Elevated blood 17alpha-hydroxyprogesterone (17OHP) level, although widely used for the screening of classical 21-hydroxylase deficiency (21OHD) in neonates, has frequently been found in some neonates without classical 21OHD, particularly preterm neonates. We studied the diagnostic value of the metabolite of 21-deoxycortisol (pregnanetriolone, Ptl) and the metabolite of 17OHP (pregnanetriol, PT) in identifying 21OHD in term and preterm neonates with elevated blood 17OHP on the newborn screening. Spot urine samples from 59 classical 21OHD neonates (50 term, 9 preterm), 83 neonates without 21OHD having transiently elevated blood 17OHP (non-21OHD) (49 term, 34 preterm), and 62 control term neonates were studied using gas chromatography/mass spectrometry in selected ion monitoring analysis for Ptl, PT, 5beta-tetrahydrocortisone (betaTHE), and 5alpha-tetrahydrocortisone (alphaTHE). Ptl and Ptl/(betaTHE+alphaTHE) showed no overlap between 21OHD and non-21OHD, and 21OHD and controls, respectively (Ptl was 0.46-124 mg/g creatinine in 21OHD term, 0.80-26.9 mg/g creatinine in 21OHD preterm, < or = 0.08 mg/g creatinine in non-21OHD term, < or =0.06 mg/g creatinine in non-21OHD preterm, and < or = 0.07 mg/g creatinine in controls). PT and PT/(betaTHE+alphaTHE) showed significant overlap between 21OHD and non-21OHD. The above data indicate that spot urine Ptl is a highly specific marker of 21OHD with a cutoff value of 0.1 mg/g creatinine, yielding an unambiguous separation between 21OHD and non-21OHD in term and preterm neonates.