A PCR test for flu is a laboratory method that detects influenza virus genetic material in a respiratory sample (nasal or throat swab). Reverse-transcription PCR converts viral RNA to DNA and amplifies specific sequences to confirm and differentiate influenza A/B and subtypes. It is highly sensitive and specific, used for diagnosis and surveillance, with results typically available within hours to a day.

Costs vary widely by country, lab, and whether testing is public or insured. In the US a flu-panel PCR (respiratory viral panel) typically costs about $50–$300+ out of pocket; private UK tests run roughly £50–£200 while NHS testing is free when clinically indicated; in India private labs charge about ₹1,000–₹5,000. Insurance or public provision often lowers or eliminates out-of-pocket cost.

Most people test positive for influenza from about one day before symptoms begin until roughly 5–7 days after symptom onset. Children and immunocompromised people can shed virus—and test positive—for longer (often up to 10 days or more). PCR tests may detect viral RNA longer than rapid antigen tests, which usually become negative once viral load falls (typically after the first few days).

Doctors usually diagnose flu from symptoms but confirm it with respiratory samples (nasal or nasopharyngeal swabs). Rapid antigen tests give results in 10–30 minutes but are less sensitive. Molecular tests (RT‑PCR or rapid molecular assays) are more accurate and used for hospitalized patients or treatment decisions. Viral culture is rarely used. Testing is recommended for high‑risk, severe, or outbreak cases.

A positive PCR test indicates detection of the virus’s genetic material (RNA or DNA) in the sample, suggesting current or very recent infection. It doesn’t directly measure contagiousness, since remnants of nonviable virus can persist after recovery. Clinical context, symptom timing, and repeat testing guide interpretation; a positive result typically prompts isolation and public-health actions like contact notification.

Influenza typically begins abruptly with high fever, chills, and sudden onset of cough (usually dry), sore throat, nasal congestion, headache, intense muscle and body aches, and marked fatigue. Children may also have vomiting or diarrhea. Severe cases cause shortness of breath, chest pain, lightheadedness, or prolonged high fever—seek medical attention if these occur. PCR testing confirms the viral infection.