What to do in case of a suspected false positive strep (streptococcal) antibody test?

Managing Suspected False Positive Strep Antibody Test Results

In cases of suspected false positive strep antibody tests, confirmation with a throat culture is recommended, as strep antibody titers reflect past rather than current infection and are not valuable for diagnosing acute pharyngitis. 1

Understanding Strep Testing Accuracy

• Rapid Antigen Detection Tests (RADTs) have excellent specificity (≥95%) compared to blood agar plate cultures, making false-positive results uncommon 1
• Despite high specificity, false-positive RADT results can still occur, with studies showing approximately 15% of positive RADTs may have negative cultures 2
• Strep antibody titers reflect past immunologic events, not current infection, and are primarily valuable for confirming prior streptococcal infections in patients suspected of having acute rheumatic fever or glomerulonephritis 1

Causes of False Positive Results

• Some false-positive results may be due to the presence of non-Group A streptococci carrying Group A carbohydrate antigen, particularly organisms from the Streptococcus milleri group 2
• Recent research suggests that many apparent "false positives" may actually be true positives that culture missed - PCR testing found that 76% of RADT "false positives" were actually GAS-positive 3
• Bacterial inhibition may cause false-negative cultures when both GAS and Staphylococcus aureus are present, leading to apparent false-positive RADTs 3

Management Algorithm for Suspected False Positive Results

1. For positive strep antibody test with minimal symptoms:

   • Recognize that antibody titers reflect past infection, not acute pharyngitis 1
   • Do not initiate antibiotics based solely on positive antibody test 4

2. For positive RADT with clinical suspicion of false positive:

   • Consider throat culture confirmation if treatment decision is uncertain 5
   • Evaluate clinical presentation using Centor criteria (fever, absence of cough, tonsillar exudates, tender cervical lymphadenopathy) 4
   • Low Centor score (0-2) with positive test may warrant culture confirmation 4

3. For negative RADT with high clinical suspicion:

   • Consider throat culture confirmation, as RADT sensitivity is only 80-90% 1
   • In adults with low clinical suspicion, confirmation of negative RADT with throat culture may not be necessary 4

Special Considerations

• Recent streptococcal pharyngitis does not appear to increase false-positive rates of RADTs, contrary to previous beliefs 6
• RADTs may actually be more reliable (sensitivity 91% vs 70%) in patients with recent streptococcal pharyngitis 6
• The modest benefits of antibiotics must be weighed against side effects, impact on microbiota, increased antimicrobial resistance, and costs 4

Common Pitfalls to Avoid

• Treating based solely on antibody test results without considering clinical presentation 4
• Assuming exposure history alone warrants treatment despite negative testing and minimal symptoms 4
• Failing to distinguish between tests for current infection (RADT, culture) and past infection (antibody titers) 1
• Overreliance on clinical judgment alone, which can have 20-29% false positive and 2-10% false negative results compared to laboratory confirmation 7