Abstract for presentation at Australia and New Zealand Society of Nephrology Annual Scientific Conference

Serum phosphate is an important determinant of corrected serum calcium in end-stage renal disease

  • Richard Singer, Fremantle Hospital, University of Western Australia, Australia
  • Mary Anne Townsend, Fremantle Hospital, University of Western Australia, Australia
  • Paul Chub, Fremantle Hospital, University of Western Australia, Australia
  • Paolo Ferrari, Fremantle Hospital, University of Western Australia, Australia

    • Approximately 20% of the total bound blood calcium is linked to various anions including phosphate. In patients with end-stage renal disease (ESRD) the range of variation of serum phosphate is significantly larger than in healthy subjects. We propose that establishing a formula to calculate albumin- and phosphate-corrected total calcium would be more appropriate to estimate biologically active free calcium in ESRD patients.
    In 47 haemodialysis patients serum ionised calcium (Caion) and pH were measured by blood gas analyser with ion-selective electrodes at the point-of-care, while bicarbonate, phosphate, albumin, magnesium, total calcium (Catot) were measured at the central laboratory. Linear regression analysis of measured variables was used to best fit adjusted calcium (Caadj) versus Caion.
    The most parsimonious multiple linear regression model (R2=0.915) of variables associated with Caion included Catot (coeff 0.802, P<0.0001), albumin (coeff -0.14, P<0.0001) and phosphate (coeff -0.046, P<0.004), while pH, bicarbonate and magnesium did not have a significant effect. Modelling of available variables yielded the following equation to adjust calcium for albumin and phosphate:
    Caadj = Catot+(0.02x(40-[albumin])+(0.07x(1.5-[phosphate])).
    Assuming a serum Catot of 2.4mmol/l and albumin of 40g/l, the Caadj would equate to 2.44mmol/l with a serum phosphate of 1.0mmol/l, 2.37mmol/l with phosphate 2.0mmol/l and 2.30mmol/l with phosphate 3.0mmol/l. These differences could influence the prescription of phosphate binders or active vitamin D.
    In conclusion, because Guidelines recommendations indicate that corrected serum calcium should be maintained within the normal range in dialysis patients, inclusion of phosphate to correct total serum calcium in ESRD patients would have relevant clinical implications.

    Conference Organiser - ICMS Pty Ltd