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Background Mutations of CYP21A2 encoding 21-hydroxylase are the most frequent cause of congenital adrenal hyperplasia (CAH) and are associated either with elevated basal or ACTH-stimulated levels of 17-hydroxyprogesterone (17OHP) in blood. Objective The study objective was to identify the most suitable of 12 different test algorithms and appropriate cut-off levels for that test to recognize patients with non-classical congenital adrenal hyperplasia (NCCAH) and carriers of clinically relevant mutations in CYP21A2. Method and Patients Between July 2006 and July 2015 ACTH-tests were conducted in 365 children and adolescents (Age 1–20 y) suspected to have NCCAH. As a reference, results from subsequent gene sequencing of CYP21A2 was used. Inclusion criteria that were used were premature pubarche with accelerated bone age, hyperandrogenism, hirsutism, or menstrual irregularities. Receiver operating characteristics (ROC) were plotted. Evaluated test algorithms were composed around 17OHP measurements by radioimmunoassays. The most suitable test was identified by the greatest area under the curve (AUC). Results Among the 12 tested algorithms, the sum of 30 min and 60 min stimulated 17OHP values (sum17OHPstim) showed the highest AUC of 0.774 for identifying heterozygous and bi-allelic mutations. A cut-off of 10.1 μg/l was advisable. Bi-allelic mutations only were best identified calculating the difference between 30 min and basal 17OHP values (Δ17OHP30). A cut-off of 9.4 μg/l was most effective. Conclusion Alternatively to the above mentioned cut-offs the difference of 60 min after stimulation to basal 17OHP (Δ17OHP60) can be used for the benefit of a combined test to identify both heterozygotes and bi-allelic patients. There are minimal decreases in sensitivity and specificity compared to an approach that applies two tests. However, it denotes a simpler approach in the clinical routine. [ABSTRACT FROM AUTHOR]