Group A Streptococcal (GAS) Testing

General

Synonym/Acronym:
strep screen, rapid strep screen, GAS pharyngitis testing.

Rationale
To detect a GAS infection such as strep throat.

Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction.

Normal Findings
(Method: Enzyme immunoassay, latex agglutination, or PCR) Negative.

Critical Findings and Potential Interventions
N/A

Overview

(Study type: Body fluid, throat swab [two swabs should be submitted for the 2–step algorithm so that a culture can be performed if the screen is negative; specific liquid-based collection systems may be required for nucleic acid amplification test (NAAT) methods, the testing facility should be consulted prior to specimen collection]; related body system: Immune and respiratory systems.)

The onset of strep throat is sudden and includes symptoms such as chills, headache, sore throat, malaise, and exudative gray-white patches on the tonsils or pharynx. Early diagnosis and treatment appear to lessen the seriousness of symptoms during the acute phase and overall duration of the infection and sequelae. A two-step algorithm using a rapid screening test followed by a confirmatory culture is widely used. A negative result does not mean absence of infection; studies have indicated a high percentage of false-negative findings in the presence of GAS infection. Administration of antibiotics is often initiated before confirmatory culture results are reported. Evidence of GAS by conventional methods disappears rapidly after the initiation of antibiotic therapy.

Concerns regarding emergence of antibiotic-resistant microorganisms have led to the implementation of mandatory antibiotic stewardship programs in most facilities in order to curb unnecessary use of antibiotics. A number of NAATs that employ molecular methods (e.g., polymerase chain reaction [PCR]) have gained U.S. Food and Drug Administration approval for use in clinical laboratories and point-of-care (office) settings. The sensitivity and specificity of PCR assays is reported to be on par with culture methods but with a much faster time to results reporting (i.e., less than 60 min for NAAT versus several days for bacterial culture). Increased awareness and availability of NAAT for GAS may eventually eliminate the need for a two-step process and help prevent unnecessary administration of antibiotics. Rheumatic fever is a possible sequelae to an untreated streptococcal infection; for additional information, refer to the studies titled, “Antideoxyribonuclease-B Streptococcal Antibody and Antistreptolysin O Antibody.”

Indications

• Assist in the rapid determination of the presence of group A streptococci.

Interfering Factors

• Rapid test kits: Polyester (rayon or Dacron) swabs are favored over cotton for best chance of detection. Fatty acids are created on cotton fibers during the sterilization process. Detectable target antigens on the streptococcal cell wall are destroyed without killing the organism when there is contact between the specimen and the fatty acids on the cotton collection swab. False-negative test results can be obtained on specimens collected with cotton tip swabs. Negative strep screens should always be followed with a traditional culture.

Other Considerations:

• Sensitivity of the method varies among manufacturers.
• Adequate specimen collection in children may be difficult to achieve, which explains the higher percentage of false-negative results in this age group.

Potential Medical Diagnosis: Clinical Significance of Results

Positive Findings In:

• Rheumatic fever
• Scarlet fever
• Strep throat
• Streptococcal glomerulonephritis
• Tonsillitis

Nursing Implications, Nursing Process, Clinical Judgement