If you suspect you have the flu, rushing to the hospital immediately is often not the best move! The “12 to 48 hours (especially 12 to 24 hours) after the onset of fever” is the Golden Time for receiving an examination and test. This is when the rapid influenza test (rapid antigen test) achieves its highest accuracy, and when anti-influenza medications can exert their maximum effect.
We will explain why going immediately after the fever starts can lead to a “false negative” result, and provide the latest guidelines for the 2024-2025 season, including what to do if more than 48 hours have passed. The general rule of thumb is: “Wait half a day to a full day after the fever starts, and go to the hospital before a full 24 hours has passed.”
Timing Guidelines for Receiving an Influenza Test
| Time Since Symptom Onset | Meaning/Accuracy of Test | Recommendation | Notes |
| 0 to 12 Hours | Viral load is often still low, leading to a high chance of a false negative. | △ (Not Highly Recommended) | Sensitivity of the rapid antigen test is low. |
| 12 to 48 Hours | Viral load is at its peak, and the positive rate of the rapid antigen test is highest. | ◎ (Most Recommended) | Almost all medical institutions recommend this timing. |
| 48 to 72 Hours | Viral load starts to decrease, but still often detectable. | ○ (Possible) | Risk of a false negative increases slightly. |
| 72+ Hours | Viral load drops sharply; the false negative rate is very high. | ▲ (Less Meaningful) | Anti-influenza medication effectiveness is also significantly reduced. |
The Conclusion: When is the Best Time to Go for a Test?
→ The most reliable time to receive an examination and test is within 12 to 48 hours (ideally around 24 hours) after the onset of fever.
Practical Examples of When to Visit (Example Schedule)
- High fever on Monday night → Visit on Tuesday morning to noon (Ideal)
- Fever onset on Saturday night → Visit on Sunday morning first thing, or Monday morning (Go earlier on Monday if approaching the 48-hour mark).
- More than 48 hours (A full two days) have passed → The test is likely to show negative, so consult with a doctor (In cases of severe symptoms, treatment may be started without a test).
Notes for the 2024-2025 Season
- The sensitivity of rapid antigen test kits is generally the same as in previous years (Type A: approx. 90%, Type B: 70-80%).
- Early visit recommendation: Elderly individuals and those with underlying health conditions are advised to visit the clinic earlier, even within 12 hours of onset, for early treatment to prevent severe illness.
- More medical institutions are now willing to prescribe anti-influenza drugs (like Xofluza, Tamiflu) without a testif the “clinical symptoms are clear,” even after 48 hours have passed.
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:
| Rank | Product Name (Manufacturer) | Targets Detected | Sensitivity (Detection Rate) (Estimate: 12-48h Post-Onset) | Result Time | Approximate Price (Per Test) | Suitability for Pregnant Women / Children | Key Features & Availability |
| 1 | Panbio COVID-19/Flu A&B | COVID + Flu A/B | COVID: Approx. 90% | Flu: Approx. 85% | 15 mins | ¥1,980 – ¥2,480 | ◎ (Shallow Nasal Swab) |
| 2 | クイックナビ-Flu+COVID | Same (COVID + Flu A/B) | COVID: 88% | Flu: 80–85% | 15–20 mins | ¥1,780 – ¥2,200 | ◎ (Designed for children’s use) |
| 3 | KBMラインチェック nCoV/Flu | Same (COVID + Flu A/B) | COVID: 85% | Flu: Approx. 80% | 15 mins | ¥1,480 – ¥1,980 | ◎ |
| 4 | イージードック Flu＆COVID-19 | Same (COVID + Flu A/B) | COVID: 87% | Flu: 82% | 15 mins | ¥2,200 – ¥2,600 | ◎ (Many cases of use by pregnant women) |
