What is the accuracy of streptococcal (strep) tests?

Accuracy of Streptococcal Tests

Rapid antigen detection tests (RADTs) have excellent specificity (≥95%) but moderate sensitivity (80-90%), meaning a positive test reliably confirms strep throat but a negative test requires culture confirmation to avoid missing true infections. 1

Test Performance Characteristics

Rapid Antigen Detection Tests (RADTs)

Specificity:

• RADTs demonstrate excellent specificity of ≥95% compared to blood agar plate cultures 1, 2
• False-positive results are rare, allowing confident treatment decisions based on positive results 1, 2
• This high specificity ensures against unnecessary antibiotic use 3

Sensitivity:

• Most RADTs have sensitivity between 80-90% when compared to throat culture 1
• A 2016 Cochrane review of 98 studies (101,121 participants) found summary sensitivity of 85.6% (95% CI 83.3-87.6%) 3
• This means approximately 14 out of 100 children with true strep throat will be missed by the rapid test 3
• Sensitivity varies significantly based on bacterial inoculum size: 95% for heavy growth versus only 40% for light growth 4

Negative Predictive Value:

• Remains stable at approximately 92% regardless of clinical features 4
• In a population with 30% GAS prevalence, 43 out of 1000 children with strep will be missed 3

Throat Culture (Reference Standard)

• Requires 18-24 hours incubation at 35-37°C 1
• Plates negative at 24 hours should be re-examined at 48 hours, as additional overnight incubation at room temperature identifies a considerable number of positive cultures that would otherwise be missed 1
• Serves as the gold standard for comparison 1, 3

Rapid Nucleic Acid Tests (RNATs) - Newer Technology

• Summary sensitivity of 97.5% (95% CI 96.2-98.3%) 5
• Summary specificity of 95.1% (95% CI 93.6-96.3%) 5
• Direct comparison studies show RNATs are significantly more sensitive than RADTs (96.8% versus 82.3%, p=0.004) with equal specificity 5
• May allow use as stand-alone tests without culture confirmation 5

Clinical Application Algorithm

For Positive RADT Results:

• Treat immediately without culture confirmation due to high specificity 1, 2
• False positives are uncommon (≤5%) 1, 2

For Negative RADT Results:

• Confirm with conventional blood agar plate culture, as a large proportion of false-negative RADTs occur in truly infected patients, not just carriers 1
• This is particularly critical in children and adolescents 6
• In adults, culture confirmation after negative RADT is not necessary due to low incidence of complications 6

Technology Evolution

First Generation:

• Latex agglutination methodology with relatively low sensitivity and unclear endpoints 1, 2

Second Generation:

• Enzyme immunoassay (EIA) techniques with sharply defined endpoints and increased sensitivity 1, 2

Third Generation:

• Optical immunoassay (OIA) and chemiluminescent DNA probes may approach throat culture sensitivity 1, 2
• EIA and OIA tests show comparable sensitivity (85.4% versus 86.2%) 3

Critical Pitfalls to Avoid

• Never rely solely on negative RADT results without culture confirmation, especially in high-risk populations 1, 2
• Do not confuse strep antibody tests (which reflect past infection) with RADTs (which detect current infection) 1, 2, 7
• Antibody titers have no value in diagnosing acute pharyngitis 1, 6
• Do not assume the number of colonies on culture plates reliably differentiates true infection from carrier state 1

Real-World Performance Variations

• RADT sensitivity varies significantly with bacterial load, with spectrum effects only occurring in patients with light inocula 4
• Individual RADT products show variable performance: one study found sensitivities ranging from 72% to 92% for different commercial tests 8
• Proper swab technique (vigorous rubbing of both tonsils and posterior pharyngeal wall) is essential for accuracy 9