A strep test can be wrong in both directions, but the two kinds of error are not equally common. The rapid test is much better at confirming strep than at ruling it out, so a negative result deserves more doubt than a positive one. Even a correct positive can mislead, because finding strep in the throat doesn’t always mean strep is causing the sore throat.
How Often a Rapid Strep Test Gets It Wrong
The rapid strep test is the 10-minute swab test done at most clinics. It looks for a piece of the strep bacterium’s outer coat on the swab. Most rapid tests are compared against a throat culture, which grows the swab in a lab for one to two days and is the usual standard for judging them.1Rapid Mis-St(r)ep. Rapid Mis-St(r)ep A culture lets a small number of bacteria multiply for a day or two before anyone looks. That growth step is why a culture can find strep that a rapid test misses, and also why it takes longer, while the rapid test can give an answer before you leave the clinic.
Misses are the bigger problem. Across tens of thousands of children, rapid tests caught about 86% of strep infections, missing about 1 in 7.2PubMed Central. Rapid antigen detection test for group A streptococcus in children with pharyngitis When strep is absent, the test correctly says negative about 95% of the time.3PubMed Central. Rapid antigen detection test for group A streptococcus in children with pharyngitis Commercial tests range from catching 70% to 95% of cases, averaging about 85%.4Clinical Microbiology and Infection. Review Rapid streptococcal testing for sore throat and antibiotic resistance So a negative is the result most likely to be wrong. That means about 14 of every 100 children with strep would go home with a negative result, and nothing in the result says which group your child is in. That is why doctors often don’t treat a negative as final for kids.
A positive is more trustworthy because the test rarely reacts when no strep is on the swab. A positive usually means strep bacteria really were in the throat, though whether they are causing the illness is a separate question, covered below.
Accuracy also varies by setting and brand. In one hospital, rapid tests caught 70% of strep in children but only about 59% in adults.5PubMed. The sensitivity and the specifity of rapid antigen test in streptococcal upper respiratory tract infections One brand in a lab comparison detected only about 5% of cases the culture found.6PubMed. Comparison of 3 Nucleic Acid Amplification Tests and a Rapid Antigen Test with Culture for the Detection of Group A Streptococci from Throat Swabs
A negative paired with a very sore throat deserves a second look rather than automatic reassurance. A negative paired with mild symptoms that are fading is less worrying, since the odds of strep were lower to begin with.
Why a Strep Test Can Miss a Real Infection
Rapid tests only turn positive when enough strep bacteria are on the swab, so a swab that doesn’t firmly touch the tonsils and the back of the throat can miss strep that is there.7Rapid Mis-St(r)ep. Rapid Mis-St(r)ep8Rapid Mis-St(r)ep. Rapid Mis-St(r)ep Not reaching those areas is widely cited as the most common cause of false negatives.9Your Family Walk In Clinic. Strep Throat Testing: What Patients Need to Know – Your Family Walk In Clinic10PubMed Central. Performance and user-friendliness of the rapid antigen detection tests QuickVue Dipstick Strep A test and DIAQUICK Strep A Blue Dipstick for pharyngotonsillitis caused by Streptococcus pyogenes in primary health care
A good swab firmly rubs both tonsils (or where they would be) and the back wall of the throat, and it takes only a few seconds. Gagging is common and usually means the swab reached the right spot, so it isn’t a sign something went wrong. A child who clamps their mouth, turns away, or squirms may only get the tongue or cheek touched, which collects little and raises the chance of a miss. Holding a young child steady and asking them to open wide and say “ahh” helps the clinician reach the right area.
Low bacterial levels can fool both kinds of test. Most false negatives in one evaluation came from samples with only light bacterial growth.11PubMed Central. Performance and user-friendliness of the rapid antigen detection tests QuickVue Dipstick Strep A test and DIAQUICK Strep A Blue Dipstick for pharyngotonsillitis caused by Streptococcus pyogenes in primary health care Even a lab culture can miss strep when bacteria are scarce, as newer tests that detect tenfold smaller amounts have shown.12PubMed Central. Detection of Group A Streptococcus from Pharyngeal Swab Samples by Bacterial Culture Is Challenged by a Novel mariPOC Point-of-Care Test
Timing in the illness can matter too. Early on there may be fewer bacteria in the throat, which makes a miss more likely. A child who tests negative on day one and then gets clearly worse is a reasonable reason to be seen again.
