this type of "false positive" hemoagglutination must be recognized. It was encountered here four times during the past year. The reaction is given by fluids which are relatively clear and which come from live embryos. Consequently, differentiation from the amorphous, coarsely granular precipitate of chicken cells sometimes associated with turbid, heavily contaminated fluids (Salk et al., 1945) is possible by inspection. Nevertheless, on each occasion that "false positive" agglutination was encountered, it was associated with bacterial contamination. Three times it was with a mixed flora identified merely as streptococci, staphylococci, Neisseria, or gram-positive rods, and once with a pure culture of a hemolytic Bacillus subtilis. The circumstances surrounding this last instance illustrate clearly the need for recognizing this "false positive" agglutination. Throat washings with nutrient broth were obtained on January 2, 1946, from a patient (M) on the fourth day of an influenzal illness later shown by agglutination-inhibition tests (Florman and Crawford, 1944) to have been caused by an influenza type B virus. The washings were stored at approximately -20 C until January 7. On that day they were slowly thawed, sufficient penicillin was added to yield a concentration of 1,000 units per ml, and each of six 11-day-old embryonated eggs