Procollagen type I N-terminal propeptide (PINP) as an indicator of type I collagen metabolism: ELISA development, reference interval, and hypovitaminosis D induced hyperparathyroidism.

A sandwich enzyme-linked immunosorbent assay (ELISA) for quantification of the N-terminal propeptide of human procollagen type I (PINP) utilizing purified alpha 1-chain specific rabbit antibodies is described. The ELISA measured the content of the alpha 1-chain of PINP independent of the molecular form of the molecule. A parallelism was found between amniotic fluid (calibrator), normal and patient serum, and purified PINP (alpha 1), as well as the high and low molecular weight forms of PINP (alpha 1). The concentration of PINP in the calibrator (second trimester amniotic fluid) was determined to 25 micrograms/mL and the detection limit was 62 pg/mL measured in amniotic fluid, and 41 pg/mL measured in serum. The interassay coefficients of variation were 4.6% (low control) and 5.3% (high control), and the corresponding intraassay parameters were 2.9% and 4.9%. Recovery studies revealed an accuracy between 93% and 105%. The normal range (n = 57) for PINP was 56 ng/mL (median) the 10th and 90th centiles being 30 and 82 ng/mL, respectively. Patients with hyperparathyroidism due to hypovitaminosis D had median serum level of 168 ng/mL with a 10th centile of 44 ng/mL and a 90th centile of 450 ng/mL, these values being significantly different from the normal range (p < 0.001). The PINP-ELISA was superior to commercially available assays for PICP and osteocalcin in separation between healthy controls and patients with osteomalaci.