Some strep types aren’t what the test looks for at all. The rapid test detects group A strep only. In one reported case, a woman with severe tonsillitis tested negative three times while her culture grew group C strep.13PubMed Central. Severe Acute Pharyngitis Caused by Group C Streptococcus
When a Negative Result Needs a Throat Culture
For children older than 3, US guidance says a negative rapid test should be followed by a throat culture.14Centers for Disease Control and Prevention. Clinical Guidance for Group A Streptococcal Pharyngitis If the culture comes back positive, the clinic should contact the family and start antibiotics.15Centers for Disease Control and Prevention. Clinical Guidance for Group A Streptococcal Pharyngitis
For other ages, a culture after a negative rapid test isn’t routinely needed.16Centers for Disease Control and Prevention. Clinical Guidance for Group A Streptococcal Pharyngitis That’s because rheumatic fever, the main complication antibiotics prevent, is very rare in those groups.17Centers for Disease Control and Prevention. Clinical Guidance for Group A Streptococcal Pharyngitis
The clinic usually takes a second swab at the same visit and sends it to a lab. If you were told only that the test was negative, it is fair to ask whether a second swab was collected.
Ask how and when you will hear back, which is usually within one to two days, and whether the clinic will call in a prescription if the culture is positive. Check that the clinic has a working phone number for you. While you wait, watch for the worsening signs described later in this article.
How a Positive Strep Test Can Mislead
A strep carrier has strep bacteria living in the throat without being sick from them. If a carrier catches a viral sore throat, the strep test can be positive even though the virus is causing the symptoms.18Centers for Disease Control and Prevention. Testing for Strep Throat or Scarlet Fever
Carriers are common. Many children with no symptoms carry strep,19PubMed. Viral Features and Testing for Streptococcal Pharyngitis and in one study, 12% of children without symptoms had strep in their throats.20PubMed Central. Are clinicians overdiagnosing strep throat and overprescribing antibiotics? Antibiotics given in that situation won’t help the actual illness.
Some clues point toward a virus despite a positive test. Cough, runny nose, hoarseness, and mouth sores suggest a virus, while strep throat usually comes without a cough. These are clues, not proof: a child with those symptoms can still have strep, and one without them can still have a virus. A child who tests positive at nearly every cold may be a carrier, and that pattern is worth mentioning to the doctor. Doctors often cannot tell for certain, so treatment is frequently given when the test is positive and symptoms fit. Unnecessary antibiotics bring side effects like rash and diarrhea, and overuse feeds antibiotic resistance, which is part of why doctors try to test only when symptoms suggest strep.
Other bacteria can also trigger the test. Normal throat bacteria can occasionally make the rapid test react,21PubMed. False-positive rapid antigen detection test results: reduced specificity in the absence of group A streptococci in the upper respiratory tract and one related strep species carried the same marker the test looks for.22PubMed. False-positive rapid antigen detection test results: reduced specificity in the absence of group A streptococci in the upper respiratory tract
What Can Happen If Strep Is Missed
Treating strep throat with antibiotics is the main way to prevent rheumatic fever.23PubMed. Prevention of rheumatic fever and diagnosis and treatment of acute Streptococcal pharyngitis: a scientific statement from the American Heart Association Rheumatic Fever, Endocarditis, and Kawasaki Disease Committee of the Council on Cardiovascular Disease in the Young, the Interdisciplinary Council on Functional Genomics and Translational Biology, and the Interdisciplinary Council on Quality of Care and Outcomes Research: endorsed by the American Academy of Pediatrics Rheumatic fever is an inflammatory illness that can follow untreated strep and can damage the heart valves. Rheumatic fever developed in about 2.4% of untreated suspected strep cases versus 0.7% of treated ones,24PubMed Central. Antibiotics for the primary prevention of acute rheumatic fever: a meta-analysis so antibiotics cut that risk by almost 70%.25PubMed Central. Antibiotics for the primary prevention of acute rheumatic fever: a meta-analysis Still, in wealthy countries rheumatic fever is now uncommon, so the absolute risk is small.
