Guidelines for the Prevention and Control of Influenza Outbreaks in Residential Care Facilities for Public Health Units in Australia
4.4 Diagnostic tests
Diagnostic testing available for influenza includes viral culture, serology, immunofluorescence assay (IFA) and other assays (including point-of-care tests) for antigen detection, and PCR for detection of influenza RNA. Tests performed will depend on what is available in laboratories in each state and territory. PCR and IF are generally the preferred initial diagnostic methods as they can provide results within 24 to 48 hours for PCR, and 2-4 hours for IFA (but at a lower level of sensitivity compared with PCR) . PCR and IFA also provide rapid sub-typing results of influenza A strains (i.e., H1N1, H3N2), as well as distinguishing between influenza A and B. Viral culture takes several days, and serodiagnosis requires at least 2 to 4 weeks, and therefore have less practical value in an outbreak setting than the rapid detection methods. Point-of-care influenza tests are rapid (providing a result in 15-30 minutes), but are not as sensitive as PCR or IFA, and are currently not recommended as the only tests undertaken for establishing the cause of an outbreak.
Optimal yield of viral culture/PCR/antigen detection occurs if specimens are obtained early in the course of the disease, preferably within the first 48 hours after the onset of symptoms. Positive results may be achieved up to one week after illness onset. The specimen should be stored at 40C and transported to the laboratory immediately if possible, and definitely within 48 hours.
4.4.1 Type of specimens to be collected
In an outbreak situation, nose and throat swabs should be collected for virus culture and/or PCR (Appendix 7), and serum for serology. Other samples that should be collected include sputum and urine, so that testing may be performed for bacterial pathogens.4.4.2 Swabs
Nose and throat swabs should be taken from residents and staff with acute symptoms (onset within the preceding 48 hours), preferably from cases with the most classical clinical presentation of the illness suspected. Ideally samples should be collected from 8 to 10 people.4.4.3 Specimen collection and transport
The PHU may assist with specimen collection if a site visit is conducted; otherwise specimens might be collected by GPs, laboratory collection staff, or nursing staff trained in collection methods and wearing appropriate personal protective equipment (PPE) (Appendix 8). Samples should be appropriately labelled before transport to the laboratory, and notes on the request form should include ‘Outbreak investigation’. A request form must also be completed by a medical officer.The laboratory should be advised as early as possible that specimens will be arriving, and contact details for person to receive the results communicated to the laboratory. The laboratory should be asked to notify results to this person by phone and/or fax whether positive or negative.
4.4.4 Results of laboratory tests
Apart from viral culture, laboratory results are usually available within 2 to 48 hours of receipt at the laboratory, depending on the test requested. The laboratory should send a hard copy of all results (negative and positive) to the requesting doctor. If this doctor is not from the PHU, the PHU should liaise with him/her about the results. If initial laboratory tests fail to detect influenza, further testing of additional cohorts of recently ill residents should be performed before concluding that the current outbreak is not caused by influenza.The PHU will verify that the facility OIMT representative has received the results. Direction will be provided at the time regarding any additional control, treatment, or prophylaxis measures to be implemented.
