Evaluation of sensitivity and specificity of rapid influenza diagnostic tests for novel swine-origin influenza A (H1N1) virus.

To the Editors: This letter presents the usefulness of the rapid influenza diagnostic tests (RIDTs) to screen patients with suspected novel swine-origin influenza A (H1N1) virus (S-OIVH1N1) during the outbreak in Bangkok, Thailand. On May 12, 2009 the first 2 laboratoryconfirmed cases of S-OIVH1N1 infection were reported from Thailand. Within a month outbreaks occurred in the community and schools. The standard for diagnosis is viral isolation or real time reverse transcriptasepolymerase chain reaction (RT-PCR), however the laboratory capacity is limited. Rapid influenza diagnostic tests (RIDTs) have been used to screen patients with suspected influenza, and they offer the advantage of providing a timely result that might impact patient care. The performance of current RIDTs in diagnosing S-OIVH1N1 is unknown. Therefore, we conducted a study to evaluate the sensitivity and specificity of RIDTs against RT-PCR for novel H1N1 in the background of seasonal influenza in a private health care setting. From June 11 to July 28, 2009, 3096 patients presented with influenza-like illness at the outpatient clinic of Bangkok Christian Hospital and were tested with RIDTs. Only 841 (27%) of the patients had a nasal swab specimen tested with both RIDTs and confirmatory RT-PCR assay. The Quick Vue Influenza A B test (Quidel Corporation, CA, USA) (N 477) or SD bioline influenza antigen test (Standard Diagnostics, Inc., Gyeonggi-do, Korea) (N 364) was performed at the hospital laboratory. The RT-PCR assay was performed at the virology unit, Department of Pathology, Ramathibodi Hospital of Mahidol University. We calculated the sensitivity, specificity, accuracy, positive and negative predictive values, as well as 95% confidence intervals calculated using binomial exact methods, for each RIDT compared with confirmatory influenza testing by RTPCR assay. All of the calculations were performed by using VassarStats (http://faculty. vassar.edu/lowry/clin1.htm). The median age was 13 years (range, 6 months to 97 years); 16.2%, 19.4%, 37.4%, and 27% were younger than 5 years, 6–10 years, 11–20 years, and 20 years. There were 459 (55%) males. Of 3096 nasal swabs tested by RIDTs, 1027 were positive for influenza A (33.2%), and 19 were positive for influenza B (0.61%). Among 841 patients, RIDTs were positive for 376 out of 429 of S-OIVH1N1 confirmed cases, and 118 out of 140 seasonal influenza A. RIDTs were positive for 85 out of 272 patients who had negative RT-PCR tests for both S-OIVH1N1 or seasonal influenza A. The sensitivity and specificity of the RIDTs were 87.6% or 50.7%, respectively. The test parameters are presented in Table 1. Previous studies have described low sensitivity in the range of 17.8% to 51% and high specificity in the range of 93.6 to 99% for detecting S-OIVH1N1 compared with RT-PCR assay. Unlike previous reports, our data show high sensitivity and moderately accuracy for detecting S-OIVH1N1 in the background of seasonal influenza virus activity. The high sensitivity of the test might be explained by the study population, of whom the majority were children who had more viral shedding, and also by the high quality of specimen collection. The low specificity and low positive predictive value of the RIDTs compared with RT-PCR for S-OIVH1N1 may be due to cocirculation of seasonal influenza in the community. Since false negative results can occur, if clinical suspicion of influenza is high in a patient who tests negative by RIDTs, empiric antiviral therapy should be administered based on severity of illness, underlying medical condition, and risk for complications. In conclusion, our findings demonstrate the results of RIDTs, which are highly sensitive and moderately accurate for detecting S-OIVH1N1 in the background of seasonal influenza virus activity. The RIDTs are simple to perform with results available within 30 minutes. The results of RIDTs have clinical and public health implications and have been used to promptly detect S-OIVH1N1 and seasonal influenza virus infections. They allow early appropriate use of antiviral therapy and implementation of outbreak-control interventions, and also obviate further unnecessary diagnostic tests.