Calcium-PTH relationship in dialysis patients: the pitfalls of using a constant dialysate calcium concentration during the dynamic tests

Abstract

The calcium-PTH relationship in uremic patients has been often studied during dialysis sessions with high or low dialysate calcium concentration (CaD). This method has been used because it is less complex and invasive than i.v. infusion of calcium salts and calcium chelating agents. However, the constancy of CaD during the tests does not allow for the control of the serum calcium profile and, given that the blood calcium concentration is only one factor of a more complex calcium-related mechanism of the PTH release, the calcium-PTH curve may become dependent on the unpredictable rate at which the ionized calcium changes. Dynamic testing of the parathyroid gland was performed in 9 dialysis patients comparing constant CaD of 1.0 and 2.0 mmol/l (A) with a linear change in CaD (B). The rate of serum calcium change remained constant over time only in experiment B. The total decrease in calcemia (0-0.38 +/- 0.03 vs -0.14 +/- 0.1 mmol/l) and PTHmax (748.25 +/- 124.76 vs 374.89 +/- 53.03 pg/ml) were significantly higher in B, whereas the total increase in calcemia (+0.26 +/- 0.03 vs +0.28 +/- 0.02 mmol/l) and the minimum value of PTH (59.15 +/- 9.53 vs 55.64 +/- 9.08 pg/ml) were similar in both experiments. The calcium-PTH curves were clearly different in A and B. The setpoint and the slope were significantly higher in A (1.196 +/- 0.01 vs 1.142 +/- 0.02 mmol/l; 840.54 +/- 96.85 vs 542.43 +/- 112.26%/mmol). For similar serum calcium values (1.084 +/- 0.01 vs 1.059 +/- 0.02 in the stimulation test and 1.325 +/- 0.02 vs 1.336 +/- 0.02 mmol/l in the inhibition test) the PTH secretion was significantly different (335.86 +/- 44.36 vs 647.65 +/- 104.09 in the stimulation test and 76.35 +/- 12.57 vs 105.03 +/- 20.59 pg/ml in the inhibition test). In conclusion, the way of inducing serum calcium change affected the calcium-PTH curve and the value of the set point and the slope was a function of the way in which the blood calcium changes were achieved. The modulated CaD dialysis was shown to be a more correct method of studying the calcium-PTH relationship in dialysis patients, as well as an alternative to the more complex and invasive infusional methodology.