Laboratory diagnosis of measles virus infection in the setting of an outbreak

Aim An outbreak of measles occurred in south western Sydney between May and October 2012. We examined proven cases to determine the performance and turn-around times of the available diagnostic methods. Methods Suspected cases had serum collected for measles IgM testing and/or nasopharyngeal aspirates (NPA) or swabs (NPS) for measles PCR and immunofluorescence staining (DFA). Demographic and clinical details of cases were obtained from the electronic medical record. Turn-around time (TAT) was defined as the time taken to notify the clinician of a positive result and was calculated separately for specimen collection time and time of specimen receipt in the laboratory. Results 173 suspected cases were examined in the study with 161 undergoing IgM testing, 83 for PCR and DFA testing. There were 86 proven cases of measles identified during the outbreak and IgM testing detected 74 of these. A further 12 cases were detected by PCR or DFA of them only one had IgM testing performed, and was negative at the time of testing.The median TAT for IgM diagnosis of cases was 18 hours (IQR 6–20 hours) from time of collection and 4 hours (IQR 2–7 hours) from time of specimen receipt in the laboratory. As expected the TAT for PCR and DFA testing was longer as these tests were sent to a reference laboratory off-site. All cases had clinical features of measles on presentation and 37% were admitted to hospital. The outbreak mainly involved teenagers (34% of cases aged 10–19 years) and infants Discussion Measles IgM testing remains the most rapid and practical diagnostic test for measles during an outbreak. An acceptable TAT was achieved which enabled prompt administration of post-exposure prophylaxis and institution of isolation procedures. The majority of diagnostic delay occurred between specimen collection and receipt in the laboratory and this was mainly due to transportation times from distantly located hospitals and the absence of testing after-hours and on weekends.