If you are in a hurry
- Even if the test is negative, influenza cannot be ruled out. Misses are especially likely soon after the fever begins
- If consciousness is altered, breathing suddenly becomes hard, or a seizure will not stop, call 119 (Japan’s emergency number) regardless of the test result
- If the fever continues despite a negative result, or symptoms get worse, be seen again and talk with us
Hello. I am Shunichi Gamo, MD, PhD, a board-certified pulmonologist and the director of Sendai Airport North Clinic.
At the fever clinic, I often hear this kind of question.
“My rapid test was negative. But my fever won’t go down and my joints hurt. Is it really not influenza?”
You feel relieved to hear “negative,” and yet you are still worried. Both feelings are natural.
To give you the conclusion first: even if the rapid test (a test on a nose or throat swab that gives a result on the spot) is negative, influenza cannot be ruled out. In particular, soon after the fever begins, the amount of virus is small and misses are likely.
In this article, as a lung specialist, I explain why misses happen, how the time since onset relates to how easily the virus is found, what doctors look at when the test is negative, when a repeat test helps, and the signs that mean you should be seen again.
Before that, a note about our fever clinic on Sundays and holidays. PCR tests for influenza and COVID-19 (a test that multiplies the virus’s genetic material to detect it) give you a result the same day.
Since October 4, 2026, our fever clinic is also open on Sundays and holidays
10:00 to 15:00 (no lunch break). No appointment is needed; we accept patients by same-day check-in. We give you your PCR test result the same day. On Sundays and holidays we run the fever clinic only, with no general outpatient care and no IV drips. We are closed on Sunday, October 25, and from November we are generally open every Sunday and holiday. Announcement: fever clinic on Sundays and holidays
Shunichi Gamo, MD, PhD
Director, Sendai International Airport North Clinic
Born and raised locally, he attended Yuriage Elementary School and Yuriage Junior High School in Natori City. After graduating from Tohoku University School of Medicine, he worked at Sendai Open Hospital and Kesennuma City Hospital, and then at Tohoku University Hospital. In April 2021, in the middle of the COVID-19 pandemic, he opened Sendai International Airport North Clinic. Holding to a fever clinic that never turns patients away, he cared for well over 20,000 patients with fever in the two years before COVID-19 was moved to Class 5 under Japan’s infectious disease law.
Society memberships and board certifications
Certified Member and Board Certified Fellow, Japanese Society of Internal Medicine
Board Certified Pulmonologist, Japanese Respiratory Society
Board Certified Allergist, Japanese Society of Allergology
Board Certified Specialist, Japanese Society of Anti-Aging Medicine
PhD in Medicine (Tohoku University)
Why can’t influenza be ruled out when the rapid test is negative?
The rapid test turns positive only when the amount of virus is above a certain level, so when the amount is small, the result can be negative even though you are infected.
The rapid test looks in the swab fluid for fragments of the virus (antigens, the proteins that make up the virus). Unlike the PCR test, which multiplies the virus’s genetic material before checking, it has no step that multiplies anything. When there are only a few fragments, the line may not appear.
A negative result despite infection is called a “false negative.” Conversely, a positive result without infection is a “false positive.”
There is a report pooling studies from around the world. It was a meta-analysis (a method that combines and analyzes the results of several studies) of 159 studies covering 26 kinds of rapid tests [1].
| Number | Meaning | Result |
|---|---|---|
| Sensitivity | The proportion of infected people correctly judged positive | 62.3% [1] |
| Specificity | The proportion of uninfected people correctly judged negative | 98.2% [1] |
| Sensitivity in adults and children | The proportion of infected people detected | Adults 53.9%, children 66.6% [1] |
| Sensitivity for type A and type B | Difference in ease of detection by type | Type A 64.6%, type B 52.2% [1] |
A sensitivity of 62.3% means that of 100 infected people, about 62 test positive [1]. The rest test negative even though they are infected. On the other hand, the specificity is high at 98.2%, so a positive result is quite likely to mean influenza [1].
The authors of this study concluded that with a rapid test, “a positive result can make the diagnosis, but a negative result cannot rule it out” [1].
In another meta-analysis (162 studies), the sensitivity of conventional rapid tests was 54.4% for type A and 53.2% for type B [2]. The specificity of every test exceeded 98% [2].
In the same study, the sensitivity of the type of antigen test in which a machine reads the result (digital immunoassay) was 80.0% for type A, and for the rapid genetic test it was 91.6% [2].
Dr. Gamo“Negative” does not mean “it isn’t influenza”; it means “this test did not find it.” We also think about your symptoms and what is going around nearby.
Why are misses more likely within 12 hours of the fever starting?
Early in the infection, the virus is still multiplying, and the amount in the swab fluid may not reach the test’s detection limit (the smallest amount it can find) [3][4].
