Streptococcus pyogenes serodiagnosis

Infection with GAS effects the production of specific antibodies against streptococcal exoenzymes, the most important of which are anti-streptolysin O (ASO) and anti-deoxyribonuclease - B (ADB). Tests for the antibodies are run in parallel.
Under normal circumstances, serum samples are processed once weekly (usually Mondays) and the turnaround time is 7 working days.
A volume of 200ul serum is required; please send serum and not whole blood. Contaminated, fatty and/or haemolysed samples give erroneous results and will not be assayed. Samples should be sealed with parafilm not sellotape, securely wrapped in cotton wool, placed in a plastic bag and packaged in a box according to the Post Office regulations.

Anti-streptolysin O Test (ASO)

Basis: Neutralisation of oxygen labile haemolytic activity of group A, C, and G streptococci by patient's antibody.
Sensitivity: High.
Reproducibility: Good at closely spaced intervals.

Clinical usefulness and limitations.
The antibody response is generally good in pharyngitis and tonsillitis but will not distinguish between infections with groups A, C, and G streptococci. Generally poor in impetigo and pyoderma. Falsely elevated titres may occur in contaminated sera or in jaundiced patients with liver disease.
The normal range is up to and including 200 units/ml. A titre of 300 units/ml indicates that further testing is required. A titre of 400 units/ml is suggestive of streptococcal infection. Titres of > 600 units/ml are indicative of infection.
A single determination may be of little significance. Serial titrations at bi - weekly intervals 4 - 6 weeks following infection yield more pertinent information. At its best, the ASO test detects antibody in only 80% of patients after streptococcal pharyngitis.

The mean normal anti-ASO levels are age dependant:

Pre-school - ≤ 200
School age - ≤ 300
Adult - ≤ 200

Anti - DNase B Test (ADB).

The ADB test measures antibodies to DNase B common to all GAS strains.

Basis: DNase B depolymerises DNA; patients antibody inhibits activity of the enzyme.
Sensitivity: High.
Reproducibility: Good at closely spaced intervals.

Clinical usefulness and limitations.
The antibody response is good in skin as well as throat infections and may be specific for GAS infection. ADB titres peak later than ASO levels, and remain elevated for several months. The ADB can therefore be of value if there is a delay in diagnosis, or if there is a long latent period between infection and post- infective complications (as may be the case in Sydenham's chorea).

The normal range is up to and including 200 units/ml. Titres of 300-400 units/ml indicate that further testing is required. Titres of 600-800 units/ml are suggestive of infection with S.pyogenes. Titres of >1200 units/ml indicate infection with S.pyogenes.
The ADB test shows elevated titres in >90% of clinically diagnosed cases of pyoderma, acute glomerulonephritis and acute rheumatic fever.

Last reviewed: 24 July 2009