5 year-old presents to clinic with erythematous pharnyx

A positive rapid strep test (RST) is considered a reliable indication for the presence of group A betahemolytic streptococcus in the pharynx

A positive RST is considered a reliable indication that group A betahemolytic streptococcus (GABHS) is present in the oropharynx.

The RST has a 90% to 95% sensitivity for detecting GABHS (much lower if antibiotics were already taken), though a positive antistreptolysin O blood test is still the only valid method for confirming a GABHS infection. A negative RST is less useful; the patient might still have undiagnosed GABHS and remain at risk for developing rheumatic fever.

Most cases of pharyngitis/tonsillitis are due to viral agents and follow a benign, self-limiting course. Overall, the presence of viral symptoms such as cough, coryza, and rhinorrhea and the absence of cervical adenopathy make GABHS (or any other bacterial pharyngitis/tonsillitis) less likely, whereas high fevers, marked cervical adenopathy, a sandpaper rash (scarlet fever) and the absence of cough, coryza, and rhinorrhea makes a diagnosis of GABHS more likely. But this distinction is only moderately reliable and the positive diagnosis of "strep pharyngitis" can only be made in the context of bacterial confirmation with a test. Unfortunately, it is clinically impossible to reliably distinguish bacterial from viral pharyngitis.

Bacterial pharyngitis due to GABHS peaks in the 5 to 15 year old group, but can occur at any age. In children within this age range, GABHS is the number one cause of bacterial phayrngitis. In adults, acute bacterial pharyngitis is typically caused by all groups (not just group A) of betahemolytic streptococci (23%), Mycoplasma pneumoniae (9%) or Chlamydia pneumoniae (8%).

GABHS must be treated within 9 days to prevent rheumatic fever - a complication that occurs in 0.3 % of patients. Another complication - poststreptococcal glomerunephritis - is not affected by antibiotics.

Exudate does NOT necessarily point to a bacterial infection. Adenovirus is common and can present with an especially severe pharyngitis/tonsillitis with a large amount of white exudates.

Pearl: Influenza (scratchy throat) and syphilis (painless lesions) are two infections of the pharynx that are less commonly associated with exudates. See the second reference for good pictures of pharyngitis from various causes.

References:

Lean, Wei Ling, et al. "Rapid diagnostic tests for group A streptococcal pharyngitis: a meta-analysis." Pediatrics 134.4 (2014): 771-781.

Subashini, Beula, Shalini Anandan, and Veeraraghavan Balaji. "Evaluation of a rapid antigen detection test for the diagnosis of group-A beta-hemolytic Streptococcus in pharyngotonsillitis." Journal of global infectious diseases 7.2 (2015): 91.