Performance and Reliability of the Enzygnost Measles

Evaluation of the Enzygnost Measles Enzyme-Linked Immuno-Sorbent Assay kit (Behring) performance to detect specific immunoglobulin M (IgM) was carried out with 3,297 single serum samples and 898 paired serum samples collected during a measles epidemic (10,184 reported cases) in Quebec, Canada. Anti-measles IgM and IgG were detected by using the Enzygnost kit with the appropriate conjugates. Complement-fixing (CF) antibody (Ab) titers were assessed by the laboratory branch complement fixation micromethod. The Centers for Disease Control's clinical measles case definition was used. A modification of the manufacturer's optical density interpretation algorithm was introduced to allow for equivocal results, in addition to positive and negative ones. These three categories differed as to their association with a significant increase in CF Ab titer and the time between the onset of symptoms and phlebotomy. The IgM positivity rate for complement fixation-confirmed measles cases was 96.6% for vaccinated subjects and 100% for nonvaccinated subjects. The daily percentage of IgM seropositivity that was detected for subjects who became IgM positive within 30 days increased gradually from 40 to 90% for sera taken 1 to 7 days after the onset of symptoms, and it plateaued at 100% for sera taken 16 to 30 days after the onset of symptoms. IgM seropositivity was strongly associated with IgG seroconversion, CF Ab titer increase, and clinical measles (P < 0.0001). Reproducibility was 100% for nonreactive sera and 99.1% for reactive sera. In conclusion, the Enzygnost Measles Enzyme-Linked Immuno-Sorbent Assay kit performed adequately to confirm measles virus infection during this epidemic. A second serum sample should be tested when an early-acute-phase serum sample is IgM negative.