What is the sensitivity and specificity of rapid antigen strep (Streptococcal) testing compared to polymerase chain reaction (PCR) strep testing?

Diagnostic Performance of Rapid Antigen Strep Testing Compared to PCR

Rapid antigen detection tests (RADTs) for Group A Streptococcus have excellent specificity (≥95%) but moderate sensitivity (80-90%) when compared to throat culture, and when compared specifically to PCR testing, RADTs demonstrate even lower sensitivity (63-73%) while maintaining very high specificity (99-100%). 1, 2

Test Characteristics: RADT vs. Throat Culture

The established performance characteristics when using throat culture as the reference standard are:

• Sensitivity: 80-90% (meaning 10-20% of true infections will be missed) 1
• Specificity: ≥95% (false-positive results are rare) 1

This high specificity means you can confidently treat based on a positive RADT result without culture confirmation. 1, 3 However, the moderate sensitivity necessitates that negative RADT results must be confirmed with conventional blood agar plate culture in children and adolescents. 1

Test Characteristics: RADT vs. PCR

When PCR is used as the reference standard (which is more sensitive than culture), RADT performance decreases:

• Sensitivity: 63-73% depending on the specific RADT platform 2
• Specificity: 99-100% across all RADT platforms 2

The careUS Strep A Plus showed the best performance with 73.3% sensitivity and 99.3% specificity compared to PCR, while other platforms (SD Bioline, BD Veritor) had sensitivities of 63-67%. 2

Clinical Implications of These Differences

The 10-27% gap between RADT sensitivity when compared to PCR versus culture reflects that PCR detects lower bacterial loads that both RADT and culture miss. 2 This means:

• A positive RADT reliably indicates GAS infection requiring treatment (due to ≥95% specificity) 1, 3
• A negative RADT misses 10-20% of culture-positive cases 1, 4
• A negative RADT misses 27-37% of PCR-positive cases 2

Evolution of RADT Technology

Newer generation RADTs show improved performance:

• First-generation latex agglutination tests had poor sensitivity and unclear endpoints 1
• Enzyme immunoassay (EIA) tests offer sharper endpoints and better sensitivity 1, 3
• Optical immunoassay (OIA) and chemiluminescent DNA probe tests may approach culture sensitivity, though conflicting data exists 1

Despite manufacturer claims of 95% sensitivity in package inserts, peer-reviewed studies consistently show approximately 85% sensitivity—a 10% overestimation. 5 This discrepancy stems from sponsored studies with methodological limitations. 5

Critical Pitfalls to Avoid

• Never rely solely on a negative RADT in children without backup culture confirmation 1, 6, 3
• Do not assume false-negative RADTs only occur in asymptomatic carriers—studies demonstrate that most false-negatives represent true infections 1, 7
• Prior antibiotic use further reduces RADT sensitivity by lowering bacterial load below detection thresholds 7
• Package insert performance data overestimates real-world sensitivity by approximately 10% 5

Practical Testing Algorithm

For children and adolescents (3-18 years):

• Perform RADT when clinical features suggest GAS pharyngitis 6
• Treat immediately if RADT is positive (no culture confirmation needed) 1, 6
• Obtain backup throat culture if RADT is negative 1, 6, 3
• Incubate cultures for 48 hours (not just 24) to maximize yield 7

For adults:

• Perform RADT when clinical features suggest GAS pharyngitis 6
• Treat if positive 6
• Backup culture after negative RADT is generally not necessary 6