There is no evidence that bacterial sore throats are more severe than viral ones or that the duration of the illness is significantly different in either case. The precise diagnosis may be of academic interest, or possibly clinically relevant in more severe cases. The most common single organism which is identified is group A beta-haemolytic streptococcus (GABHS).

Diagnosis can be attempted on clinical findings or by laboratory or near patient testing. The most commonly used tests in worldwide terms are culture of throat swabs and rapid antigen testing (RAT).

3.1 Clinical Diagnosis

Precise clinical diagnosis is difficult in practice. The clinical picture in an individual sore throat is of limited assistance in distinguishing between a bacterial and a viral aetiology. Several studies have attempted to differentiate between these, based on symptom complexes including tonsillar exudate, anterior cervical lymphadectomy, absence of cough, pharyngeal erythema, level of pyrexia and pain, etc., but the results are conflicting and inconclusive.^16-19

Studies for sensitivity and specificity suggest that reliance on clinical diagnosis will miss 25-50% of GABHS pharyngitis cases and that 20-40% of those with negative throat cultures will be labelled as having GABHS.²⁰ Evidence level IIb

Clinical examination should not be relied upon to differentiate between viral and bacterial sore throat.

3.2 Throat Culture

A positive throat culture for GABHS makes the diagnosis of streptococcal sore throat likely but a negative culture does not rule out the diagnosis. There are cases where streptococcus is isolated from sore throats but there is no serological evidence of infection.²¹ There is also a high asymptomatic carrier rate for GABHS of up to 40%.^{21, 22} The flora of bacteria recovered from the surface of the tonsil correlates poorly with that of those deep in the tonsillar crypts which are most likely to be causing the infection.^{23, 24} Symptoms also correlate poorly with results of throat swab culture.²⁵ Evidence level III

Throat swabs are neither sensitive or specific for serologically confirmed infection, considerably increase costs, may medicalise illness, and alter few management decisions.²⁶ Evidence level III

Throat swabs should not be carried out routinely in sore throat.

3.3 Rapid Antigen Testing (RAT)

Rapid antigen testing (RAT) is commonly used in North America to identify GABHS. The sensitivity of RAT measured against throat swab culture is wide and varies between 61% and 95%, although specificity may be better at 88-100%.^27-31 However, throat swab culture as a gold standard for comparison is questionable when compared with antistreptolysin O (ASO) titre, which is not in itself clinically useful in managing acute sore throats. A UK study among 23 GPs and 250 patients showed that the sensitivity of the test was 63% and specificity 91.7% compared with 74% and 58% respectively for clinical assessment. Use of the test changed prescribing decisions very little. The test cost £4 and took 10 minutes to report.³² Evidence level III

Rapid antigen testing should not be carried out routinely in sore throat.