Streptococcal Infections - Diagnosis

How was your streptococcal (GAS) infections diagnosed?

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How are group A streptococcal (GAS) infections diagnosed?

After a history and physical examination, many clinicians presumptively diagnose strep throat from its symptom production and throat appearance (see Fig. 2). However, cultures from the throat or other site of infection form the basis of definitive testing. For example, GAS organisms will grow on sheep blood agar plates that contain two different antibiotics and cause beta hemolysis (complete sheep blood red cell lysis to form a clear area) of the sheep red blood cells (see Fig. 3). In addition, there are rapid tests (RADT or rapid antigen detection test) that take only a few minutes to complete that detect a carbohydrate surface antigen produced by GAS, with specificity of about 95% or better and fairly good sensitivity of about 80%-90%.

Because there are many other groups of Streptococcus spp., positive identification of the infecting bacteria is necessary to separate out other bacteria that may cause some similar symptoms but may require a different workup, different treatment, and produce different complications.

These tests help distinguish GAS from Streptococcus pneumoniae and other organisms.

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