Serodiagnostic approach for group a streptococcal infection and rheumatic fever

Abstract

Current assays for anti-Streptolysin O (ASO) and anti-DNAse B are accurate and precise. The Royal College of Pathologists of Australasia (RCPA) serology quality assurance program documents interlaboratory coefficient of variation (CV) of 5–10% and good interchangeability of results between methods over the diagnostically informative range. The difficulty is in interpreting the results. At the age of 5, 80% of New Zealand children have detectable ASO and 55% have anti-DNAse B;most of the remainder seroconvert by the age of 10. Population antibody levels decline a little by the age of 15 but most remain seropositive. Sera referred for diagnostic testing at Waikato Hospital, NZ, show lifelong seropositivity, with 93% having detectable ASO and 68% detectable anti-DNAse B beyond the age of 40. The distribution of seropositive antibody levels is unimodal;there is no distinct population with higher levels pathognomonic of recent infection. Published data on temporal decline in levels after acute infection is sparse. Serology cannot be relied upon to diagnose recent infection. Paired sera may demonstrate a recent immunological response, so efforts should be made to obtain a second sample. Interpretive comments on laboratory reports should convey the modest contribution to specificity that serology can offer in the diagnosis of rheumatic fever and post streptococcal glomeruonephritis.