Comparison of nasopharyngeal aspirate and nasal swab specimens for detection of respiratory syncytial virus in different settings in a developing country

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Abstract
OBJECTIVE To compare detection of respiratory syncytial virus (RSV) for diagnostic purposes using nasopharyngeal aspirate (NPA) and nasal swabs (NS) in different clinical settings in a community study in Guinea‐Bissau. METHOD During 1996–98 paired specimens were obtained from 635 children under 5 years of age (median: 274 days; interquartile range: 144–453 days) with symptoms of lower respiratory infections (LRI). The specimens were analysed by an enzyme‐linked immunosorbent assay for RSV antigen in Guinea‐Bissau and re‐analysed in Denmark using the same assay. The gold standard for RSV antigen detection was defined as any test being positive. RESULTS RSV antigen was detected in 84 (13%) children, the prevalence being 19% (41/219) among infants aged < 6 months, 12% (22/184) in infants aged 6–11 months, and 9% (21/230) in older children. Sensitivity of antigen detection was higher in NPA (92% in analyses in Guinea‐Bissau and 98% in Denmark) than in NS (63% in analyses in Guinea‐Bissau, 71% in Denmark). Specificity of RSV antigen detection was equally high in NPA and NS (99–100%). Time since onset of symptoms was significantly shorter in RSV antigen positive than negative samples. Sensitivity did not depend on clinical setting or age of the child. CONCLUSION Using NS samples was associated with a 27–31% reduction in sensitivity compared with NPA specimens. As NPAs are costly and considered a nuisance by the population, it might be cost‐effective in larger epidemiological studies to lose 25–30% in sensitivity but be able to collect samples from a much larger population.

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