Passive hemagglutination assays (PHA) may be used to detect IgA antibodies to confirm clinical diagnoses of suspected IgA anaphylactic transfusion reactions. Passive hemagglutination inhibition assays (PHIA) may be used to identify IgA-deficient blood donors whose plasma- containing components are transfused to prevent anaphylactic transfusion reactions in prospective recipients at risk because of the presence of IgA antibodies. Using a standard PHA, we detected class- specific anti-IgA in 76.3% of 80 IgA-deficient patients with a history of an anaphylactic transfusion reaction, and in 21.7% of 97 asymptomatic IgA-deficient blood donors or their IgA-deficient family members. Using PHIA, we confirmed IgA deficiency (< 0.05 mg/dL) for the donors of 525 plasma-containing blood components that were transfused without acute clinical reactions to 48 IgA-deficient recipients with anti-IgA and/or a history of an anaphylactic transfusion reaction. The frequency of IgA-deficiency with class-specific anti-IgA among 32,376 random blood donors was 0.08% (1/1,200). The combined use of PHA for detecting anti-IgA and PHIA for measuring IgA concentration provides an effective and safe strategy for the diagnosis and prevention of IgA anaphylactic transfusion reactions. However, PHA for anti-IgA lacks specificity for identifying persons who are truly at risk for significant anaphylactic transfusion reactions. The consequence is an overdiagnosis of IgA anaphylactic transfusion reactions and an overestimation of the number of persons at risk for IgA anaphylactic transfusion reactions because of the detection of an IgA antibody in their serum.