Problems in the diagnosis of Q fever by complement-fixation tests.

For the selection of a suitable antigen concentration for use in routine complement-fixation tests with Q-fever antisera, a rigid system of antigen units is unsatisfactory. The optimum antigen dilution should be judged after inspection of the results of full "chess-board" titrations with a variety of antisera. Non-specific reactions may occur with sera from patients with primary atypical pneumonia or sera which have deteriorated during storage. These may be detected with a typhus antigen or some similar control antigen.Nine Mile and Henzerling strain antigens, which are widely used for routine diagnosis, were compared in tests with 868 sheep sera, 1055 human blood-donor sera, 66 human convalescent sera, and 20 guinea-pig sera. Considerable discrepancies were found in the results obtained with the two antigens.Irrespective of antigen concentration, the Nine Mile strain was more sensitive than the Henzerling strain for the detection of Q-fever antibody in human and guinea-pig sera and in some sera from Welsh sheep. With antibody in Kentish sheep sera, on the other hand, the Henzerling antigen was markedly more sensitive than the Nine Mile strain antigen. It is concluded that these two strains cannot be regarded as interchangeable, and the choice of antigen must depend on the geographical area and the species to be tested.