How Accurate Is the Rapid Strep?

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The diagnostic accuracy of rapid streptococcal antigen testing is such that a negative test rules out disease and should not be treated; a positive test rules it in and should be treated.

Why does this matter?
Streptococcal pharyngitis (strep throat) is usually treated with antibiotic therapy.  The original intent was to reduce the likelihood of acute rheumatic fever.  However, that is uncommon now in the US.  Most are treated with antibiotics to reduce the chance of suppurative complications of strep throat, such as peritonsillar abscess, "retropharyngeal abscess, lymphadenitis, otitis media, mastoiditis, sinusitis, meningitis, bacteremia, endocarditis, and pneumonia. Nonsuppurative complications include acute rheumatic fever, rheumatic heart disease, scarlet fever, acute glomerulonephritis, Sydenham’s chorea, pediatric autoimmune neuropsychiatric disorders, and streptococcal toxic shock syndrome."  But how good is a rapid strep test?

"Just a little tickle in the back of your throat," he says deceptively.
This was a quick systematic review of 105 studies to ascertain the overall diagnostic accuracy of rapid strep testing.  The overall sensitivity of enzyme immunoassay was 85.4%, specificity 95.8%.  Overall sensitivity for optical immunoassay was 86.2%, specificity 93.7%.

With these numbers in mind, let's do an exercise.  The prevalence of strep throat in children is about 30% +/-.  If the rapid strep test is positive, the post-test probability is now 88%; if negative, 6%.  What this means is that you can avoid treatment if negative and provide treatment if positive.  In other words, the rapid strep is an accurate diagnostic test.

Source
How Accurate Is Rapid Antigen Testing for Group A Streptococcus in Children With Pharyngitis?  Ann Emerg Med. 2017 Aug 31. pii: S0196-0644(17)30918-6. doi: 10.1016/j.annemergmed.2017.07.018. [Epub ahead of print]

Peer reviewed by Thomas Davis, MD.