Yes, the strep test will likely be inaccurate after 2 days of cefdinir
After 2 days of cefdinir treatment, both rapid antigen detection tests (RADT) and throat cultures are likely to show false-negative results because the antibiotic will have begun reducing the bacterial load of Group A Streptococcus in the pharynx, even though treatment is incomplete.
Why This Matters
Cefdinir is a third-generation cephalosporin that is highly effective against Group A Streptococcus (GAS). Studies demonstrate that cefdinir achieves rapid bacterial eradication—68% of patients treated with cefdinir showed complete eradication of GAS by day 2 of therapy 1. This rapid reduction in bacterial burden directly impacts the accuracy of diagnostic testing.
The Mechanism of False-Negative Results
- Bacterial load reduction: Cefdinir begins killing streptococcal bacteria within hours of the first dose, progressively reducing the number of organisms present in the throat
- Test detection thresholds: Both RADTs and throat cultures require a sufficient quantity of bacteria to produce positive results
- Timing sensitivity: By day 2 of treatment, the bacterial concentration may fall below the detection threshold of these tests, even though viable organisms may still be present
Clinical Implications
The IDSA guidelines explicitly state that "follow-up posttreatment throat cultures or RADT are not recommended routinely" 2. This recommendation exists precisely because antibiotics alter the pharyngeal bacterial flora and reduce test sensitivity.
What You Should Do Instead
If you're trying to determine whether the child actually has strep throat:
- Don't retest during active treatment - The results will be unreliable and may mislead clinical decision-making
- Consider the clinical context - If the child was started on cefdinir empirically without prior testing, and you now want confirmation, recognize that you've missed the diagnostic window
- Complete the antibiotic course - If strep was suspected enough to start treatment, finish the full 10-day course 2, 3
- Post-treatment testing is rarely indicated - Only consider testing 2-4 weeks after completing therapy in special circumstances (e.g., recurrent infections, concern for chronic carrier state) 2
Important Caveats
- RADTs have high specificity (95-99%) 4, 5, so a positive result even on treatment would still be meaningful, though less likely to occur
- Sensitivity drops significantly - The baseline RADT sensitivity of 86% 4 will be substantially lower after antibiotic exposure
- Throat culture is similarly affected - While culture is more sensitive than RADT in untreated patients, it too requires viable bacteria and adequate bacterial load
The Bottom Line
Testing after 2 days of cefdinir is not recommended because the antibiotic has already begun eradicating the bacteria, making both false-negative RADT and culture results highly probable. If diagnostic confirmation was needed, it should have been obtained before starting antibiotics 6, 2.