Albumin-corrected calcium and ionized calcium in stable haemodialysis patients

Background. It is ionized calcium that is physiologically active and under homeostatic control; however, total calcium is more conveniently measured. Formulae for correction of calcium to account for albumin binding have not been validated in a dialysis setting. Methods. We measured ionized calcium simultaneously with total calcium (t[C a]), albumin, total protein and pH before dialysis in 50 stable outpatients and convalescent inpatients. Results. Although 92% of patients were taking calcium supplements and 70% t aking alphacalcidol, 11 patients (22%) had ionized hypocalcaemia. To facili tate comparison of calculated ionized calcium, measured total calcium (t[Ca ]), and 'corrected' calcium (c[Ca]), with the criterion measure of ionized calcium, all measurements were converted to z scores, standardized on the n ormal range for each variable. Results are expressed as intraclass correlat ion coefficients (ICC: 0, all differences are due to error; 1, all differen ces are due to between patient variation). Conclusions. None of the published formulae greatly improved the test chara cteristics beyond simply using the total calcium. A correction formula in w idespread use (Payne), quoted in reference texts, agreed less well with ion ized calcium than did the unadjusted measured calcium. Correction formulae should be abandoned in favour of the use of uncorrected calcium. In cases o f doubt, ionized calcium should be directly measured.