When Should I Get Tested for the Flu After a Fever Starts? The “12 to 36 Hours” Window is the Strongest! The Iron Rule for Avoiding False Negatives

When you have a high fever, the immediate urge is to rush to the hospital, but the accuracy (positive rate) of the influenza test is heavily influenced by the time elapsed since the onset of the fever (the start of the fever). If you go less than 12 hours after the fever starts, you are often sent home, as the positive rate for the rapid antigen test is very low. As of 2025, the iron rule for getting a confirmed diagnosis is to aim for the “12 to 36 hours after onset,” which provides the most accurate results.

How Many Hours After Onset Should You Get Tested?

Conclusion: What is the Best Time Frame?

→ Within 12 to 48 hours after onset. 　Especially the 12 to 36-hour window is the most accurate and best time!

Specific Examples (Fever Onset = 0 Hours):

• Fever of 38.5°C at 8 PM (Night) → 8 AM to 8 PM the next day (12–24 hours after) is the super-best time.
• Fever onset at 12 AM (Midnight) on Saturday → Sunday noon to Monday noon (12–36 hours after) is the target.
• 72+ hours have passed → Even if the test is “negative,” it is often considered unreliable.

Common Phrases Doctors Use (Actual Scenarios)

• “It hasn’t been 12 hours yet; please come back tomorrow.”
• “It will be more accurate if you wait about 24 hours.”
• “It’s been over 48 hours, but your symptoms are typical, so I’ll prescribe the medicine.”

The Iron Rule to Remember in One Sentence

“Wait at least 12 hours—ideally about 24 hours—after the fever starts, and if you go within 48 hours, you can’t go wrong!”

At-Home Testing Kit Ranking (Combined COVID-19 and Influenza)

Here is the English translation of the ranking table for at-home diagnostic kits that test for both COVID-19 and Influenza A/B: