Flu is, in most cases, diagnosed clinically (based on history and symptoms) by a doctor. The doctors, usually, do not recommend any tests. Treatment is started based on clinical signs and symptoms.
Medical history and physical examination: The doctor will take a detailed history of your symptoms. Some of the questions that the doctor may ask include:
- Have you been in contact with a person with flu-like symptoms?
- How high has your fever been (some research shows that high fever of acute onset is likely to be due to inﬂuenza)?
- Do you have other symptoms, such as body pain, malaise, headache?
On physical examination the doctor will do a detailed general examination with specific attention to your respiratory system (lungs).
Laboratory tests: In most cases, lab tests are not required to diagnose flu. If the infection is severe or the diagnosis is doubtful, the doctor may do tests. Tests that may be done include virus culture or a blood test to detect influenza virus antigen. Sample for examination is obtained by swabbing the nose and throat.
- Viral culture: Influenza virus may be detected by culture of the clinical sample (such as throat swab). Viral culture is a definitive way for laboratory diagnosis of inﬂuenza virus, but the culture results are usually available after four to ﬁve days (range, two to 14 days). Therefore, viral culture has limited utility in patient care. Viral cultures, however, are done as virus isolation is important from research and global public health perspective. It can help to identify and monitor changes in inﬂuenza virus strain and make vaccine for newer strains of influenza virus.
- Direct Fluorescent antibody tests: Infection with influenza virus may be diagnosed using monoclonal antibodies against inﬂuenza virus antigen. The test result may be availed at the same day, though the accuracy of the test result is influenced by many factors including the quality of clinical sample used, expertise of the person examining the sample and lab equipment.
- Rapid diagnostic tests for inﬂuenza: Currently, there are over six rapid diagnostic tests for inﬂuenza for general use. These tests detect inﬂuenza virus antigen in the clinical sample sent to the laboratory. They can detect inﬂuenza virus within approximately 30 minutes after the sample is processed. This test can help to guide treatment of a patient with flu-like symptoms. The test may be recommended in people with severe infection or those who are likely to develop complications. Rapid flu test can help to decide the need for anti-viral drugs (these drugs are effective when taken within 48 hours of the first flu symptoms).