Does the rapid streptococcal (strep) antigen detection test have a high rate of false positives?

From the Guidelines

Rapid strep swabs have a very low rate of false positives, typically less than 5%, making them highly reliable when showing a positive result. This is based on the high specificity of RADTs, which is approximately 95% when compared with blood agar plate cultures 1. The low false-positive rate means that when the test indicates strep throat, it's usually correct, and therapeutic decisions can be made with confidence on the basis of a positive test result 1.

However, it's essential to note that while false positives are rare, the sensitivity of most RADTs is 70%–90%, which means they can sometimes miss actual strep infections 1. This is why healthcare providers may still send a throat culture to confirm a negative rapid test result, especially in children where treating strep throat is crucial to prevent complications like rheumatic fever.

Key points to consider:

• RADTs are highly specific, with a specificity of approximately 95% 1
• False-positive test results are highly unusual, and therefore therapeutic decisions can be made with confidence on the basis of a positive test result 1
• The sensitivity of most RADTs is 70%–90%, which can lead to false negatives 1
• A negative RADT should be accompanied by a follow-up or back-up throat culture in children and adolescents, while this is not necessary in adults under usual circumstances 1

From the Research

Rapid Strep Swab Accuracy

• The rapid strep swab has been studied for its accuracy in detecting group A beta-hemolytic streptococcal pharyngitis, with varying results:
  • A study from 2002 found that the rapid strep test showed no difference in specificity (0.96 vs 0.98) compared to the culture method, and its use might allow earlier treatment in patients with recent streptococcal pharyngitis 2.
  • A systematic review and meta-analysis from 2010 found that the rapid antigen-detection test had a sensitivity of 0.85 and a specificity of 0.96, with a likelihood ratio (+) of 22.21 and a likelihood ratio (-) of 0.15 3.
  • Another study from 2010 found that the rapid antigen detection sensitivity and specificity were 64.6% and 96.79%, respectively, with a positive predictive value of 80.95% and a negative predictive value of 92.82% 4.

False Positives

• The studies suggest that the rapid strep swab has a low rate of false positives:
  • The 2002 study found that the rapid strep test had a specificity of 0.96, indicating a low rate of false positives 2.
  • The 2010 systematic review and meta-analysis found that the rapid antigen-detection test had a specificity of 0.96, indicating a low rate of false positives 3.
  • The 2010 study found that the rapid antigen detection test had a specificity of 96.79%, indicating a low rate of false positives 4.

Factors Affecting Accuracy

• The accuracy of the rapid strep swab may be affected by various factors, including:
  • Recent streptococcal pharyngitis: A study from 2002 found that the rapid strep test was more reliable in patients who had recent streptococcal pharyngitis 2.
  • Age: A study from 2010 found that the sensitivity and positive predictive value of the rapid antigen detection test were lower in adults than in children 4.
  • Specimen collection technique and education of healthcare workers: A study from 2010 found that these factors were important for maximizing the sensitivity of the rapid strep test 4.