Timing is forgiving. Antibiotics still prevent rheumatic fever when started within several days, so waiting a day or two for a culture is generally safe. Most sore throats improve within a week either way.
A worsening throat can mean something else. A pus pocket beside the tonsil is a complication often caused by group A strep.26PEMBlog. Briefs: Is this a peritonsillar abscess? In one reported case, a teen had two negative rapid tests while developing an abscess.27PEMBlog. Briefs: Is this a peritonsillar abscess?
Certain signs after a negative test call for a return visit:
- Trouble opening the mouth fully, or pain when trying to, which can be a sign of a pus pocket forming near the tonsil.
- A muffled voice, as if talking with hot food in the mouth.
- Drooling or being unable to swallow saliva, which means the throat is badly swollen and needs prompt care.
- Pain or fever that is getting worse after two to three days instead of easing.
Trouble breathing is an emergency and needs immediate care, not a scheduled visit.
Using Symptoms to Judge a Strep Result
Many doctors use the Centor score, which gives one point each for fever, swollen or pus-covered tonsils, tender neck glands, and no cough. Some guidelines suggest testing at a score of 3, while US infectious disease guidance supports testing adults at 2.28PubMed Central. Diagnostic Accuracy of Centor Score for Diagnosis of Group A Streptococcal Pharyngitis among Adults in Primary Care Clinics in Malaysia Even with a score of 3 or more, only about 12% to 40% of adults actually have strep.29PubMed Central. Predicting streptococcal pharyngitis in adults in primary care: a systematic review of the diagnostic accuracy of symptoms and signs and validation of the Centor score
To score your own symptoms, note whether there is fever, white or pus-covered patches on the tonsils, tender lumps in the front of the neck, and whether there is a cough. A score of 0 or 1 makes strep less likely, and many doctors won’t test, though a doctor may still test a child based on the exam or a known strep exposure. A high score alongside a negative rapid test is a good reason to ask whether a culture or molecular test makes sense.
The score was designed to decide who should be tested, not to diagnose strep by itself. The usual version adjusts for age, so a child’s score is read a little differently from an adult’s.
Doctors differ in how they respond. Some may treat high-score adults without testing, and others always test first. Asking which approach they’re using is reasonable.
Are Home Strep Tests and Newer Tests More Reliable
Getting a good throat swab takes skill that clinicians are trained in.30Cleveland Clinic. Why Home Strep Tests Aren’t Reliable Even when instructions are followed carefully, home tests can miss strep,31Cleveland Clinic. Why Home Strep Tests Aren’t Reliable and a swab that doesn’t firmly reach the tonsils and back of the throat is the main cause of misses described earlier.
If you have a home result, a positive still needs a clinician, because antibiotics require a prescription and the clinician can weigh whether the symptoms fit. A negative home test in someone with bad symptoms is not reassuring. Bring the child in for an exam and a clinic test rather than repeating the home test.
Some clinics offer molecular (PCR) strep tests that read the bacterium’s genes in under an hour. Several such tests caught nearly 100% of cases culture found.32PubMed. Comparison of 3 Nucleic Acid Amplification Tests and a Rapid Antigen Test with Culture for the Detection of Group A Streptococci from Throat Swabs You can ask whether your clinic offers one. It may cost more or take longer than the rapid test, but a negative result can replace the wait for a culture. It shouldn’t be used to check whether treatment worked, because PCR can stay positive for days after treatment begins, with a median of 4 days.33PubMed. Duration of Group A Streptococcus PCR positivity following antibiotic treatment of pharyngitis