Countercurrent Immunoelectrophoresis in the Diagnosis of Haemophilus influenzae Type b Infection: Relationship of Detection of Capsular Antigen to Age, Antibody Response, and Therapy

Sixty-one patients with Haemophilus influenzae type b infection were studied for the presence of capsular antigen by countercurrent immunoelectrophoresis. Antigen was present (≥10 ng/ml) in admission blood or cerebrospinal fluid samples of 92% of patients with meningitis and could be detected in blood for up to 17 days. Despite bacterial cultures reverting to negative, detectable antigen was present at two to three days in 75% of patients, and at four to five days in 54%. No difference in persistence of antigen was found in patients with meningitis who were receiving ampicillin or chloramphenicol. Antigenemia was less frequently detected in patients without meningitis who were studied at the time of hospital admission (7/22, P <.01). However, antigen was detected 24 hours after onset of therapy in one initially negative patient; in four other patients who lacked circulating antigen, bone or joint aspirates or nasopharyngeal secretions were antigen positive. Circulating antigen was less frequently detected in individuals 2 years of age or older and those with convalescent hemagglutinating titers greater than or equal to 1:4. We propose that older patients may develop early anticapsular antibody responses to their infection that interfere with detection of circulating antigen.