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The diagnosis is usually based on clinical considerations, that is, the patient’s symptoms and whether they are compatible with flu virus infection; whether or not it is flu season; and epidemiological data such as how the patient has been in contact with people infected with influenza.
Diagnostic tests are unnecessary for most people with flu because the results would not affect or alter their treatment, which generally consists of maintaining hydration levels, fever control measures and rest.
Population groups at risk of developing complications due to flu (pregnant women, children under 5 years, people over 65 years, people with chronic illnesses such as asthma, heart disease, lung disease, diabetes, patients with a weak immune system) may undergo other diagnostic tests in order to start antiviral therapy early on to help alleviate the symptoms and reduce the severity of the infection and its complications, which can be serious or fatal for these groups.
Diagnostic tests for Flu
Diagnostic tests for the influenza virus are carried out on respiratory samples which should be collected at the earliest opportunity. These samples include:
Nasopharyngeal swab. These are done using two special swabs to collect samples from the nose and throat; they do not cause any damage, but the swabs may tickle and make you want to sneeze. A very thin swab is inserted in a nostril and rotated three times to collect as many cells as possible. A second, slightly thicker swab is used to collect a sample from the throat following the same procedure of wiping the swab three times to try and gather the maximum number of cells.
Nasal lavage. This technique is used in small children because it is straightforward and easy to do. The sampling method uses a syringe to flush 1–1.5 mL of sterile saline solution into one nostril while holding the other closed. Suction is then applied to the syringe to recover the saline solution and nasal discharge. The sample is sent to a laboratory in a container filled with a special culture media for viruses to maintain sample quality.
Bronchoalveolar lavage. This method is carried out in patients in the Intensive Care Unit.
Several diagnostic tests can be used to identify the type of influenza virus:
Virus isolation. This involves cultivating the respiratory sample and isolating the virus.
Detection of viral proteins. These tests entail binding antibodies with proteins on the virus. If viral proteins are detected in the respiratory sample, it confirms the patient has influenza.
Detection of viral genetic material. Tests called molecular assays are used to extract any viral genetic material present in respiratory samples. In this case, if viral ribonucleic acid (RNA) is isolated from the patient’s sample, it confirms the presence of the flu virus.
Due to the increased number of patients attending emergency services, rapid antigen detection tests that return results in 30 minutes are used during the flu season. These rapid influenza diagnostic tests can only detect type A and B viruses. They are limited by their low sensitivity and must be interpreted together with the patient’s clinical signs.