Establishing Cutoffs for Streptococcal Antibody Titers in Uganda: Implications for Rheumatic Fever Diagnosis

Background: Central to rheumatic fever (RF) diagnosis is evidence of streptococcal exposure, specifically antistreptolysin O (ASO) and antideoxyribonuclease B (ADB) antibodies. It is unknown if these antibody titers should be adjusted to the background exposure rates of GAS or if published standards should be used. Here, we establish the normal values of ASO and ADB in Uganda and examine RF case detection using published vs. population-specific thresholds. Methods: Participants (age 0-50 years) were recruited. ASO was measured in-country by nephelometric technique. ADB samples were sent to Australia (PathWest) for ADB determination by enzyme inhibition assay, and 80% upper limit values by age were established. The published standard values for ASO (200IU/ml) and ADB (375IU/ml) were compared to the Ugandan 80% upper limit of normal values (ULN) for RF case detection in children 5-15 years. Findings: Of the 428 participants, 16 were excluded (9 sore throat, 1 skin sores, 5 fever, 4 echocardiograms showing occult RHD), and 183 of the remaining were children 5-15 years. The median ASO titer in this age group was 220 IU/ml, with the 80th percentile value of 389 IU/ml. The median ADB titer in this age group was 375 IU/ml, with the 80th percentile value of 568 IU/ml. Application of new Ugandan cutoffs to 528 children enrolled in our prospective RF study, reduced the number of definite RF cases to 120/528 (22·7%), as compared to 173/528 (32·8%) using published normal values. Interpretation: The 80th percentile ULN for ASO and ADB are higher in Uganda than in other countries. Applying these higher values to RF diagnosis in Uganda results in higher diagnostic specificity, but some unknown loss in sensitivity. Implications of over-diagnosis and missed cases will be explored through a longitudinal follow-up study of children in the RF research program. Funding Statement: This work was supported by the American Heart Association (#17SFRN33670607), the DELTAS Africa Initiative (DEL-15-011 to THRiVE-2), and General Electric. Declaration of Interests: There were no conflicts of interest in conducting this study. Ethics Approval Statement: Institutional review board (IRB) approval was obtained from the Research and Ethics Committee at Makerere University School of Medicine, the Children’s National Medical Center IRB, and the Uganda National Council for Science and Technology IRB. We approached all participants for their consent, and the study was explained in detail by a research nurse in the participants’ primary language. All participants provided written informed consent before information was collected and children were enrolled only if written consent was obtained from a parent or guardian and written assent by children aged 8 years or older.