The most vexing issue with the rapid influenza antigen test is the “false negative” (being negative when you actually have the flu). A false negative not only delays appropriate treatment but also complicates the decision to return to school or work.
Why do false negatives occur? It’s due to a variety of factors, including the time elapsed since the onset of the fever, the virus type (especially Type B), and even the quality of the specimen collection.
Complete Summary of Influenza Rapid Test “False Negatives” (As of 2025)
| Situation | Estimated Probability of a False Negative | Experience in the Clinical Setting |
| 0–12 Hours Post-Onset | 40–70% | “Too early to show up” is extremely common. |
| 12–24 Hours Post-Onset (Best Timing) | 5–20% | Rarely occurs (almost 0% for Type A). |
| 24–48 Hours Post-Onset | 15–35% | Sharp increase from here for Type B. |
| 48–72 Hours Post-Onset | 40–70% | A negative result is barely trusted. |
| 72+ Hours Post-Onset | 70–90% or higher | Almost 100% false negative. |
| Type B Influenza + | 20–30% higher | Much more likely to occur than with Type A, even at the same timing. |
False Negative Ranking TOP 7 (Actual Clinical Frequency)
| Rank | Cause | Probability of False Negative Increase |
| 1st | More than 48 hours elapsed since onset | +50–80% |
| 2nd | Type B Influenza | +20–40% |
| 3rd | Less than 12 hours since onset (Too early) | +40–60% |
| 4th | Nasal congestion resulting in a sparse specimen | +30–50% |
| 5th | Children (Swab cannot reach deep into the nose) | +20–40% |
| 6th | Kit performance is below average | +10–20% |
| 7th | Already taken Xofluza (Baloxavir Marboxil) | Almost 100% negative (Undetectable within 24 hours). |
Patterns Leading to Immediate Suspicion of a “False Negative”
Doctors immediately suspect a false negative when:
- The entire family is positive for Influenza A, but the patient is negative.
- The patient has a fever of 38.5°C or higher + body aches + sudden fever onset, but the result is negative.
- The patient was negative yesterday but returns today with the same symptoms.
- The result is negative during a period when Type B is highly prevalent.
→ If even one of these four points applies, 90% of doctors will suspect a “false negative” and prescribe medication without further testing.
The Strongest Method to Avoid False Negatives (3 Golden Rules)
- Visit the clinic 12–36 hours after onset (This is the most important rule!!).
- Be honest and state, “My nose is congested” → The doctor will often swab the throat as well.
- Strongly tell the doctor, “I am certain I have the flu” despite the negative result → They will offer a retest the next day or prescribe medication without a test.
“If you wait a full day (24 hours) after the fever starts and go before a full two days (48 hours) have passed, the chance of a false negative is virtually zero!” Otherwise, consider “negative” not as “not the flu,” but as “still uncertain.”
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) |
