Investigations
1st investigations to order
clinical diagnosis
Test
The diagnosis of scarlet fever is mainly clinical, based on the presence of the typical clinical features (i.e., scarlatiniform rash, fever [>38°C (>100.4°F)], and sore throat).
In countries such as the UK, where rapid antigen detection tests (RADTs) for scarlet fever are not readily available, test confirmation of group A streptococcus infection is not required before starting antibiotics in patients with a clinical diagnosis of scarlet fever.[25]
In countries where RADTs for scarlet fever are available, a positive test result may be required before starting antibiotics. Refer to your local protocol for specific guidance in your area.
Result
no tests are usually necessary
rapid antigen detection test (RADT) for group A streptococcus (GAS)
Test
In countries where RADTs for scarlet fever are available, a positive test result may be required before starting antibiotics.
For example, a RADT for GAS from a throat swab is recommended in:
All children aged 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 a scarlatiniform rash, by the Infectious Diseases Society of America and the American Academy of Pediatrics.[2] See Management.
Patients ≥15 years old with a McIsaac score or a Centor score of ≥3, by international guidelines.[30]
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][37][38] These tests may have a lower specificity in children recently treated for GAS.[39]
Positive testing in the absence of characteristic symptoms of pharyngitis (fever, lack of cough, tonsillar exudates, and anterior cervical adenopathy) likely represents colonisation 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
Investigations to consider
bacterial culture (throat, skin, wound)
Test
In countries such as the UK, where rapid antigen diagnostic tests (RADTs) for scarlet fever are not routinely used or recommended, consider taking a throat swab for culture of group A streptococcus (GAS) in the following circumstances:[25]
Uncertainty about the clinical diagnosis
Suspected case as part of an outbreak: the local health protection team should advise primary care if a local outbreak is suspected and when testing is appropriate
True allergy to penicillin, to determine antimicrobial susceptibility, depending on clinical judgement. GAS can be resistant to non-penicillin options such as macrolides and clindamycin
Regular contact with vulnerable people who are at high risk of complications, such as healthcare workers.
Take swabs before starting antibiotics but do not delay starting treatment while waiting for culture results. See Management.
In countries where RADTs for scarlet fever are available, such as the US, a culture of throat swab is recommended in children and adolescents (aged 3 to 15 years) who have a negative RADT result.[2][3]
Throat cultures are not recommended 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, is very low in adults with acute pharyngitis.[2]
A culture of other body sites (e.g., superficial skin lesion, open wound) is recommended as initial test in patients with suspected non-pharyngitis 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 non-focal 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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