Diagnosis of Flu
The diagnosis of flu is often made clinically, based on symptoms and history, particularly during known outbreaks or flu season. Flu typically presents with sudden onset fever, chills, fatigue, cough, and muscle aches. Because symptoms overlap with other viral infections, doctors sometimes use tests to confirm the presence of influenza virus—especially in high-risk patients or those with complications.
Clinical Assessment
Healthcare providers assess:
Symptom onset and severity
Exposure to flu, such as recent contact with someone who is ill
Time of year, especially during known influenza outbreaks in the community
The combination of high fever, body aches, dry cough, and fatigue often points strongly to flu, even without testing.
Rapid Diagnostic Tests
Rapid influenza diagnostic tests (RIDTs) can detect flu virus proteins in 10–15 minutes
They are quick but may miss some cases (lower sensitivity), especially early in infection
Useful in settings like emergency rooms or care homes to guide treatment decisions
Molecular Tests (PCR)
Reverse transcription polymerase chain reaction (RT-PCR) is more accurate than RIDTs
Detects small amounts of viral RNA and can identify the specific strain
Often used in hospitals or reference labs
Turnaround time is usually a few hours to a day
Testing Is Most Helpful When:
The patient is in a high-risk group (elderly, pregnant, chronically ill)
Hospital admission is required
The illness may affect public health responses (e.g., in an outbreak)
Another diagnosis needs to be ruled out (e.g., COVID-19 or bacterial pneumonia)
Differential Diagnosis
Doctors must rule out:
Common cold, which has milder symptoms
COVID-19, which overlaps heavily with flu and often requires concurrent testing
RSV, especially in children and the elderly
Pneumonia, particularly if chest pain, productive cough, or shortness of breath is present
In most mild cases, lab confirmation is not necessary unless symptoms persist or worsen.
Diagnosis of Flu
The diagnosis of flu combines clinical judgement with selective testing, depending on severity, context, and risk. Early identification supports timely treatment and helps prevent spread to others.
