Assay of rubella antibody by passive hemagglutination and by a modified indirect immunofluorescence test

The passive hemagglutination (PHA) test and a modified indirect immunofluorescence (IF) test were compared with the standard hemagglutination-inhibition (HI) test for the assay of rubella antibody. For sera from healthy individuals with or without detectable rubella antibody the agreement between the new and the standard procedure was 96.4% for the PHA and 98.1% for the IF test. In rubella patients with serologic confirmation by HI, the IF test detected significant rises in 219 (99.1%) of 221 patients and the PHA test detected antibody conversion in 68 (93%) of 73 patients. In some circumstances the PHA or the IF test may facilitate a presumptive or definitive diagnosis. Absence of PHA reactions in single sera from patients with HI antibody was often (41% of such cases) an early indication of current rubella infection. IF antibody increased more gradually than HI antibody in rubella patients, and diagnostic IF titer rises were observed in several cases when HI titers had reached a plateau.