There are 56 studies in which volunteers were infected with influenza virus and followed. This is a review (a paper that summarizes earlier studies) pooling data on 1,280 people [4]. Virus shedding rose sharply 0.5 to 1 day after infection and peaked on day 2 [4].
However, these results come from healthy adult volunteers. The same data are said not to exist for children and older adults [4].
In Japanese studies as well, the shorter the time since symptoms began, the lower the sensitivity of the rapid test tended to be. The main studies are summarized in the table.
| Study | Population and method | Early period | Later period or overall |
|---|---|---|---|
| Sato et al. 2022 [3] | 177 nasopharyngeal (deep nasal) swab samples from suspected cases. 51 samples were positive for type A or B on the genetic test | Under 6 hours from onset: 41.7% and 50.0% for the 2 products | 24 to 48 hours from onset: 100.0% for both |
| Nakao et al. 2014 [5] | Pediatric outpatients. Rapid antigen test compared with a genetic test | 6 hours after fever: 37.5% | 12 hours: 60.9%, overall: 70.8% |
| Boku et al. 2013 [6] | Done in the outpatient clinic by the doctor at the exam | Within 12 hours of fever: 44.4% | Overall: 58.3% |
| Mikamo et al. 2022 [7] | 413 people. Rapid antigen test compared with a genetic test (cobas Liat) | Under 12 hours from onset: the genetic test was significantly more sensitive | Rapid antigen test 79.7%, genetic test 99.5% |
The test kits and patients differ from study to study, so the numbers cannot be compared directly. Even so, in the three studies by Sato, Nakao, and Boku, the sensitivity in the early period stayed between 37.5% and 60.9% [3][5][6].
Sato and colleagues state that the rapid antigen test “may give many false negatives early in infection or when the virus is scarce” [3].
The 12-hour line is not a boundary beyond which the test becomes unusable. The tendency is that the more time passes, the more easily the virus is found.
Dr. GamoEven right after the fever starts, please be seen if you are miserable. At the exam we can sort out the situation and decide together on fever reducers, how to take fluids, and when to repeat the test or come back.
When the test is negative, what does the doctor look at to decide whether it is influenza?
The doctor judges overall, combining not only the test result but also symptoms, what is going around, people you have had contact with, and findings at the exam [8].
The statement from the Japanese Association for Infectious Diseases covers patients with influenza that is “confirmed or suspected.” It says that starting an antiviral drug (a drug that stops the virus from multiplying) within 48 hours of onset to try to relieve symptoms is treatment in line with the drug’s indication [9].
In other words, treatment is not only for people who test positive.
According to the Ministry of Health, Labour and Welfare, when an antiviral drug is started within 48 hours of onset, the fever period is usually shortened by 1 to 2 days, and the amount of virus shed from the nose and throat also falls [10].
Whether to use a drug is not essential for everyone; it is decided by the doctor’s judgment [8]. Older adults, people with weak immunity, and children are thought to be more prone to becoming seriously ill [8].
The Japanese Association for Infectious Diseases says that even when the doctor decides not to use an antiviral drug, it is necessary to tell the patient to be seen right away if symptoms get worse [9].
When you are seen, please tell us the following. Even with a negative result, they are major clues for the diagnosis.
- The time symptoms began (for example, “chills from around 10 p.m. last night,” as best you know)
- The course of the fever (the highest temperature, and the time you took a fever reducer)
- Other symptoms, such as joint pain, cough, sore throat, and shortness of breath
- Influenza going around in your family, workplace, or school, and people you have had contact with
- Underlying conditions, pregnancy, and the medicines you usually take
When can a repeat test find something?
If the first test may have been too early and symptoms such as fever continue, a repeat test after some time can sometimes find the virus.
Among the studies, one reported that the sensitivity of the rapid antigen test was 100.0% at 24 to 48 hours after onset [3]. Another reported that a sensitivity of 37.5% at 6 hours after fever rose to 60.9% at 12 hours [5].
On the other hand, there are also situations in which a repeat test is less likely to help.
| Situation | Repeat test | How to think about it |
|---|---|---|
| The first test was within a few hours of the fever starting. The fever continues | May help | The virus tends to be easier to find as time passes [3][5] |
| Influenza is going around in your family or workplace | Discuss at the exam | If suspicion is strong, the doctor considers treatment even when the test is negative [9] |
| Older age, underlying conditions, pregnancy, infants and young children | Be seen again early and discuss | These groups are thought to be more prone to severe illness, so it is important not to wait too long [8] |
| Repeating the same test a few hours later | Less likely to help | If the interval is short, how easily it is found may not change much (because the virus tends to become easier to find as more time passes [3][5]) |
| No fever and mild symptoms. Nothing going around nearby | Less likely to help | Other causes, such as other cold viruses, are considered |
“Less likely to help” does not mean the visit is a waste. Even when the test is negative, we can look for the cause of the fever at the exam and give treatment to ease the miserable symptoms. When the fever is high and hard to bear, it can also be brought down with acetaminophen (for example Calonal).
