2025 Southeastern Residency Conference

Title: Does corrected calcium adequately reflect calcium levels in critically ill patients?


Authors: Allison Krueger, Caitlin Thomas


Objective: Review methods to assess patients’ calcium status and whether corrected calcium accurately represents the status of critical care patients.


Self Assessment Question: Which of the following is TRUE regarding the original Payne corrected calcium equation?


Background: Abnormalities in calcium status are common among critically ill patients, and disturbances in calcium status have been linked with increased mortality and morbidity. Accurate representation of calcium status is key in managing patients in the intensive care unit (ICU). There are two main ways of measuring calcium in a blood sample: total serum calcium (totCa) and ionized calcium (measurement of unbound calcium). Since only approximately half of the serum calcium is biologically active under normal conditions, equations were developed to estimate that value using serum calcium levels before labs were capable of directly testing ionized calcium. The most well know adjustment is the modified Payne equation that “corrects” totCa from reduced albumin levels. A formula by Pftizenmeyer et al. in 2007 was designed to “correct” totCa for very elderly patients in a facility that does not utilize iCa. The purpose of this study is to evaluate the accuracy of albumin corrected calcium and total serum calcium compared to ionized calcium at discerning calcium homeostasis in patients requiring critical care.


Methods: This is a single-center, retrospective cohort study that was deemed exempt from Institutional Review Board approval. It was conducted in a large, tertiary level, community teaching hospital with patients across seven adult ICUs. The electronic medical record was reviewed for inclusion in the study. Patients were included if the following labs were collected with 10 minutes of each other: ionized calcium, serum calcium, and serum albumin. Patients were excluded if they received albumin within the 24 hours prior to the lab collection and/or received intravenous calcium within the 12 hours prior to lab collection. The primary outcome is to assess total calcium and modified Payne corrected calcium for noninferiority to ionized calcium.  


Results: A total of 25 patients were included in this study. The mean levels for iCa, totCa, and corCa were 1.12 mmol/L, 8.4 mg/dL, and 9.5 mg/dL respectively. There was a statistical difference between the three mean levels (F = 11.35, p < 0.001). There was a statistical difference between the three methods at categorizing calcium status (Q = 7.98, p = 0.0185). Total calcium was found to be the outlier. Bland-Altman analysis of totCa shows a mean difference of 0.13 mmol/L (95% CI -0.06 – 0.32). Bland-Altman analysis of corCa shows a mean difference of 0.01 mmol/L (95% CI -0.15 – 0.17).


Conclusion: Total calcium was inferior to iCa when it comes to assessing patient calcium status. This study found that corCa compared to iCa was noninferior at predicting normocalcemia. Furthermore, the Bland-Altman analysis shows that, when comparing methods for getting an accurate value, both total calcium and corrected calcium have too wide of variations to accurately rely on them.