Nasal swabs offer a sensitive sampling method for the detection of respiratory viruses in children. Respiratory syncytial virus (RSV) is an exception and it is detected more often in nasopharyngeal aspirates (NPA) than in nasal swabs when it is searched for using immunoassays or viral culture.1,2 Therefore, more laborious and painful NPA have been the first‐choice specimen for high‐yield recovery of RSV by conventional methods. We wanted to examine whether the use of reverse transcription‐polymerase chain reaction (RT‐PCR) increases the usefulness of nasal swabs by comparing the performance of nasal swab‐RT‐PCR with NPA‐immunoassays in the detection of RSV infections in children.
We studied 112 children admitted to the Department of Paediatrics, Turku University Hospital, Turku, Finland between November 2003 and February 2004 when there was an RSV epidemic in Finland. The committee on ethics of the hospital district approved the study protocol, and informed consent was obtained from the parents of all participating children.
An NPA and a nasal swab were obtained from opposite nostrils.1 NPAs were tested for RSV antigen at the point‐of‐care (POC, n = 101) using the Novitec RSV Rapid Test (Hiss Diagnostics, Freiburg, Germany) and in the laboratory using time‐resolved fluoroimmunoassay (TR‐FIA, n = 112).3 Swabs were stored frozen at −70°C until the end of the study period, when they were subjected to RT‐PCR4 and a fluorometric hybridisation assay with a sequence specific probe (n = 112). All NPAs from patients with a negative or missing result in any of the above assays (n = 45) were tested using RT‐PCR. The gold standard was defined as a concordant positive result by immunoassays or, when at least one of them was negative, the result of RT‐PCR in NPA.
A result indicating confirmed RSV infection was obtained in 83 of the 112 patients (table 1).
Table 1 Detection of RSV by various methods.
| Results obtained | No. of results | |||
|---|---|---|---|---|
| Swab‐RT‐PCR | NPA‐TR‐FIA | NPA‐POC | NPA‐RT‐PCR | |
| Positive | Positive | Positive | Not done | 67 |
| Positive | Positive | Not done | Positive | 4 |
| Positive | Positive | Negative | Positive | 8 |
| Positive | Negative | Negative | Positive | 2 |
| Negative | Positive | Positive | Positive | 1 |
| Negative | Negative | Not done | Positive | 1 |
| Negative | Negative | Negative | Negative | 18 |
| Negative | Negative | Not done | Negative | 6 |
| Negative | Negative | Positive | Negative | 5 |
Nasal swab specimens were tested using reverse transcription‐polymerase chain reaction (Swab‐RT‐PCR, n = 112) and corresponding nasopharyngeal aspirates from the same patients were tested using time‐resolved fluoroimmunoassay (NPA‐TR‐FIA, n = 112), immunochromatographic point‐of‐care test (NPA‐POC, n = 101) and RT‐PCR (NPA‐RT‐PCR, n = 45).
One patient was positive by RT‐PCR in NPA only; he was the twin brother of a girl with both samples positive for RSV. Apparent false‐positive POC test results were obtained in five samples, all yielding other viral agents in the laboratory testing (data not shown). The sensitivity, specificity, positive predictive value and negative predictive value were 98%, 100%, 100% and 94% for the nasal swab‐RT‐PCR, 96%, 100%, 100% and 91% for the NPA‐TR‐FIA, and 87%, 78%, 93% and 64% for the NPA‐POC, respectively. In agreement with earlier results,1,2 nasal swabs tested for RSV by TR‐FIA showed only 50% sensitivity (data not shown). The overall agreement between the nasal swab‐RT‐PCR and the gold standard was 98%, with a κ value of 0.95 which indicates excellent agreement beyond chance. In 20 children with the nasal swab obtained on the day following the NPA, the results were in total agreement. The RT‐PCR results were not affected by the interval between collecting and freezing the swab (table 2).
Table 2 Stability of the nasal swab.
| Interval (days) | No. of patients | No. of positive swabs | No. of positive NPAs |
|---|---|---|---|
| 0 | 44 | 30 | 30 |
| 1 | 42 | 30 | 32 |
| 2–5 | 26 | 21 | 21 |
| Total | 112 | 81 | 83 |
The interval is the time between collection and freezing of the swab. The swabs were stored frozen at −70°C for 1–6 months until tested for RSV using reverse transcription‐polymerase chain reaction (RT‐PCR). The positive findings were compared with those in corresponding nasopharyngeal aspirates (NPAs) tested using time‐resolved immunoassay (TR‐FIA) and/or RT‐PCR.
The collection of a nasal swab is simple and well tolerated for repeated sampling. Our study demonstrates that it is useful for the specific diagnosis of RSV infection when a sensitive amplification method is used for the detection of viral RNA.
Acknowledgements
We thank Anne Nurmi, RN, for taking care of the patients and Johanna Vänni for technical assistance.
Footnotes
Funding: This work was supported by the Paediatric Research Foundation, Finland.
Competing interests: None.
Published Online First 11 July 2007
References
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