Congenital adrenal hyperplasia is suspected from the newborn’s external genitalia and confirmed by measuring the adrenal steroids.
Confirming the diagnosis
CAH should be suspected when there is a newborn with ambiguous or abnormal external genitalia, since in the affected female newborn androgen excess virilizes the external genitalia, and in any such newborn the diagnostic tests for CAH are mandatory.
Traditionally the laboratory tests in these neonates were a 24-hour urine collection to measure the urinary excretion of adrenal steroids and their metabolites; because that is impractical, serum assays for the adrenal steroids are now used instead. Because serum sampling is invasive in a newborn, a simple random urine test in the first days after birth is suggested as well.
In some countries there is a screening test for CAH, based on microfilter paper that detects 17-hydroxyprogesterone, the precursor that accumulates behind the enzyme block in 21-hydroxylase deficiency, from a random blood sample taken from the heel.
The gold standard for the hormonal diagnosis of CAH is the corticotropin (ACTH) stimulating test, in which the serum levels of 17-hydroxyprogesterone and Δ4-androstenedione are measured at baseline and 60 minutes after the injection of corticotropin.
In a patient strongly suspected of CAH, the hormonal diagnostic test should be done 48-72 hours after birth, because 17-hydroxyprogesterone is normally high in the first day of life and most infants in the first 24 hours therefore test positive even without the disease. While awaiting the result, glucocorticoid replacement therapy should be initiated.