Q: Can an early flu test be a false negative? My child still has a high fever days later, should we retest?

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:

My child developed a fever on Wednesday at 5 PM. We had a flu test done on Thursday at 10 AM, and it came back negative. However, a severe high fever started on Saturday. It is now Sunday, and the high fever is persisting even with alternating fever-reducing medications. Although the initial test was done about 17 hours after the fever first started, is it possible that the test on Thursday missed the flu? Should I take my child back to the clinic to get retested today?

Answer:

Yes, there is a strong possibility that the flu was not detected during the test on Thursday.

Rapid flu tests are most accurate when performed 24 to 48 hours after the onset of symptoms. Since the test on Thursday morning was done only about 17 hours after the fever started on Wednesday evening, it may have been too early to catch the virus, resulting in a false negative.

Given the current situation—a high fever persisting for several days despite alternating medications, and symptoms worsening after an initial negative test—I highly recommend taking your child to the clinic for a retest today. Here is why a retest is a good idea:

1. Timing: Enough time has now passed since the onset of symptoms for a flu test to be highly accurate.

2. Symptoms: A persistent, unresponsive high fever strongly suggests the possibility of the flu.

3. Treatment: If your child is diagnosed with the flu, your doctor can prescribe antiviral medication. While these medications are most effective when started within the first 48 hours, they can still provide benefits up to 5 days after symptom onset.

When you visit the doctor, be sure to provide:

• A record of your child's fever (temperatures and the times they were taken).

• A list of any other symptoms (such as cough, runny nose, body aches, or headache).

• The exact times and dosages of the fever-reducing medications you have administered.

⚠️ Seek immediate emergency care if your child shows any of the following signs:

• Difficulty breathing or chest pain

• Decreased consciousness, extreme lethargy, or inability to wake up

• Signs of dehydration (no tears, no wet diapers for several hours, dry mouth)

• A persistent fever of 40°C (104°F) or higher that does not come down with medication

Even if a repeat flu test comes back negative, the prolonged high fever could be caused by a different viral or bacterial infection. Please consult your pediatrician today for an accurate in-person evaluation and diagnosis.

For extra peace of mind and clear guidance when tracking your child's symptoms, the FeverCoach app is always there to help.

fevercoach.us

Apple App Store

Google Play

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