Rapid streptococcus antigen tests

Definition

A rapid streptococcus antigen test is used to quickly diagnose strep throat , a contagious infection of the pharynx caused by a Group A streptococcus (Streptococcus pyogenes), commonly referred to as GAS.

Purpose

Rapid streptococcus antigen tests are used to identify a strep throat. While a throat culture remains the gold standard for diagnosis of group A streptococcus infection, this procedure takes 24-48 hours for results. A rapid strep test takes only five minutes to perform. Since streptococci are sensitive to penicillin and related drugs, antibiotic therapy can be initiated immediately when the test is positive. A positive test result eliminates the need for bacterial throat culture.

Precautions

An untreated strep throat can increase the patient's risk of developing scarlet or rheumatic fever , which is associated with meningitis ; and diseases affecting the heart , skin, kidneys , and joints. False-negative results occur with this test with a frequency ranging from 4-39%. Therefore, negative test results should be confirmed by throat culture.

Description

Approximately 10-19% of all sore throats are caused by group A beta-hemolytic streptococci. The majority of strep throat infections occur in children between the ages of five and fifteen, although adults with weakened immune systems are also at risk. The highest incidence of strep throat occurs during the winter and early spring months. Rapid streptococcal tests utilize antibodies to detect streptococcal antigens. There are four different direct streptococcal antigen detection platforms available in a wide number of different commercial products. These are latex agglutination; optical immunoassay; double antibody sandwich immunoassay; and immunochromatographic detection. The majority of rapid assays used today are based on double antibody sandwich and/or immunochromatography because these techniques do not require mixing, multiple reagent addition, or washing. An example of a combined double antibody sandwich immunochromatography method is described below.

The first step of a rapid strep test is the extraction of specific Group A streptococcal carbohydrate antigen from the swab. The swab is placed in a test tube to which the extracting reagents have been added. The swab is rotated vigorously in the solution while pressing the tip against the sides of the test tube. After all fluid is pressed from the swab, it is discarded and the extract is applied to a nitrocellulose membrane containing both immobilized antibodies and nonimmobilized antibodies to different regions of the Group A strep antigen. The nonimmobilized antibodies are conjugated to dyed colloidal gold particles. If Group A streptococcal carbohydrate antigen is present in the extract, the conjugated antibodies bind to it, forming antigen-antibody complexes. These migrate along the membrane until they reach the reaction zone containing immobilized antibodies to the same Group A strep antigen. These antibodies capture the antigen-antibody complexes, forming a colored band or line (usually pink or blue) in the reaction zone area.

Specificity of these tests is approximately 97-98%. Few causes of false positives have been reported. False positives are possible when the patient's throat contains a heavy growth of Staphylococcus aureus. False-negative test results are commonly reported and often occur when insufficient antigen is obtained from the swab extraction procedure.

Preparation

All rapid group A strep tests require a sample from the infected patient's throat. The sample is obtained by depressing the tongue and swabbing the back of the throat and tonsils, while avoiding the tongue, saliva and lips. The swab should come in contact with all the inflamed areas, vesicles and pustular tonsils. Swabs made of rayon or Dacron should be used. Swabs containing cotton, calcium alginate, or wooden shafts, or that have been placed in transport medium containing charcoal are not recommended.

Aftercare

There are no aftercare concerns with this test.

Complications

There are no complications associated with this test.

Results

Normal results are negative. However, these assays tend to have low sensitivity, and thus it is recommended that all negative tests be followed by culture of a different throat swab on a blood agar plate for the isolation of beta-hemolytic streptococci. A positive result indicates an infection with group A streptococcus, and does not require culture follow-up.