An Antistreptolysin O (ASO) screen is a blood test that detects antibodies to Group A Streptococcus (GAS) bacteria. These bacteria cause conditions like strep throat and impetigo. The test measures the body’s immune reaction to a past encounter with these bacteria, rather than indicating a current infection. This helps healthcare providers determine if a recent streptococcal infection has occurred, even if it went undiagnosed or untreated.
Reasons for an ASO Screen
Doctors order an ASO screen when a patient develops symptoms suggestive of a post-streptococcal complication, often weeks after an infection. This test is not used to diagnose an active strep throat infection, which is identified through a rapid strep test or throat culture. Instead, the ASO test helps determine if a recent Group A Streptococcus infection triggered subsequent health issues.
An ASO test investigates symptoms of acute rheumatic fever. This condition can develop after an untreated strep throat and affects the heart, joints, brain, and skin. Patients might experience joint pain, inflammation, and involuntary movements, leading a doctor to consider a past strep infection as the cause.
The ASO screen is also used when symptoms suggest post-streptococcal glomerulonephritis (PSGN). This kidney disorder can follow either strep throat or a streptococcal skin infection, like impetigo. Individuals with PSGN may exhibit fatigue, swelling in the face and extremities, and blood in their urine, leading a physician to look for evidence of a preceding strep infection.
Interpreting Test Results
The results of an ASO screen are reported as a titer, indicating the concentration of antistreptolysin O antibodies in the blood. A low or negative ASO titer suggests that a recent Group A Streptococcus infection is unlikely. This finding helps rule out post-streptococcal complications when other symptoms are present.
Conversely, a high or rising ASO titer indicates a probable recent streptococcal infection. An elevated titer means the body has produced a significant amount of antibodies in response to the bacteria. However, a single high ASO test result alone does not diagnose a specific condition like rheumatic fever or glomerulonephritis.
A healthcare provider uses the ASO test result as one piece of evidence, alongside a physical examination, the patient’s reported symptoms, and other laboratory tests. For more conclusive information, a doctor may order two ASO tests several weeks apart. Observing whether the antibody level is rising, falling, or remaining stable provides a clearer picture of the infection’s timeline and its potential link to current symptoms.