Nephrol Dial Transplant (2000) 15: 1841-1846
© 2000 European Renal Association-European Dialysis and Transplant Association
Albumin-corrected calcium and ionized calcium in stable haemodialysis patients
1 Division of Nephrology, Dalhousie University, Halifax, NS, 2 Department of Clinical Epidemiology and Biostatistics and 3 Division of Nephrology, McMaster University, Hamilton, ON, Canada
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[Ca]), 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% taking alphacalcidol, 11 patients (22%) had ionized hypocalcaemia. To facilitate 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 normal range for each variable. Results are expressed as intraclass correlation coefficients (ICC: 0, all differences are due to error; 1, all differences are due to between patient variation).
Conclusions. None of the published formulae greatly improved the test characteristics beyond simply using the total calcium. A correction formula in widespread use (Payne), quoted in reference texts, agreed less well with ionized calcium than did the unadjusted measured calcium. Correction formulae should be abandoned in favour of the use of uncorrected calcium. In cases of doubt, ionized calcium should be directly measured.
Keywords: adjustment; albumin; calcium; correction; measurement; validation
Correspondence and offprint requests to: Dr Catherine M. Clase, Division of Nephrology, Room 5088 Dickson Building, 5820 University Avenue, Halifax, Nova Scotia B3H 1V8, Canada.
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