Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. Is it acceptable for a clinical laboratory to calculate ionized (free) calcium if calcium ion-selective electrode is not available? Are results of calculated ionized (free) calcium of acceptable accuracy in clinical practice? And what is the recommended formula for performing this calculation?

A. July 2020—Total serum calcium exists in three distinct forms: approximately 10 percent bound to a variety of anions; approximately 40 percent bound to plasma proteins, mainly albumin; and approximately 50 percent in the ionized, or free, form, which is physiologically active.¹ Ionized calcium can be directly measured using calcium ion-selective electrode, which is considered the gold standard. However, this method is not readily available in many clinical laboratories. Instead, ionized calcium is sometimes measured using total calcium that has been adjusted for albumin and/or total protein.

Over several decades, many formulas have been proposed for performing this calculation, including Payne’s formula (adjusted calcium = calcium − albumin + 4.0).² However, numerous studies have shown that these formulas have poor accuracy in calculating ionized calcium in various clinical populations,^3-5 with some studies even calling for completely abandoning these correction formulas.^6-9 Given these findings and the potential for patient over- or undertreatment when using correction formulas, calculated ionized calcium is not of acceptable accuracy for use in clinical practice.

Furthermore, Payne, et al., investigated whether these formulas for adjusting total calcium for albumin could be transferred between laboratories and found the regressions on total calcium for albumin differed significantly between labs. Payne, et al., recommend that laboratories use internal data to derive their own adjustment formulas and monitor their validity over time.¹⁰

 

 

1. Klemm KM, Klein MJ. Biochemical markers of bone metabolism. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. Elsevier; 2017:188–190.
2. Payne RB, Little AJ, Williams RB, Milner JR. Interpretation of serum calcium in patients with abnormal serum proteins. Br Med J. 1973;4:643–646.
3. Ladenson JH, Lewis JW, Boyd JC. Failure of total calcium corrected for protein, albumin, and pH to correctly assess free calcium status. J Clin Endocrinol Metab. 1978;46:986–993.
4. Gøransson LG, Skadberg Ø, Bergrem H. Albumin-corrected calcium or ionized calcium in renal failure? What to measure? Nephrol Dial Transplant. 2005;20(10):2126–2129.
5. Smith JD, Wilson S, Schneider HG. Misclassification of calcium status based on albumin-adjusted calcium: studies in a tertiary hospital setting. Clin Chem. 2018;64(12):1713–1722.
6. Clase CM, Norman GL, Beecroft ML, Churchill DN. Albumin-corrected calcium and ionized calcium in stable haemodialysis patients. Nephrol Dial Transplant. 2000;15(11):1841–1846.
7. Slomp J, van der Voort PH, Gerritsen RT, Berk JA, Bakker AJ. Albumin-adjusted calcium is not suitable for diagnosis of hyper- and hypocalcemia in the critically ill. Crit Care Med. 2003;31(5):1389–1393.
8. Morton AR, Garland JS, Holden RM. Is the calcium correct? Measuring serum calcium in dialysis patients. Semin Dial. 2010;23(3):283–289.
9. Lian IA, Åsberg A. Should total calcium be adjusted for albumin? A retrospective observational study of laboratory data from central Norway. BMJ Open. 2018;8(4):e017703.
10. Barth JH, Fiddy JB, Payne RB. Adjustment of serum total calcium for albumin concentration: effects of non-linearity and of regression differences between laboratories. Ann Clin Biochem. 1996;33(1):55–58.

Matthew Thomas, MD
Anatomic and Clinical Pathology
Postgraduate Year Four
Robert J. Tomsich Pathology and Laboratory Medicine Institute
Cleveland Clinic
Junior Member, CAP Clinical Chemistry Committee

Q. Under checklist requirement COM.04250, “Comparability of Instruments and Methods—Nonwaived Testing,” what is the minimum number of samples that should be analyzed and which acceptance criteria should be used for the comparison? In addition, what parameters in the complete blood count do not apply for comparison purposes?

A. The intent of performing a comparison study of all nonwaived test methods testing the same analyte is to compare test results generated via different methodologies, instruments, or testing areas under a single CAP/CLIA number. There are no minimum number of samples required; however, the number of samples used should challenge the test system. Examples of samples that should be included are those from normal and abnormal results, positive and negative results, or samples that include various concentrations of the analyte being tested.

This requirement is not applicable to calculated parameters. For example, when performing CBC testing on a hematology analyzer, it is only necessary to compare the directly measured parameters. However, the laboratory may choose to compare all CBC parameters.

The comparison study must include acceptability criteria. Statistically defined acceptability limits should be used for quantitative assays. Some laboratories may choose to use published allowable total error limits, which may include percent or standard deviation limits. It is the laboratory director’s responsibility to approve the acceptance criteria used in his or her laboratory.