Abstract
Objectives: The sensitivity of laboratory confirmation of invasive meningococcal disease (IMD) by culture or PCR is affected by prior antibiotic treatment and decreasing use of early lumbar puncture. Serological diagnosis of IMD is not widely used because of reliance on paired serum samples. The application of single point estimations of IgM antibodies in the diagnosis of IMD was explored. Design: Outer membrane proteins from a mix of commonly encountered meningococcal serotypes were partially purified and used as an antigen in an enzyme immunoassay for the detection of IgM antibody. The cut-off for the assay was derived using sera from blood bank donors and the accuracy then evaluated with sera from patients with culture-confirmed IMD, other bacterial infections and culture-proven nasopharyngeal colonisation with Neisseria meningitidis. Results: The coefficient of variability of the assay was < 10% in negative, mid- and high-range positive sera and the specificity of the assay was at least 93%. In sera collected from 49 adult patients at various times after positive blood or CSF culture-confirmed IMD, the assay had a sensitivity of 100% in specimens collected between 5 and 18 days. At the time of isolation of meningococci from either blood or CSF, eight of 29 sera were IgM-positive, but beyond 70 days no positive results were detected. No differences were seen in the IgM responses in patients from whom different serogroups of N. meningitidis were recovered. Conclusions: Serological examination by single point IgM enzyme immunoassay (EIA) offers the possibility of an expanded laboratory confirmation of IMD in adults for samples taken between 5 and 18 days after onset.