Can a point of care (POC) strep test still be positive after 24 hours on antibiotic therapy?

Point-of-Care Strep Test Positivity After 24 Hours of Antibiotics

A point-of-care strep test can still be positive after 24 hours of antibiotic therapy, though the majority of patients (approximately 83-93%) will have converted to negative by this time. Testing patients shortly after antibiotic initiation should generally be avoided due to the risk of false-positive results from persisting bacterial antigens.

Test Performance During Antibiotic Treatment

The likelihood of a positive test decreases rapidly but not completely within the first 24 hours:

• Approximately 36% of patients remain culture-positive the morning after starting antibiotics, but this drops to 17% by 24 hours 1
• A systematic review found that 6.9% of patients remain culture-positive on day 1, declining to 5.4% on day 2, and 2.6% by days 3-9 2
• For penicillins specifically, day 1 positivity is approximately 6.5% 2

Critical Clinical Considerations

False-negative results are a recognized concern when antibiotics are given before or at the time of specimen collection 3. The IDSA guidelines explicitly state that antibiotic exposure shortly before testing can produce false-negative results 3.

However, false-positive results are the more relevant concern when testing after treatment initiation:

• Rapid antigen tests may detect persisting bacterial antigens from non-viable bacteria after effective antibiotic treatment 4
• One study specifically noted that false-positive antigen results were often seen in patients previously diagnosed and/or treated for GAS, recommending that "follow-up testing with the RAT of GAS patients who previously tested as positive should be avoided due to increased FP results" 5

Practical Algorithm for Testing Decisions

Do not perform point-of-care strep testing if:

• The patient has received antibiotics within the previous 24 hours 3
• The patient is being evaluated for test-of-cure after completing treatment (routine post-treatment testing is not recommended) 6
• The patient was recently treated for strep pharyngitis and remains asymptomatic 6

Consider testing only if:

• The patient presents with new or worsening symptoms despite 24+ hours of appropriate antibiotic therapy, suggesting possible treatment failure
• There is clinical suspicion of a new infection episode (not the same illness)

Evidence Quality and Nuances

The research on this topic has methodological limitations. A 2023 systematic review noted that only 6 of 43 included studies had low risk of bias, and there was "imprecision of pooled estimates" 2. The studies primarily evaluated throat cultures rather than rapid antigen tests specifically.

One recent study (2023) found 91% agreement between RADT and culture at follow-up within 21 days of penicillin treatment 4. However, this study showed that 27 of 316 patients had positive RADT with negative culture, suggesting the RADT may detect non-viable organisms 4.

Common Pitfalls to Avoid

• Do not test patients who are clinically improving on antibiotics - this may lead to unnecessary treatment changes or extended therapy based on false-positive results 5
• Do not use positive tests after antibiotic initiation to guide isolation decisions - patients are considered non-contagious after 24 hours of appropriate antibiotic therapy regardless of test results 3
• Do not interpret a positive test after antibiotics as treatment failure without clinical correlation - the test may be detecting antigens from dead bacteria 4