Short communication
Performance of laboratory diagnostics for the detection of influenza A(H1N1)v virus as correlated with the time after symptom onset and viral load

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Abstract

Background

To diagnose influenza A(H1N1)v virus infection, accurate and rapid detection are important. However, there is scanty data on the performance of various laboratory diagnostics.

Objective

To compare the performance of rapid antigen test (RAT), viral culture and RT-PCR for the detection of influenza A(H1N1)v virus and to correlate their performance with the time after symptom onset and viral load.

Study Design

From May 1, 2009 to June 25, 2009, respiratory samples were collected from 5740 individuals suspected of having influenza A(H1N1)v infection. The performance of viral culture and RT-PCR were investigated and correlated with the time after symptom onset. The sensitivity of RAT ESPLINE influenza A & B-N (Fujirebio Inc, Tokyo) was evaluated using a subset of 60 samples from patients diagnosed as having influenza A(H1N1)v infection.

Results

Using respiratory samples from 587 patients diagnosed with influenza A(H1N1)v infection, comparison of laboratory diagnostics showed viral culture and RT-PCR gave comparable results with overall sensitivity of 93.9% and 98.1%, respectively. For RAT, when testing a subset of 60 samples collected ≤3 days following symptom onset, the sensitivity was 62%.

Conclusions

Although viral shedding is prolonged and of higher titre in influenza A(H1N1)v infection, RAT showed a low sensitivity of 62% among patients presenting ≤3 days after symptom onset. Viral culture showed comparable performance with RT-PCR and with sensitivity better than that documented for seasonal influenza.

Section snippets

Background

The current outbreak of novel influenza A (H1N1)v virus [henceforth: influenza A(H1N1)v virus, where v stands for variant, as proposed by the World Health Organization] was first detected in Mexico in late March 2009 and spread worldwide rapidly. The first case in Hong Kong was detected in a visitor from Mexico on May 1, 2009. The infection continued to be introduced by overseas visitors/returnees for over 1 month when the first local outbreak occurred on June 10, 2009.

Objectives

We compared the performance of RAT, viral culture and RT-PCR tests for the detection of influenza A(H1N1)v virus and correlated the sensitivity with the time after symptom onset among patients diagnosed as having influenza A(H1N1)v infection.

Clinical specimens

Nasopharyngeal aspirate (NPA), combined throat and nasal swabs (TNS) nasopharyngeal swabs (NPS) and throat swabs (TS) were obtained from individuals fulfilling both the clinical and epidemiological criteria for suspected influenza A(H1N1)v infection.1 Specimens were placed in viral transport media and transported to the laboratory within 24 h after collection.

Data collection

Influenza A(H1N1)v virus infection was made notifiable disease on April 27, 2009 in Hong Kong. All patients admitted to hospital with

Patient descriptions

During the first 8 weeks of influenza A(H1N1)v outbreak, 5740 patients were tested and 596 patients were diagnosed as having influenza A(H1N1)v infection with a positive result on viral culture and/or RT-PCR assays. Information on date of symptom onset was available for 587 of these patients. They ranged in age from 5 months to 70 years, 60% were in the age group between 11 and 20 years and 99% were younger than 60 years, the mean age of patients was 21 years. Respiratory samples comprised 229

Discussion

Our data show, for the first time, comparison of RAT, viral culture and RT-PCR for the detection of A(H1N1)v as correlated with time after symptom onset and viral load. In the present study, influenza A(H1N1)v virus replicated well in MDCK cells producing cytopathic effect (CPE) as early as 2 days after inoculation, with an average of 3.5 days after inoculation (N = 551; confidence interval, CI = 3.4–3.6), much earlier than influenza A(H1N1) and A(H3N2) viruses that took 5.2 days (N = 47; CI = 4.7–5.7)

Conflicts of interest

None.

Acknowledgements

We thank all staff of the Virology Division, Public Health Laboratory Services Branch, and Surveillance and Epidemiology Branch, Centre for Health Protection, for technical assistance and epidemiological information.

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