Congenital Adrenal Hyperplasia

Vijayakumar Madhava
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Congenital Adrenal Hyperplasia - Diagnosis
Estimation of 17- hydroxyl progesterone
Elevated 17 OHP level is the gold standard test of 21- hydroxylase deficiency. Both basal and stimulated levels are markedly elevated in salt loosing ad simple virilizing forms. Basal levels are usually > 2000 ng/dL and they increase to > 5000 – 10,000 ng/d L after ACTH. In milder late onset variety, basal levels may be mildly elevated but the stimulated values will be high (1500 – 10,000 ng/ dL). Ideal time of measurement of 17- OHP is before 8.00 AM
17 OHP level in a neonate
17 OHP levels decease in utero as the gestational age advances. Hence its levels are high in a preterm. Levels are also higher in SGA babies and babies under stress (birth asphyxia, cardiac or pulmonary disease) resulting in false positive results. Levels are high in cord blood (1000 – 3000 ng/ dL) . After birth, 17-OHP levels fall dramatically in the first 48 hours and reach a baseline value. Hence a false high value may be obtained if the measurement is done before 48 hours.

Usually cortisol response will be subnormal to ACTH stimulation. Basal plasma ACTH levels will be very high in classic forms of CAH but may be normal or have mild elevation in milder forms. Urinary excretion of 17- ketosteroids will be elevated, but this test is rarely used now a days. While doing this test 24- hour collection is a must and concomitant measurement of creatinine excretion should be done.

Plasma renin activity is an immunoassay of the amount of angiotensin 1 generated per ml of serum per hour at 370C. Usually PRA is detected twice, once in the morning after overnight supine position and again after maintenance of upright posture for 4 hours. Levels of PRA will be high in children with CAH. Elevated PRA and a reduced ratio of aldosterone to PRA indicates impaired aldosterone synthesis and can differentiate salt wasting variety from simple virilizing forms of CAH.

CAH is ideal for newborn screening because this condition is relatively common and unless picked up in the new born period and effectively managed, many succumb without a proper diagnosis. Girs may be picked up because of their atypical genitalia.
Measurement of 17 – hydoxy progesterone (17-OHP) level is the screening tool used in newborn screening programs. 17- OHP levels are normally high at birth. Its level falls during the first few days after birth in normal babies but progressively increases in children with CAH. Hence accuracy is poor in the first few days. Preterm, sick babies, SGA and stressed neonates have higher 17-OHP levels and may give false positive results. Hence the screening test is usually done after 48 hours. Once a positive result is obtained, it should be confirmed by a second assay.

In places where facilities are available, CYP21A2 mutations can be detected in DNA extracted from the dried blood spots in the neonatal period itself.


References
Congenital Adrenal Hyperplasia Congenital Adrenal Hyperplasia 03/18/2016
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