A repeat test is not necessarily the same rapid test. We may choose a genetic test with higher sensitivity [2][7]. Which test to choose is something to discuss at the exam.
The cause of the fever may not be influenza. If you have a cough and chest pain when you breathe in, we also consider tests for pneumonia. If you have a severe sore throat, a strep throat test may be needed.
[Signs to be seen quickly] Even with a negative result, when do you need 119, an emergency department, or another visit?
Even if the test is negative, if consciousness, breathing, or fluids are clearly off, hurry regardless of the result.
Call 119 right away
If even one of the following applies, call 119 without hesitation. This is the same at night and on holidays.
- The person does not answer when called, or is dazed (altered consciousness)
- Breathing suddenly became hard, the shoulders heave when breathing, or the lips are bluish
- A seizure will not stop
Go to an emergency department the same day, even at night or on a holiday
In the following cases, go to an emergency department without waiting for morning. If the person is in distress and cannot be moved, call 119.
- The person can hardly take any fluids, or has not passed urine for half a day or more
- Even at rest, breathing is clearly faster than usual or looks hard
- The person is limp, the skin is pale or ashen gray, and the hands and feet are cold
Be seen again the same day (the next morning if it is night)
The Ministry of Health, Labour and Welfare Q&A also urges seeing a medical facility early if you feel quite unwell, for example with a persistent high fever, trouble breathing, or altered consciousness [8].
- The fever continues despite a negative result
- A fever that had come down has gone up again
- The cough has become stronger, the chest hurts, or you are short of breath
- It is getting harder to take fluids or food
- Influenza is going around in your family or workplace
- You are an older adult, have an underlying condition, are pregnant, or have an infant or young child, and you are worried
If the condition changes during the night, go back to the “119” and “emergency department” guides above and decide again at that point.
Children and teenagers who are recovering at home after being diagnosed with influenza and starting treatment should not be left alone for at least 2 days after treatment begins. This is because abnormal behavior, such as suddenly running off or trying to dash out of the room, can occur [8].
When the test is negative, how should I spend my time at home and avoid passing it to others?
Bring a miserable fever down with acetaminophen (for example Calonal), take fluids, and rest. Even if the test is negative, keep thinking of others with a mask and hand washing.
The Ministry of Health, Labour and Welfare Q&A recommends taking plenty of fluids. Tea, soup, or whatever you feel like drinking is fine [8].
People with influenza are said to shed virus from the nose and throat from the day before onset to 3 to 7 days after onset [10]. Even with a negative test, as long as influenza is suspected, it is safer to avoid places where people gather.
- Keep up hand washing, frequent ventilation, and wearing a mask
- Do not share towels or cups with family members
- Even after the fever goes down, do not overdo it until you feel yourself again
- For work or school, follow the doctor’s instructions
For infection control at home, see tips for not spreading infection within the household; for school and daycare, see the guide to time away from school; and for talking about medicine, see what to tell the doctor about antiviral drugs.
Frequently asked questions
- My test right after the fever started was negative. How many hours later should I repeat it?
-
There is no fixed time. If the fever continues, note the time your symptoms began and be seen again. The rapid test tends to find the virus more easily as time passes [3], and the doctor looks at your symptoms and what is going around to judge whether a repeat test is needed.
- Why is an antiviral drug sometimes prescribed even when the test is negative?
-
Because the statement from the Japanese Association for Infectious Diseases says that starting an antiviral drug within 48 hours of onset is treatment in line with the drug’s indication, not only for patients with confirmed influenza but also for those who are suspected [9]. Whether to use it is decided by the doctor, based on your symptoms, what is going around, your underlying conditions, and so on.
- The fever continues but the test stays negative. Is it not influenza?
-
It may not be influenza, but it cannot be decided by the rapid test alone [1]. There are other causes, such as COVID-19, pneumonia, and strep throat. If the fever continues, be seen again and talk with us.
- Could I pass it to my family even if the test is negative?
-
With influenza, that is possible. A negative rapid test cannot rule it out [1]. As long as it is suspected, keep up the mask and hand washing, and do not share towels and similar items.
- Can I get tested or ask for advice on Sundays and holidays?
-
Yes. We run the fever clinic on Sundays and holidays from 10:00 to 15:00 (no lunch break). No appointment is needed; we accept patients by same-day check-in with the QR code at the fever clinic entrance. We are closed on Sunday, October 25. On Sundays and holidays we do not offer general outpatient care or IV drips.
