RSV tests: One is good for kids, another adults

Synopsis: The Binax NOW chromatographic assay was found to be the optimal method for detection of respiratory syncytial virus (RSV) in upper respiratory secretions of children, while direct fluorescent antibody (DFA) testing was optimal in adults.

Source: Ohm-Smith MJ, et al. Evaluation of the Binax NOW, BD Directigen, and BD Directigen EZ assays for detection of respiratory syncytial virus. J Clin Microbiol 2004; 42:2,996-2,999.

Ohm-Smith and colleagues at San Francisco General Hospital evaluated four assays for the detection of RSV in nasal and/or nasopharyngeal wash, aspirate, or swab specimens from adults and children with suspected viral respiratory infection during the 2002-2003 respiratory illness season. The results were compared to those obtained by cell culture.

All 180 specimens were examined by DFA testing, BD Directigen RSV EIA (D-RSV), and cell culture. Eighty-eight also were tested by the BD Directigen EZ, and 118 by the Binax NOW chromatographic assays. Fifty-three (29%) of the 180 specimens were obtained from adults. Of the 180 specimens, 26% were culture-positive for RSV, including 34% of those collected from children and 8% from adults. Of 149 samples containing an adequate number of cells, there was agreement between DFA and culture in 96%. The overall sensitivity of DFA was 93%, while those of the NOW, EZ, and D-RSV assays were 89%, 59%, and 77%, respectively. The specificities of the assays were 97%, 100%, 98%, and 96%, respectively.

The results obtained with the assays were comparable when applied to samples obtained from children, with the NOW assay being the most sensitive and specific. With samples from adults, DFA was the only effective test among those evaluated.

Comment by Stan Deresinski, MD, FACP

The diagnosis of RSV infection is important for epidemiologic and clinical reasons. Hospitalized patients require adequate isolation and, although only selected high-risk patients are candidates for antiviral therapy, a confirmed diagnosis allows the avoidance of unnecessary antibacterial therapy in others. This study found that rapid assays performed well in children, but the NOW assay provided optimal results in this patient group. In stark contrast, only the DFA provided useful results in adults. Unfortunately, DFA, in contrast to other rapid methods studied, is more labor-intensive and requires highly trained technologists: individuals in short supply, especially in off-hours.

Also, a recent study found DFA had a sensitivity of only 23% in adults with RSV infection.1 That investigation differed from the one reviewed here, in that the gold standard was either a positive culture, serology, or PCR, while the current one relied only on culture. Nonetheless, the current guidelines of the Infectious Disease Society of America, for the management of immunocompetent adults with community-acquired pneumonia, state: "[RSV] antigen detection tests are readily available but are insensitive for detecting infections in adults and are not generally recommended for adults."2

Ohm-Smith, et al. agree that while the Binax NOW assay appears to be optimal for use in children, none of the rapid kit assays other than DFA, were useful in adults. Their data do suggest that DFA has value in adults for whom they suggest a two-step procedure: "Laboratories should consider performing direct immunofluorescence and/or culture testing of specimens from adults who give negative results with any of the rapid kit tests."

References

1. Casiano-Colon AE, et al. Lack of sensitivity of rapid antigen tests for the diagnosis of respiratory syncytial virus infection in adults. J Clin Virol 2003; 28:169-174.

2. Mandell LA, et al. Updated practice guidelines for the management of community-acquired pneumonia in immunocompetent adults. Clin Infect Dis 2003; 37:1,405-1,433.

Stan Deresinski, MD, FACP, is Associate Chief of Infectious Diseases, Santa Clara Valley (CA) Medical Center.