Tonsillopharyngitis - Acute Diagnostics

Laboratory Tests and Ancillaries

GABGS Pharyngitis Testing  

It is important to note that the clinical presentation of GABS and viral pharyngitis greatly overlap. Patients who have clinical and epidemiological findings suggestive of GABS pharyngitis should be tested for the presence of group A Streptococci in the pharynx. Diagnostic testing of contacts of infected patients is not routinely recommended.  

Throat Swab Culture

Throat swab culture is the gold standard for the confirmation of clinical diagnosis of GABs pharyngitis. It is recommended for those with history of contact with symptomatic persons with GABS pharyngitis, recurrent GABS infection, symptomatic patients at high risk for rheumatic fever. If done correctly, the culture of a single throat swab on a blood agar plate has a sensitivity of 90-95%. It is less expensive than RADT and more readily available. Results may take 24-48 hours. However, it does not differentiate between illness and carrier states. False-negative results may be seen in patients who received antibiotic therapy shortly before or at the time the swab was obtained.   

Proper Techniques of Obtaining a Throat Swab 

In doing a throat swab, it is important to swab the surface of both tonsils, tonsillar fossae, and posterior pharyngeal wall. The mouth, uvula, and oropharynx are not included. Lastly, it is optimally done at the onset of symptoms and before antimicrobial therapy is started.  

Rapid Antigen Detection Test (RADT)  

RADT was developed for the identification of GABS directly from the throat swabs. It may be considered in patients with modified Centor criteria scores of ≥3 or when 2 viral features (eg fever, tonsillar exudates or swelling, swollen anterior cervical nodes, absence of cough) are present. It is more expensive than throat swab cultures, but results are available faster (within minutes). It has a sensitivity of 80-90% and a specificity of 90-99%. Just like throat swab cultures, RADT does not differentiate between illness and carrier states. A rapid test can lead to earlier initiation of definitive therapy with the following advantages such as reduced risk of GABS, reduced acute morbidity associated with illness, and earlier return of patients to work or school. Lastly, a negative RADT result for an adult patient does not need to be confirmed with a throat culture because of the low incidence of streptococcal infection and the low risk of rheumatic fever in this age group.  

Nucleic Acid Amplification Test  

Nucleic acid amplification test is more sensitive and specific than RADT and culture, particularly when bacterial burden is low. It must be noted that high sensitivity may increase the risk of false positives wherein GABS carriage rather than infection is detected.  

Other Tests  

Testing for other etiologies (eg coronavirus disease 2019 [COVID-19], influenza, dengue, etc) will be helpful as antiviral therapies might be needed in high-risk individuals. Local prevalence and/or patient exposure must be considered when deciding to test for COVD-19. Rapid testing or reverse transcriptase-polymerase chain reaction (RT-PCR) test should be used to identify, isolate, and treat patients who are suspected of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. In some places, rapid testing kits for influenza and other respiratory viral infections may be available.  Dengue testing may be considered in febrile patients with erythematous throat.