Strep Throat (GAS) (cont.)
John Mersch, MD, FAAP
John Mersch, MD, FAAP
Dr. Mersch received his Bachelor of Arts degree from the University of California, San Diego, and prior to entering the University Of Southern California School Of Medicine, was a graduate student (attaining PhD candidate status) in Experimental Pathology at USC. He attended internship and residency at Children's Hospital Los Angeles.
Steven Doerr, MD
Steven Doerr, MD
Steven Doerr, MD, is a U.S. board-certified Emergency Medicine Physician. Dr. Doerr received his undergraduate degree in Spanish from the University of Colorado at Boulder. He graduated with his Medical Degree from the University Of Colorado Health Sciences Center in Denver, Colorado in 1998 and completed his residency training in Emergency Medicine from Denver Health Medical Center in Denver, Colorado in 2002, where he also served as Chief Resident.
In this Article
Is strep throat contagious?
Strep throat is contagious most commonly via direct person-to-person contact. Passage of airborne droplets and/or saliva from the infected individual to another is the most likely mechanism of contracting strep throat. As such, close living quarters (home, classroom, day care centers, college dorms, etc.) provide an ideal environment for passage of GAS bacteria from one person to another. The risk of contracting a strep throat infection is approximately 40% in household environments. Most infectious disease specialists believe that a patient is no longer contagious after 24 hours of effective antibiotic therapy. Spread of strep bacteria via food borne transmission is less common than direct person-to-person exchange of droplets or saliva. The exact likelihood of developing strep throat from family pets is unknown, but most experts believe it is minimal.
How is the diagnosis of strep throat established?
There are two common tests used to establish whether an individual has a strep throat infection. The traditional test, a throat culture, involves the doctor obtaining a swab from the back of the throat and tonsils. It takes approximately 36 to 48 hours to determine the presence or absence of the GAS bacteria. This test is considered the "gold standard" in diagnosing strep throat.
Many medical offices, clinics and hospital emergency departments use a rapid strep test (RADT - rapid antigen detection test) in an effort to provide a more timely diagnosis for their patients. The throat swab is obtained similarly to that for a traditional throat culture, but a RADT "kit" is employed to help determine the presence or absence of the strep bacteria. The turn around time is generally 10 minutes. The RADT is less precise when compared with a throat culture. Most RADT tests are about 95% accurate in determining the presence of strep bacteria. Most guidelines recommend that a negative rapid strep test be confirmed with a throat culture. Of note, antibiotic pretreatment prior to obtaining a throat swab for strep bacteria can interfere with throat culture results, and thus the ability to correctly diagnose and treat a strep throat infection.
The above tests are designed to determine whether an individual currently has a strep throat infection. Sometimes it may be necessary to determine whether a patient has had a recent history of a strep infection. In that case, two blood tests are available: (a) an ASO (anti streptolysin O) titer, and/or (b) an anti DNase titer. The antibodies that these tests are sensitive to are not commonly present until a few weeks following an acute strep infection. These tests may be ordered when evaluating a patient who may have developed delayed complications (such as rheumatic fever or glomerulonephritis) after a strep throat infection.
Medically Reviewed by a Doctor on 2/4/2015
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