Rapid Strep Test (RADT) — Group A Streptococcal Pharyngitis

Test Characteristics

Metric	Value
False-negative rate	14% (sensitivity 86%)
False-positive rate	4.6% (specificity 95.4%)
Bayes factor (positive test)	20×
Bayes factor (negative test)	1/5×
Base rate	15% of adults with sore throat; 30% of children with sore throat

Interpreting Results

Scenario	Prior	+ Result	− Result
Adult with sore throat	15%	20 × 15% ≥ 100%15%× 20100%	1/5 × 15% = 3%15%÷ 53%
Child aged 5–15 with sore throat	30%	20 × 30% ≥ 100%30%× 20100%	1/5 × 30% = 6%30%÷ 56%

20 × 15% ≥ 100%15%× 20100%: exact posterior is 78%. 20 × 30% ≥ 100%30%× 20100%: exact posterior is 90%. Positive is very reliable. The 6% residual in children explains why IDSA recommends backup culture for negative RADTs in children.

• + result: risk goes from 15% to 78% (adult) or 30% to 90% (child) — strong confirmation
• − result: risk drops meaningfully — helps avoid unnecessary antibiotics

Sources:

• Cochrane review. Rapid tests for strep throat. 105 evaluations, 58,244 participants.
• PLOS One systematic review, 2014. PLOS One.
• CDC. Clinical guidance for GAS pharyngitis.