Screening for Non-classic Congenital Adrenal Hyperplasia Revisited: Proposal for a New Serum 17-hydroxyprogesterone Threshold for which a Cosyntropin Stimulation Test is Indicated

Introduction:
The 250µg-cosyntropin stimulation test (CST) is used to diagnose non-classic congenital adrenal hyperplasia (NCCAH). The current recommendation to perform CST is when follicular 17-hydroxyprogesterone (17OHP) is >6nmol/L. This cut-off is derived from radioimmunoassay (RIA) data. Recently, a validated enzyme-linked immunosorbent assay (ELISA) has replaced RIA. This study aimed to determine the RIA and ELISA-based 17OHP cut-offs at which CST should be performed.

Methods:
A retrospective study at Maccabi Healthcare Services. Data were retrieved from adult females with suspected NCCAH, referred for CST during 2001–2020. NCCAH was defined as post-CST 17OHP >30 nmol/L. Serum 17OHP levels were assayed by direct RIA from 1/2000-3/2015, and by ELISA from 4/2015-12/2020. For each assay group, a ROC curve was generated and optimal pre-CST 17OHP threshold determined.

Results:
Cosyntropin testing was performed in 2409 female subjects (1564 in RIA and 845 in ELISA). The mean(±SD) age was 24.1±7years. NCCAH was diagnosed in 74(4.7%) of the RIA group and 63(7.5%) of the ELISA group. The mean(±SD) pre- and post-CST 17OHP levels were lower in the RIA group as compared to the ELISA group (4.1±6.4vs.5.9±9.0 and 9.9±15.3vs.12.3±17.3, respectively,p<0.0001). The optimal pre-CST 17OHP cut-off values predicting NCCAH were 6.05 nmol/L in the RIA group (sensitivity=93.2%, specificity=91.7%) and 8.16 nmol/L in the ELISA group (sensitivity=93.7%, specificity=92.3%). When pre-CST 17OHP of 6 nmol/L was used in the ELISA group, specificity decreased to 84%.

Conclusions:
The optimal RIA-based pre-CST 17OHP cut-off was comparable with that recommended in the guidelines. The results suggest adopting a higher 17OHP cut-off when using ELISA.