Document exposure to group A streptococcal antigen, particularly streptolysin O. A marked rise or persistently elevated level indicates that a focus of streptococcal infection or poststreptococcal sequelae is present. Over 80% of patients with acute rheumatic fever and over 95% of patients with acute glomerulonephritis have elevated ASO levels.
< 5 years: 0-99 IU/ml; 5-15 years: 0-249 IU/ml; Adult: 0-199 IU/ml
ASO is a poor indicator of streptococcal pyoderma (75% false negatives) even if glomerulonephritis develops. This test is not useful in determining strep as the causative agent in poststreptococcal pharyngitis. It is recommended that in cases of suspected strep pyoderma the 'Streptozyme Screen' be ordered.
- Antistreptolysin O Antibody
- ASO (Anti-Streptolysin O)
- Streptolysin O Antibodies
- ANTI-STREPTOLYSIN O
Collect specimen in an SST tube. Centrifuge, aliquot serum into a plastic vial and refrigerate up to 48 hours or freeze for longer storage.
The most convincing serologic evidence suggestive of a recent group A streptococcal infection is a doubling rise between acute and convalescent sera. Increases occur about 1 week after infection and peak 2-4 weeks later. In the absence of complication or reinfection, the ASO will usually fall to preinfection levels within 6-12 months. Final diagnosis is determined by both clinical and laboratory findings. If only a single specimen is available, or in the absence of a doubling rise (or fall) the upper `limit of normal' is a useful guide for determining the significance of the ASO. The upper `limit of normal' value represents the highest ASO level obtained in 85% of a `normal' population with no apparent recent streptococcal infection. Thus an ASO above the upper limit has an 85% chance of being due to a recent streptococcal infection. It should be recognized that an ASO on a single specimen below the upper limit of normal cannot definitely rule out a streptococcal infection since a significant increase may occur even though the levels do not exceed the `normal' range.