Rapid Strep Test
Encyclopedia
The Rapid Strep Test or rapid antigen detection test (RADT), is an in-office test done by a clinician in order to determine whether or not a patient has streptococcal pharyngitis (colloquially called "strep throat
" in American English
), which is a group A streptococcal infection
of the pharynx
and possibly other parts of the body.
Streptococcal pharyngitis is a common infection caused by the Streptococcus pyogenes
bacterium, also known as group A Streptococcus or beta-hemolytic Streptococcus. It is generally treated aggressively because of the potential serious complications of any group A streptococcal infection. In order to treat pharyngitis
adequately it is helpful to identify the infection, but none of the symptoms are specific to streptococcal pharyngitis; they can also be caused by other diseases, so testing is often used.
One of the most common tests for streptococcal pharyngitis is the rapid strep test. This is a test performed in a doctor's office. The throat is swabbed, and results are generally available in 10 or 15 minutes. However, one problem with the test is that, though it has high specificity of approximately 95 to 98%, the sensitivity is only 75 to 85%. This means that the odds of a false positive are lower than the odds of a false negative and one can be more confident about a positive result than a negative result. If the rapid test is negative, a follow-up culture (which takes 24 to 48 hours) might be performed. A negative culture could suggest a viral infection, infection with other typical bacterium such as Haemophilus influenzae
, or an infection with an atypical bacterial organism like Mycoplasma
. Viruses do not respond to antibiotics. Most of the other organisms are not treated aggressively because they do not generally cause the long term sequelae of group A Streptococcus and are considered to be self-limited, though some of them can still respond to antibiotics.
The rapid strep test works by detecting the presence of a carbohydrate antigen unique to group A Streptococcus. This may account for some of the problems with the rapid strep test sensitivity. Gargling, eating or other infusion of liquids into the mouth prior to the test may affect the results. If the test is performed before sufficient organisms are present in the throat, or late in the infection when most of the strep germs have been cleared by the immune system, or if it is performed after someone has been partially treated with antibiotics that kill the organism, then the rapid strep test is less likely to detect the organism.
If the rapid strep test detects strep, the infection should be treated with appropriate antibiotics to prevent long-term damage and sequelae. Should the rapid strep test fail to detect streptococcal pharyngitis, the clinician might still treat the throat infection based on his or her own judgment.
Strep throat
Streptococcal pharyngitis, streptococcal tonsillitis, or streptococcal sore throat is a type of pharyngitis caused by a group A streptococcal infection. It affects the pharynx including the tonsils and possibly the larynx. Common symptoms include fever, sore throat, and enlarged lymph nodes...
" in American English
American English
American English is a set of dialects of the English language used mostly in the United States. Approximately two-thirds of the world's native speakers of English live in the United States....
), which is a group A streptococcal infection
Group A streptococcal infection
The group A streptococcus bacterium is a form of β-hemolytic Streptococcus bacteria responsible for most cases of streptococcal illness. Other types may also cause infection...
of the pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
and possibly other parts of the body.
Streptococcal pharyngitis is a common infection caused by the Streptococcus pyogenes
Streptococcus pyogenes
Streptococcus pyogenes is a spherical, Gram-positive bacterium that is the cause of group A streptococcal infections. S. pyogenes displays streptococcal group A antigen on its cell wall. S...
bacterium, also known as group A Streptococcus or beta-hemolytic Streptococcus. It is generally treated aggressively because of the potential serious complications of any group A streptococcal infection. In order to treat pharyngitis
Pharyngitis
Pharyngitis is an inflammation of the throat or pharynx. In most cases it is quite painful, and is the most common cause of a sore throat.Like many types of inflammation, pharyngitis can be acute – characterized by a rapid onset and typically a relatively short course – or chronic....
adequately it is helpful to identify the infection, but none of the symptoms are specific to streptococcal pharyngitis; they can also be caused by other diseases, so testing is often used.
One of the most common tests for streptococcal pharyngitis is the rapid strep test. This is a test performed in a doctor's office. The throat is swabbed, and results are generally available in 10 or 15 minutes. However, one problem with the test is that, though it has high specificity of approximately 95 to 98%, the sensitivity is only 75 to 85%. This means that the odds of a false positive are lower than the odds of a false negative and one can be more confident about a positive result than a negative result. If the rapid test is negative, a follow-up culture (which takes 24 to 48 hours) might be performed. A negative culture could suggest a viral infection, infection with other typical bacterium such as Haemophilus influenzae
Haemophilus influenzae
Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...
, or an infection with an atypical bacterial organism like Mycoplasma
Mycoplasma
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
. Viruses do not respond to antibiotics. Most of the other organisms are not treated aggressively because they do not generally cause the long term sequelae of group A Streptococcus and are considered to be self-limited, though some of them can still respond to antibiotics.
The rapid strep test works by detecting the presence of a carbohydrate antigen unique to group A Streptococcus. This may account for some of the problems with the rapid strep test sensitivity. Gargling, eating or other infusion of liquids into the mouth prior to the test may affect the results. If the test is performed before sufficient organisms are present in the throat, or late in the infection when most of the strep germs have been cleared by the immune system, or if it is performed after someone has been partially treated with antibiotics that kill the organism, then the rapid strep test is less likely to detect the organism.
If the rapid strep test detects strep, the infection should be treated with appropriate antibiotics to prevent long-term damage and sequelae. Should the rapid strep test fail to detect streptococcal pharyngitis, the clinician might still treat the throat infection based on his or her own judgment.