Diagnostic Use
Anti-streptolysin O (ASO) and anti-DNase B (ADNaseB) are used to confirm antecedent infection by Group A streptococci in the investigation of rheumatic fever or acute glomerulonephritis.
Interpretation
ASO antibody begins to rise about 1 week following infection, takes about 3-6 weeks to peak, around 6-12 weeks to decline and generally approaches pre-infection level by around 6-12 months. Anti DNase B takes about 2 weeks to rise following infection, 6-8 weeks to peak, starts to decline by around 3 months and will persist for a long period.
ASO may remain normal in about 15-20% of patients with rheumatic fever. ASO response to Group A streptococcal skin infection is generally poorer than anti-DNase B. Testing both ASO and Anti-DNase B improves sensitivity.
Elevated ASO can also be encountered in non-Group A streptococcal disease (group C and G), chronic liver disease and hypergammaglobulinaemia.
To define a recent Group A streptococcal infection, streptococcal serology should be requested during the acute phase and in the convalescent phase about 14-28 days later. Whilst traditionally a 2-fold or more rise in ASO titre from doubling dilution tests had been used as evidence of a preceding streptococcal infection, the magnitude of change required in streptococcal serology from modern immunoturbidimetry assays (the method in use at CMDHB laboratory, report in kIU/L) is not well defined.
The upper limit of normal for streptococcal serology = value exceeded by 20% of a normal population. This level varies with the epidemiology in that particular population e.g. prevalence of impetigo. It also varies with age (higher in the age range from 5 to 15 yrs), season (winter to spring months generally higher level) and the analytical method used.