Pharyngitis laboratory findings

Advantages

• Rapidity of the test: Rapid identification and treatment of patients with GAS pharyngitis can reduce the risk of spread, allowing the patient to return to school or work sooner, and can reduce the acute associated morbidity.^[1]
• High specificity: RADTs currently available are highly specific (approximately 95%) when compared with blood agar plate cultures.^[2]
• False positive test results are highly unusual, and therefore therapeutic decisions can be made with confidence on the basis of a positive test result.^[3]

Disadvantages

• Sensitivity is low: Because the sensitivities of the various RADTs are <90% and because the proportion of acute pharyngitis due to GAS in children and adolescents is sufficiently high (20%-30%), a negative RADT should be accompanied by a follow-up or back-up throat culture in children and adolescents, while this is not necessary for adults under usual circumstances.^[3]

• Can not differentiate acutely infected persons from asymptomatic streptococcal carriers with intercurrent viral pharyngitis.

Description about the test

• Adults with 2 or more Centor criteria should have RADT
• A positive RADT establishes the diagnosis for GAS pharyngitis in conjunction with supportive clinical and epidemiological evidence.
• If RADT is positive but is not associated with clinical evidence of infection, it identifies a Streptococcus carrier who is chronically colonized.
• If the streptococcal infection is suspected and RADT is negative, follow-up with a throat culture is warranted due to the possibility of false-negative results.
• RADT has 70% to 90% sensitivity and 90% to 100% specificity.

Advantages

• High sensitivity: The culture of a single throat swab on a blood agar plate is 90%– 95% sensitive for detection of GAS pharyngitis.

Disadvantages

• A major disadvantage of throat cultures is the delay (overnight or longer) in obtaining results.
• Can not differentiate acutely infected persons from asymptomatic streptococcal carriers with intercurrent viral pharyngitis.

Variables that affect culture results

• Culture methods: Use of anaerobic incubation and selective culture media may increase the proportion of positive culture results.^[4]
• Manner in which the swab is obtained: Throat swab specimens should be obtained from the surface of either tonsil (or tonsillar fossae) and the posterior pharyngeal wall. Other areas of the oral pharynx and mouth are not acceptable sites. An uncooperative child without immobilizing the neck may obtain a specimen that is neither adequate nor representative.
• Duration of Incubation

Description about the test

• Throat culture is the gold standard for the diagnosis of GAS pharyngitis

• Should be done in adults at high risk for severe infections (immunocompromised patients and those with diabetes mellitus or who use steroids) in whom RADT may be negative.
• Sensitivity is between 90% and 95%, and specificity is from 95% to 99% when the swab is collected appropriately.
• Throat culture results will serve as a guide for the completion of treatment.
• If Neisseria gonorrhoeae is suspected, the diagnosis should be confirmed by culture on Thayer-Martin medium or validated nucleic acid amplification testing.

Advantages

• Measurement of anti-streptococcal antibody titers is often useful for diagnosis of the nonsuppurative sequelae of GAS pharyngitis, such as acute rheumatic fever and acute glomerulonephritis.^[5]

Disadvantages

• Testing of antibody is not useful in the diagnosis of acute pharyngitis because antibody titers of the 2 most commonly used tests, antistreptolysin O (ASO) and anti- DNase B, may not reach maximum levels until 3–8 weeks after acute GAS pharyngeal infection and may remain elevated for months even without an active GAS infection.