Anti-streptolysin O (ASO) is an antibody produced in response to the streptolysin O toxin produced by Group A streptococcus (GAS) bacteria. GAS is responsible for most strep throat infections and if left untreated may result in post-streptococcal complications such as rheumatic fever and glomerulonephritis. Diagnostic reporting of ASO results is based on clinical cut-offs with reference to population studies. The Royal College of Pathologists of Australasia Quality Assurance Programs (RCPAQAP) has a Streptococcus serology module which includes reporting ASO both quantitatively (IU/mL) and qualitatively (significant / non-significant) linked to an associated clinical scenario. RCPAQAP undertook a 5 year retrospective review of the program data to identify any trends in performance and reporting of ASO.
Data from 6 survey specimens with varying levels of ASO (258 sets of data from 43 laboratories) from 2013 to 2017 was reviewed. Statistical analysis only included data sets where participants returned results for all six surveys. Clinical notes for the selected specimens all stated, “a symptomatic 15-year-old patient”. Quantitative and qualitative results were analysed against the cut-off values provided by each laboratory. Assessment of assigned results was determined by a consensus of ≥ 80%.
Three specimens had a consensus result of “non-significant”, two were “significant”, and no consensus was achieved for one specimen. Inconsistencies were identified in the interpretation of the quantitative results against the nominated cut-off. In addition, significant variation was noted in the cut-off values provided both between (1IU/mL to >480IU/mL) and within different method groups (e.g. 116-300IU/mL).
In conclusion, participants provided a variety of cut-off values for the same clinical notes over the 5 year study period. This poses the question on how clinical cut-off values are being determined and reviewed to ensure consistent results are being provided to clinicians. The variation in cut-off values (even within the same method group) highlights a lack of standardisation in this area and the need for the pathology profession to act on harmonising reporting practices.