Tests
1st tests to order
rapid antigen detection test (RADT) for group A streptococcus (GAS)
Test
Perform a RADT for GAS from a throat swab in:
All children ages 3-14 years who present with GAS pharyngitis (e.g., sudden onset of sore throat, tonsillopharyngeal inflammation as noted by erythema, pharyngeal exudates, swelling, palatal petechiae) and/or presence of a scarlatiniform rash.[2] However, do not give antibiotics until a RADT or a throat culture is positive.[2] See Management.
Patients ≥15 years old with a McIsaac score or a Centor score of ≥3.[24] [ Sore Throat (Pharyngitis) Evaluation and Treatment Criteria (McIsaac) Opens in new window ]
RADTs for GAS offer the advantage of immediate point-of-care testing and are about 70% to 90% sensitive and 95% specific compared with throat culture.[2][31][32] These tests may have a lower specificity in children recently treated for GAS.[33]
Positive testing in the absence of characteristic symptoms of pharyngitis (fever, lack of cough, tonsillar exudates, and anterior cervical adenopathy) likely represents colonization and is not clinically relevant; therefore, testing should be used only when the clinical symptoms are consistent with GAS disease.
Result
positive in GAS infection
Tests to consider
bacterial culture (throat, skin, wound)
Test
Order a culture of throat swab in children and adolescents ages 3-15 years with suspected scarlet fever who have a negative rapid antigen detection test (RADT) result for group A streptococcus (GAS).[2][3]
Do not order throat cultures in adults with a negative RADT result for GAS.[2][3] The incidence of GAS pharyngitis and the risk of subsequent complications, such as rheumatic fever, are very low in adults with acute pharyngitis.[2]
Order a culture of other body sites (e.g., superficial skin lesion, open wound) as initial test in patients with suspected nonpharyngitis GAS infection (e.g., skin and soft-tissue infection, sepsis, or streptococcosis in children <3 years old).[3]
Streptococcus pyogenes strains that cause scarlet fever may also cause other focal (e.g., skin and soft tissue) and nonfocal infections.
Result
growth of GAS
polymerase chain reaction for group A streptococcus (GAS)
Test
Polymerase chain reaction testing has high sensitivity and specificity comparable to that for throat culture, but it is more expensive and less readily available than rapid antigen detection tests for GAS.[3]
Result
positive in GAS infection
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