Comparison between Serum Calcium Levels Measured Using Direct Ion-selective Electrodes and Photometric Method in Automated Analyzers
Parul Arora, Sudip K Datta, Ashish D Upadhyay
Direct ISE, Free calcium, Ionic calcium, oCPC method, Total calcium,Albumin
Citation Information :
Arora P, Datta SK, Upadhyay AD. Comparison between Serum Calcium Levels Measured Using Direct Ion-selective Electrodes and Photometric Method in Automated Analyzers. Indian J Med Biochem 2020; 24 (1):12-15.
Introduction: Serum calcium is measured by photometric methods or ion-selective electrodes (ISEs). The ISEs measure free ionized calcium (FCa), which is not bound to proteins like albumin and is corrected using algorithms to calculate the total calcium, TCa (TCa_calc). The TCa obtained by photometry (TCa_meas) requires correction for albumin by several formulae to obtain the corrected Ca (TCa_corr).
Aims and objectives: In this study, we aim to find the agreement between TCa levels calculated from direct ISE results (TCa_calc) and TCa levels obtained by spectrophotometric methods after correction using formulae given in the literature (TCa_corr) at different levels of serum albumin.
Materials and methods: In this study, 332 serum samples were analyzed for TCa and albumin on Roche Modular P800 and FCa by direct ISE on XI-921 (Caretium) and converted to TCa_calc. The results of TCa_calc and TCa_corr were compared using paired t test.
Results: Significant difference was observed between TCa_calc (2.45 ± 0.34 mmol/L) and TCa_meas (2.07 ± 0.27 mmol/L). The TCa_meas was corrected for albumin using several commonly used formulae. However, significant differences still existed between TCa_calc and TCa_corr. The cases were further subdivided into three groups on the basis of serum albumin; however, significant differences were observed between TCa_calc and TCa_corr values in all subgroups.
Conclusion: Caution should be exercised while interchangeable usage and interpretation of serum calcium levels from direct ISE vis-à-vis photometric methods.
Clinical significance: With the infiltration of point-of-care devices in casualties and intensive care units, awareness needs to be created among clinicians regarding the potential misinterpretations of the tests involved. Regulatory guidelines to the same effect may also be considered.
Orrell DH. Albumin as an aid to the interpretation of serum calcium. Clin Chim Acta Int J Clin Chem 1971;35(2):483–489. DOI: 10.1016/0009-8981(71)90224-5.
Payne RB, Little AJ, Williams RB, et al. Interpretation of serum calcium in patients with abnormal serum proteins. Br Med J 1973;4(5893):643–646. DOI: 10.1136/bmj.4.5893.643.
Berry EM, Gupta MM, Turner SJ, et al. Variation in plasma calcium with induced changes in plasma specific gravity, total protein, and albumin. Br Med J 1973;4(5893):640–643. DOI: 10.1136/bmj.4.5893.640.
Clase CM, Norman GL, Beecroft ML, et al. Albumin-corrected calcium and ionized calcium in stable haemodialysis patients. Nephrol Dial Transplant 2000;15(11):1841–1846. DOI: 10.1093/ndt/15.11.1841.
Ladenson JH. Calcium determination in primary hyperparathyroidism. J Bone Miner Res 1991;6(Suppl 2):S33–S41. DOI: 10.1002/jbmr.5650061411.
Gauci C, Moranne O, Fouqueray B, et al. Pitfalls of measuring total blood calcium in patients with CKD. J Am Soc Nephrol 2008;19(8):1592–1598. DOI: 10.1681/ASN.2007040449.
Gøransson LG, Skadberg Ø, Bergrem H. Albumin-corrected or ionized calcium in renal failure? what to measure? Nephrol Dial Transplant 2005;20(10):2126–2129. DOI: 10.1093/ndt/gfh988.
Björkman MP, Sorva AJ, Tilvis RS. Calculated serum calcium is an insufficient surrogate for measured ionized calcium. Arch Gerontol Geriatr 2009;49(3):348–350. DOI: 10.1016/j.archger.2008.11.014.
Mir A, Goyal B, Datta S, et al. Comparison between measured and calculated free calcium values at different serum albumin concentrations. J Lab Physicians 2016;8(2):71. DOI: 10.4103/0974-2727.180785.
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(6):986–993. DOI: 10.1210/jcem-46- 6-986.
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. DOI: 10.1136/bmjopen-2017-017703.
Jassam N, Gopaul S, McShane P, et al. Calcium adjustment equations in neonates and children. Ann Clin Biochem 2012;49(Pt 4):352–358. DOI: 10.1258/acb.2011.011060.
Jain A, Bhayana S, Vlasschaert M, et al. A formula to predict corrected calcium in haemodialysis patients. Nephrol Dial Transplant 2008;23(9):2884–2888. DOI: 10.1093/ndt/gfn186.
Gouri A, Dekaken A. A comparison of corrected serum calcium levels to ionized calcium levels in haemodialysis patients. Ann Biol Clin 2012;70(2):210–212. DOI: 10.1684/abc.2012.0693.
Larsson L, Ohman S. Serum ionized calcium and corrected total calcium in borderline hyperparathyroidism. Clin Chem 1978;24(11):1962–1965. DOI: 10.1093/clinchem/24.11.1962.
Dickerson R, Alexander K, Minard G, et al. Accuracy of methods to estimate ionized and “corrected” serum calcium concentrations in critically ill multiple trauma patients receiving specialized nutrition support. J Parenter Enter Nutr 2004;28(3):133–141. DOI: 10.1177/0148607104028003133.
Boink AB, Buckley BM, Christiansen TF, et al. Recommendation on sampling, transport, and storage for the determination of the concentration of ionized calcium in whole blood, plasma, and serum. IFC scientific division, working group on ion-selective electrodes (WGSE). J Int Fed Clin Chem 1992;4(4):147–152.
Peduruhewa S, Warawita D. Pit falls in interpretation of electrolyte results. Sri Lanka J Diabetes Endocrinol Metab 2012;2:118–120. DOI: 10.4038/sjdem.v2i2.4784.