Q: How many hours after a fever starts is the best time to get a flu test?
- Myeongchan Kim, MD
- Jul 8
- 2 min read
Updated: Jul 21
Medically reviewed by Sang Hyun Ahn, MD
Content edited by Myeongchan Kim, MD
This post is a dramatization of a question and answer session experienced by our actual medical reviewers. Please note that this content is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment.
Question:
I have an inquiry about flu testing. How many hours after a fever starts should we wait before getting a flu test? I also heard that the test might not detect the virus if the fever goes down. Is this true?
Answer:
I understand your question regarding the timing of a flu test. Proper timing is crucial for the most accurate test results.
Optimal Timing for a Flu Test:
• The test is most accurate when performed 12 to 48 hours after symptoms begin.
• At least 12 hours should pass after the onset of symptoms for the virus to be properly detected.
• The possibility of a false-negative result increases after 48 to 72 hours.
Fever and Test Results:
Testing is still effective even if the fever drops:
• The virus remains in the body even if the fever resolves.
• However, the detection rate naturally decreases 3 to 4 days after symptoms begin.
• It is highly recommended to take the test before starting any antiviral medications.
When Getting Tested:
When you visit the clinic, be sure to inform your healthcare provider of the exact time your symptoms started, along with any other symptoms you are experiencing (such as coughing or body aches). If a rapid antigen test comes back negative but flu-like symptoms persist, a PCR test may be considered.
Antiviral medications are most effective when started within 48 hours of symptom onset. Therefore, if you suspect the flu, please visit a doctor promptly to get tested and receive a proper clinical evaluation.
For extra peace of mind and clear guidance when tracking your child's symptoms, the FeverCoach app is always there to help.
