The aminoterminal propeptide of type I procollagen: Evaluation of a commercial radioimmunoassay kit and values in healthy subjects

Objectives: Evaluation of the performance of a radioimmunoassay kit for measuring serum concentrations of the aminoterminal extension peptide of human type I procollagen, S-PINP. Design and Methods: S-PINP concentrations in 229 healthy subjects, 140 females, aged 3.8–81 years, and 89 males, aged 0.9–71 years, were measured with the kit. Because PINP and PICP (the carboxy-terminal propeptide of type I procollagen) are formed in equimolar concentrations, we also calculated the PICP/PINP ratio and compared the S-PINP values to those of S-PICP, which have been shown to correlate with bone formation rate. Results: The sensitivity of the assay was 2.3 μg/L, the spiking recovery ranged from 95.5 to 100.3%, the dilution recovery from 79.3 to 103.1%. The intra-assay imprecision was 2.3 to 3.5% (CV), the interassay imprecision within one reagent lot 2.5–5.2% and, between several reagent lots, 2.7 to 6.1% (CV). S-PINP in females over 20 years old ranged from 12 to 90 μg/L (x = 39.7, SD = 14.7), in males over 25 years old, from 22 to 89 μg/L (x = 49.9, SD = 15.8); the PICP/PINP ratio ranged from 1.5 to 5.2 and from 1.8 to 4.9, respectively. In females under 20 years old, S-PINP ranged from 52 to 820 μg/L, in males aged 25 years or younger from 35 to 1404 μg/L; the PICP/PINP ratio was 0.44–2.3 and 0.38–2.8. In females under 20 years and males under 25 years, there was a significant negative correlation between S-PINP and age: r = −0.70, p < 0.001 for females, r = −0.52, p = 0.004 for males. In different groups of healthy subjects, the correlation of S-PINP and S-PICP was significant (r = 0.67–0.86, p < 0.001). Conclusion: The assay performance is good. The significant positive relationship between S-PINP and S-PICP suggests that S-PINP also reflects bone formation rate. Because the clearance of PINP is probably less sensitive to hormonal changes, PINP may prove to be superior to PICP as a marker of bone formation.