Possible parathyroid calcium-sensing set point shift in patients undergoing maintenance hemodialysis.

Dear Editor, In chronic kidney disease–mineral and bone disorder (CKD–MBD), monitoring blood biochemical parameters is recommended, and albumin-corrected total serum calcium concentration (cCa) is generally used as a parameter of blood calcium [1]. However, several factors in the blood make cCa unreliable to indicate the physiological activity of blood calcium. We found six maintenance hemodialysis (MHD) patients who had hypocalcemia of cCa <8.4 mg/dL repeatedly while having plasma intact parathyroid hormone concentration (iPTH) not over the recommended range [2]. For them, calcimimetics and vitamin D analogues had not been administered. Since cCa might not have indicated the physiological activity of blood calcium, correlations between cCa, plasma ionized calcium concentration (iCa), and iPTH were examined in them. Correlations were analyzed by Pearson's correlation test in the SPSS software package (IBM Corp., Armonk, NY). Table 1 shows the characteristics and biochemical parameters of the patients. The age and dialysis vintage ranged 59–75 years and 6–92 months, respectively. Calcium bicarbonate was administered to five patients, and lanthanum bicarbonate was also administered to two of them. In one, three, and two patients, iPTH (normal range [NR]: 10–65 pg/mL) was 10–65, 65–130 and 130–260 pg/mL, respectively. In the simultaneous measurement, cCa (NR: 8.4–10.1 mg/dL), iP (NR: 3.5–5.5 mg/dL), and iCa (NR: 1.16–1.30 mmol/L) ranged 7.04–8.22 mg/dL, 3.4–7.2 mg/dL, and 0.89–1.11 mmol/L, respectively. While cCa and iCa correlated with each other (r = 0.90, p = 0.016), neither cCa nor iCa correlated with iPTH significantly (p = 0.14 and 0.09, respectively). The positive correlation between cCa and iCa suggests that cCa could indicate the physiological activity of blood calcium and be a parameter of CKD–MBD in them. While two patients had hyperphosphatemia of >5.5 mg/dL, hyperphosphatemia is an iPTH-elevating factor and would not have influenced the finding of iPTH not over the recommended range in them [3]. Thus, the finding suggests that some HD patients have iPTH not over the recommended range under hypocalcemia. The iPTH not over the recommended range under hypocalcemia would be resulted from a shift of the serum-calcium-sensing set point to a hypocalcemic state in their parathyroid. Since MHD patients with chronic hypocalcemia will have a steeper iCa increase during each dialysis session compared