What is the blood test to check for strep throat?

There is No Blood Test to Diagnose Acute Strep Throat

The diagnosis of strep throat requires either a rapid antigen detection test (RADT) or throat culture from a pharyngeal swab—blood tests for streptococcal antibodies reflect past infections, not current acute pharyngitis, and have no value in diagnosing active strep throat. 1

Why Blood Tests Don't Work for Acute Diagnosis

• Antibody titers (such as anti-streptolysin O or ASO) measure past immunologic events, not present infection, making them completely inappropriate for diagnosing acute pharyngitis 1
• These antibody tests are only valuable for confirming prior streptococcal infections in patients suspected of having complications like acute rheumatic fever or acute glomerulonephritis 1, 2
• ASO titers remain elevated for extended periods after infection and are present in most healthy individuals over age 5 due to frequent streptococcal exposures throughout life 3

The Correct Diagnostic Approach

For acute strep throat diagnosis, you must obtain a pharyngeal swab specimen and perform one of the following:

Rapid Antigen Detection Test (RADT)

• Provides results within minutes, allowing immediate treatment decisions 1
• Has excellent specificity (≥95%), meaning positive results are highly reliable and require no confirmation 1, 4
• Sensitivity ranges 80-90%, so negative results must be confirmed with throat culture 1, 4
• Newer EIA-based tests offer sharper endpoints and better sensitivity than older latex agglutination methods 1

Throat Culture on Blood Agar

• Remains the gold standard for diagnosis 1
• Requires 24-48 hours for results (incubate full 48 hours to avoid missing positive cultures) 5
• Should be used to confirm all negative RADT results 1, 5

Clinical Algorithm

1. Obtain pharyngeal swab from both tonsils (or tonsillar fossae) and posterior pharyngeal wall 5
2. Perform RADT for immediate results 1
3. If RADT positive: Treat with confidence—no confirmation needed due to ≥95% specificity 1, 4
4. If RADT negative: Send throat culture for confirmation, as 10-20% of true infections are missed by RADT 1, 5

Critical Pitfalls to Avoid

• Never order ASO titers or other streptococcal antibody tests to diagnose acute pharyngitis—they only indicate past exposure 1, 2
• Do not skip culture confirmation of negative RADTs, as studies show many false-negative results represent true infections, not just carriers 1, 5
• Do not assume recent antibiotic use makes testing unnecessary—it actually increases false-negative RADT rates by reducing bacterial load, making confirmatory culture even more important 5
• Ensure proper swabbing technique, as inadequate sampling significantly reduces test yield 5