Objective

In newborn screening programs for CAH, 17--hydroxyprogesterone (17OHP) cut-off levels are based on birth weight or on gestational age. We investigated which approach would result in the greatest specificity and sensitivity.

Study design

For the determination of 17OHP, the AutoDELFIA Neonatal 17OHP assay was used in filter paper blood of 9,492 newborns. The relationships between 17OHP and birth weight and between 17OHP and gestational age were studied by regression analysis. Reference curves with a specificity of 99.95% were constructed with the LMS method. Median cut-off levels for birth weight and for gestational age according to the 99.95% reference curves were calculated

Results

Regression analysis showed that gestational age is a better predictor of 17OHP than birth weight (R² was 50.6% vs. 35.8%, respectively). At a specificity of 99.95%, the calculated median 17OHP cut-off level was lower for gestational age (12.6 µg/L (38 nmol/L)) than for birth weight (17.6 µg/L (54 nmol/L)) thus leading to a greater sensitivity.

Conclusion

This study demonstrates that gestational age is a better predictor of 17OHP in newborns and will result in greater specificity than birth weight despite the fact that the determination of gestational age might be less reliable than birth weight.