Serum intact parathyroid hormone and ionised calcium concentration in children with renal insufficiency

Abstract

We report our experience of the use of an immunoradiometric assay for intact parathyroid hormone (i-PTH) and the measurement of plasma ionised calcium concentration (PCa²⁺) in 73 children with chronic renal insufficiency (CRI); plasma creatinine concentration (PCr) 52–856 μmol/l.

There was a poor correlation between i-PTH and PCr (r=0.10,n=552) compared with that for C-terminal PTH and PCr (r=0.60,n=248), suggesting that the i-PTH assay is independent of renal function in this group of treated children. A clear response of i-PTH to a low total plasma Ca (tPCa) and PCa²⁺ was observed. There was a significant positive correlation between both tPCa and PCa²⁺ (r=0.50,n=389) and the fraction of Ca²⁺ (the fraction of tCa which was ionised) and PCa²⁺ (r=0.50,n=389). The finding of a low or normal PCa²⁺ with a low calculated fraction of Ca²⁺ was frequently observed, i. e. the measured tPCa was unexpectedly high, suggesting complexing of Ca²⁺ by accumulated anions in CRI. There was a poor relationship between the plasma albumin concentration and both bound plus complexed Ca (tPCa minus PCa²⁺) and the fraction of Ca²⁺ (r=0.15 and −0.17, respectively). The positive predictive value for a raised i-PTH of a tubular reabsorbtion of phosphate of less than 80% was 0.87, and of an alkaline phosphatase greater than 800 U/l was 0.37. We conclude that the immunoradiometric assay for i-PTH should replace indirect assessments of secondary hyperparathyroidism, and that measurement of the Ca²⁺ concentration is necessary to evaluate the true Ca status in children with CRI.