Context The diagnosis of the polycystic ovary syndrome requires the exclusion of nonclassic congenital adrenal hyperplasia (NCAH).

Objective To evaluate the actual prevalences of 21-hydroxylase and 11-hydroxylase deficiencies among women presenting with hyperandrogenic complaints.

Settings Academic hospital.

Patients Two-hundred seventy consecutive unselected women presenting with hyperandrogenic symptoms were prospectively recruited.

Interventions Basal and adrenocorticotropin-stimulated 11-deoxycortisol and 17-hydroxyprogesterone concentrations were measured.

Main outcome measures The prevalences of 21-hydroxylase and 11-hydroxylase deficiencies were calculated, and the diagnostic performance of basal serum 17-hydroxyprogesterone levels for the screening of NCAH was evaluated by receiver operating characteristic curve (ROC) analysis.

Results Six of the 270 patients had 21-hydroxylase deficient NCAH that was confirmed by CYP21 genotyping, whereas no patient was diagnosed with 11-hydroxylase deficiency, for an overall NCAH prevalence of 2.2% (95% confidence limits : 0.5%–3.9%). According to ROC analysis, a single basal serum 17-hydroxyprogesterone determination has a 0.97 (95% confidence interval: 0.934–1.008) chance of detecting NCAH in hyperandrogenic women. In our experience, the most appropriate cut-off value for the detection of NCAH is a 17-hydroxyprogesterone above 1.7 ng/mL, showing a 100% sensitivity and a 88.6% specificity. Five of the six 21-hydroxylase deficient NCAH patients carried a severe CYP21 allele requiring genetic counseling and highlighting the importance of ruling out this disorder among hyperandrogenic patients.

Conclusions The prevalence of NCAH among hyperandrogenic patients from Spain is 2.2%. Basal serum 17-hydroxyprogesterone measurements have an excellent diagnostic performance, yet the cut-off value should be established in each laboratory to avoid false negative results.