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
Albumin, Direct ISE, Free calcium, Ionic calcium, oCPC method, Total calcium
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.
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