Summary [If the fever continues despite a negative result, be seen again]
- Even if the test is negative, influenza cannot be ruled out. Misses are especially likely soon after the fever begins
- If consciousness is altered, breathing suddenly becomes hard, or a seizure will not stop, call 119 regardless of the test result
- If the fever continues despite a negative result, or symptoms get worse, be seen again and talk with us
- Tests within 12 hours of onset are especially likely to miss. With some time, the virus tends to become easier to find
- The doctor judges overall, combining the test result, symptoms, and what is going around
- If you can hardly take fluids, have not passed urine for half a day, or are breathing fast, go to an emergency department even at night
- When you are seen, tell us the time symptoms began, the course of the fever, your underlying conditions, and what is going around
You do not have to decide alone that “it’s negative, so I’m fine.” When the fever continues, or when you feel that something is different from usual, please feel free to be seen at any time. Please leave the medical care to us.
That’s all.
Disclaimer
This article is intended to provide general medical information and does not replace an examination, diagnosis, or treatment by a doctor. The content is based on medical knowledge at the time of publication, but the appropriate response differs from person to person. If you are worried about your symptoms, please consult a medical institution.
References
- Chartrand C, Leeflang MM, Minion J, Brewer T, Pai M. Accuracy of rapid influenza diagnostic tests: a meta-analysis. Ann Intern Med. 2012 Apr;156(7):500-11. doi:10.7326/0003-4819-156-7-201204030-00403 PMID: 22371850
- Merckx J, Wali R, Schiller I, Caya C, Gore GC, Chartrand C, et al. Diagnostic accuracy of novel and traditional rapid tests for influenza infection compared with reverse transcriptase polymerase chain reaction: a systematic review and meta-analysis. Ann Intern Med. 2017 Sep;167(6):394-409. doi:10.7326/M17-0848 PMID: 28869986
- Sato Y, Nirasawa S, Saeki M, Yakuwa Y, Ono M, Kobayashi R, et al. Comparative study of rapid antigen testing and two nucleic acid amplification tests for influenza virus detection. J Infect Chemother. 2022 Jul;28(7):1033-6. doi:10.1016/j.jiac.2022.04.009 PMID: 35428574
- Carrat F, Vergu E, Ferguson NM, Lemaitre M, Cauchemez S, Leach S, et al. Time lines of infection and disease in human influenza: a review of volunteer challenge studies. Am J Epidemiol. 2008 Apr;167(7):775-85. doi:10.1093/aje/kwm375 PMID: 18230677
- Nakao A, Hisata K, Matsunaga N, Fujimori M, Yoshikawa N, Komatsu M, et al. The clinical utility of a near patient care rapid microarray-based diagnostic test for influenza and respiratory syncytial virus infections in the pediatric setting. Diagn Microbiol Infect Dis. 2014 Apr;78(4):363-7. doi:10.1016/j.diagmicrobio.2013.11.005 PMID: 24582577
- Boku S, Naito T, Murai K, Tanei M, Inui A, Nisimura H, et al. Near point-of-care administration by the attending physician of the rapid influenza antigen detection immunochromatography test and the fully automated respiratory virus nucleic acid test: contribution to patient management. Diagn Microbiol Infect Dis. 2013 Aug;76(4):445-9. doi:10.1016/j.diagmicrobio.2013.04.029 PMID: 23743175
- Mikamo H, Koizumi Y, Yamagishi Y, Asai N, Miyazono Y, Shinbo T, et al. Comparing the cobas Influenza A/B nucleic acid test for use on the cobas Liat System (Liat) with rapid antigen tests for clinical management of Japanese patients at the point of care. PLoS One. 2022 Oct;17(10):e0276099. doi:10.1371/journal.pone.0276099 PMID: 36301841
- Ministry of Health, Labour and Welfare. Q&A on the comprehensive measures against acute respiratory infections (ARI) for winter, fiscal 2026 (in Japanese). Ministry of Health, Labour and Welfare website. Accessed October 2026. https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/kenkou/kekkaku-kansenshou/infulenza/QA2026.html
- The Japanese Association for Infectious Diseases. Statement of the Japanese Association for Infectious Diseases: on the use of anti-influenza drugs (in Japanese). Website of the Japanese Association for Infectious Diseases. October 24, 2019. Accessed October 2026. https://www.kansensho.or.jp/uploads/files/guidelines/191024_teigen.pdf
- Ministry of Health, Labour and Welfare. Influenza (general page) (in Japanese). Ministry of Health, Labour and Welfare website. Accessed October 2026. https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/kenkou/kekkaku-kansenshou/infulenza/index.html
▶ Overview of this topic: Fever clinic on Sundays and holidays (from October 4, 2026, 10:00 to 15:00)
Related articles on fever
For the fever clinic on Sundays and holidays, see the announcement about the fever clinic on Sundays and holidays; for how to visit, see the page on the fever clinic pre-visit questionnaire and same-day check-in.
