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Albumin-corrected calcium and ionized calcium in stable haemodialysis patients
C M Clase1, G L Norman, M L Beecroft
1Division of Nephrology, Dalhousie University, Halifax, NS, Canada.
Insights
Total calcium measurement is not a reliable indicator of ionized calcium levels in dialysis patients. Published correction formulas do not improve accuracy; direct ionized calcium measurement is recommended when results are uncertain.
Area of Science:
- Nephrology
- Clinical Chemistry
Background:
- Ionized calcium is the physiologically active form, but total calcium is routinely measured.
- Existing formulas to correct total calcium for albumin binding lack validation in dialysis patients.
Purpose of the Study:
- To evaluate the accuracy of corrected calcium formulas compared to measured ionized calcium in dialysis patients.
- To determine if standard correction formulas improve calcium assessment in this population.
Main Methods:
- Simultaneous measurement of ionized calcium, total calcium, albumin, total protein, and pH before dialysis.
- Study included 50 stable outpatients and convalescent inpatients undergoing dialysis.
- Measurements were standardized using z scores for comparison.
Main Results:
- Despite calcium supplementation, 22% of patients had ionized hypocalcemia.
- Published correction formulas showed poor agreement with measured ionized calcium.
- The Payne formula, commonly used, performed worse than uncorrected total calcium.
Conclusions:
- Correction formulas for calcium in dialysis patients are not recommended.
- Uncorrected total calcium measurements are more reliable than corrected values.
- Direct measurement of ionized calcium is advised for accurate assessment in dialysis 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[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.