You should test for strep when you have a sore throat with fever, swollen tonsils, or tender lymph nodes in your neck, especially if you don’t have a cough. A sore throat alone isn’t enough to warrant testing, and a sore throat with classic cold symptoms usually points to a virus instead. The decision comes down to a handful of specific signs that raise or lower the odds of a strep infection.
The Four Signs That Point to Strep
Doctors use a simple checklist called the Centor score to decide whether a sore throat needs a strep test. Each of these signs is worth one point:
- Fever (above 100.4°F or 38°C)
- Swollen or white-patched tonsils
- Tender, swollen lymph nodes at the front of your neck
- No cough
With zero or one of these signs, the chance of strep is only about 7 to 12%. At that level, testing typically isn’t recommended because even a positive result might just reflect someone carrying the bacteria without an active infection. With two or three signs, the probability jumps to 21 to 38%, and a test makes sense. With all four, the likelihood reaches roughly 57%.
A modified version of this scoring system also factors in age. Children between 3 and 14 get an extra point because strep is far more common in that age group. Adults over 45 actually lose a point. With the age adjustment, a child who checks every box has a 50 to 70% chance of strep.
Signs That Suggest a Virus Instead
Coughing is the single most useful clue that your sore throat is viral, not bacterial. If you’re coughing, sneezing, have a runny nose, or sound hoarse, a virus is the more likely culprit. These symptoms don’t completely rule out strep, but they make it much less probable and often tip the balance away from testing.
Other viral giveaways include red, watery eyes, mouth sores, and a sore throat that came on gradually rather than hitting suddenly. Strep tends to arrive fast: you feel fine in the morning and miserable by afternoon, with throat pain, difficulty swallowing, and sometimes a headache or stomach pain (particularly in kids).
Why Age Changes the Approach
Strep causes about 30% of sore throats in children but only 5 to 15% of sore throats in adults. That difference matters for two reasons.
First, children over age 3 who test negative on a rapid strep test should have a backup throat culture sent to the lab. The rapid test catches strep quickly, usually within minutes, but its sensitivity is around 65%, meaning it misses roughly one in three true infections. A throat culture takes one to two days but is far more accurate. The CDC recommends this backup step specifically for children because untreated strep in kids can lead to rheumatic fever, a serious inflammatory condition that can damage the heart.
For adults and teens, a negative rapid test is generally considered reliable enough on its own. Rheumatic fever is extremely rare in these age groups, so the risk of a missed diagnosis is much lower. Your doctor may still order a culture if your symptoms are strongly suggestive, but it’s not routine.
Children under 3 rarely get strep pharyngitis and almost never develop rheumatic fever, so testing in toddlers is uncommon unless there’s a specific reason, like a sibling with a confirmed case.
Testing After Exposure
If someone in your household has confirmed strep, you don’t need to rush out for a test unless you develop symptoms. Carrying strep bacteria in your throat without symptoms is common, especially in children, and a positive test in that scenario can lead to unnecessary antibiotics. The general recommendation is to watch for signs (sore throat, fever, swollen tonsils) and test only if they appear.
The exception is households where someone has a history of rheumatic fever or where strep keeps bouncing back and forth between family members. In those situations, testing asymptomatic contacts can make sense.
How the Tests Work
The rapid antigen test is a throat swab that detects proteins on the surface of strep bacteria. Results come back in 5 to 15 minutes. Its biggest strength is specificity: when it says positive, it’s right about 97% of the time. Its weakness is sensitivity, at roughly 65%, so false negatives happen.
A throat culture involves the same swab but grows the bacteria in a lab dish over 24 to 48 hours. It’s the gold standard for accuracy but takes longer. If your rapid test is negative and your doctor orders a culture, you’ll typically get a call within a couple of days if it comes back positive.
Timing Your Test
The best time to test is one to three days after your sore throat starts. Testing too early, within the first few hours of symptoms, can produce a false negative because there may not be enough bacteria to detect yet. Waiting too long isn’t ideal either, since starting antibiotics within the first 48 hours of symptoms shortens the illness and reduces the risk of spreading it to others.
If you’ve already been taking antibiotics for another reason or have been gargling with antiseptic mouthwash, mention this before your test. Both can suppress bacteria enough to cause a falsely negative result.