SERUM INTACT PARATHYROID-HORMONE AND IONIZED CALCIUM-CONCENTRATION IN CHILDREN WITH RENAL-INSUFFICIENCY

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 mu mol/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 be tween 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 pre dictive 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/1 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.