Will Two Doses of Amoxicillin Alter a Rapid Strep Test Result?
No, two doses of amoxicillin will not meaningfully interfere with rapid streptococcal antigen detection test (RADT) results, and testing can proceed without concern for false-negative results from brief antibiotic exposure.
Evidence from Bacterial Clearance Studies
A single dose of amoxicillin (50 mg/kg) administered to children with confirmed Group A Streptococcus (GAS) pharyngitis resulted in negative RADT results in only 9% of patients when retested 12–23 hours later, meaning 91% remained RADT-positive despite one full dose of antibiotic 1.
Even among the 10% who converted to negative RADT after one dose, seven of these ten patients showed only a marked decrease in bacterial colonies (from 3–4+ to 1+) rather than complete eradication, indicating the antibiotic effect was incomplete 1.
Real-time PCR quantification of GAS bacterial load after amoxicillin treatment showed that at 1–3 days after starting therapy, bacterial copies remained detectable in the vast majority of patients across all dosing regimens (once-daily, twice-daily, or three-times-daily), with negative PCR results occurring in only 0–18% of patients at this early timepoint 2.
Clinical Implications for Testing
RADTs have ≥95% specificity for detecting GAS, making false-positive results rare; the test detects bacterial antigens that remain present even after brief antibiotic exposure 3.
The standard recommendation is that routine follow-up throat cultures or RADTs are not recommended for asymptomatic patients who have completed a full course of antimicrobial therapy, because appropriate treatment effectively eradicates GAS 3.
Guidelines do not mention any waiting period or concern about false-negative RADTs due to recent antibiotic exposure when discussing diagnostic testing strategies 3.
Practical Guidance
Proceed with RADT testing without delay even if the patient has received one or two doses of amoxicillin; the test will remain accurate for detecting GAS pharyngitis 1, 2.
If clinical suspicion for GAS remains high despite a negative RADT after antibiotic exposure, consider that in children and adolescents a backup throat culture is recommended after any negative RADT (regardless of antibiotic exposure) due to the 80–90% sensitivity of RADTs 3.
The primary concern with antibiotic exposure is not test accuracy but rather the clinical decision to test at all—if antibiotics were already started empirically, the diagnostic value of subsequent testing is diminished because treatment decisions have already been made 3.