ABSTRACT
@#Introduction: Total calcium concentration is widely used to assess body calcium status although limited by many confounding factors. Thus, this study aimed to derive and internally validate an albumin-adjusted calcium equation for a selected Malaysian population. Method: This cross-sectional study involved 1011 adults at an emergency department of a tertiary hospital. Patients who had total calcium, ionised calcium and albumin measurements taken simultaneously were included. Derivation of the albumin-adjusted calcium equation was based on the adjustment equation obtained from the Association for Clinical Biochemistry and Laboratory Medicine 2015 position paper. Additionally, the equation was internally validated and compared with ionised calcium (gold standard) and the conventional Payne’s equation. Results: The newly derived equation = total calcium + 0.017 (41.35 – albumin). Internal validation exhibited the amount of shrinkage of 0.049. It tends to overestimate the adjusted calcium by a mean difference of 0.029 mmol/L compared to Payne’s equation. The comparison between Payne’s equation and the new equation with ionised calcium reclassified 402 and 486 patients, respectively into different calcium status. When both equations were compared, calcium status classification significantly differed in all and hypoalbuminaemic subjects by 90 and 16 patients, respectively. Conclusion: Locally derived albumin-adjusted calcium equation differed statistically in calcium status classification when compared to the Payne’s equation. However, to confirm this significance, the result must be compared to ionised calcium under strict, controlled preanalytical conditions. In terms of clinical significance, there was no difference in classification of calcium status between Payne’s and the new equation at medical decision limits.
ABSTRACT
@#Introduction: One commonly used equation which continues to be widely mentioned in text books and hence familiar to clinical people is total calcium + 0.02 (40 – albumin). This equation was derived using cresophthalein complexone and bromocresol green (BCG) methods for measuring serum total calcium and serum albumin respectively. However this equation maybe invalid when applied to calcium and albumin results generated by alternative assays. Hence we aim to derive an albumin-adjusted calcium equation specific to our laboratory’s total calcium and albumin methodologies. Materials and Methods: A total of 3,175 adult University Malaya Medical Centre (UMMC) patients deemed free of any calcium metabolism disorders were selected and divided into two groups for derivation and validation. Simple linear regression associating total calcium and albumin was constructed from the data in the derivation group. The new albumin-adjusted calcium equation was validated in the validation group. Differences in calcium status classification following adjustments based on existing and new albumin-adjusted calcium equation was compared in a 469 hypoalbuminaemic patients. Result: The new albumin adjusted calcium equation was: total calcium + 0.014 x (39-albumin). Of the 469 hypoalbuminemic patients, 78 were classified differently based on new equation. Based on the new equation, 55 normocalcemic patients were classified as hypocalcemic and 22 were classified as normocalcemic instead of hyperclacaemic. Conclusion: Based on the newly derived albuminadjusted calcium equation 17% of patients had different adjusted calcium classifications. This could potentially impact in the management. It is recommended that laboratories derive equations specific to their calcium/albumin methods and analytical platforms.
ABSTRACT
@#Introduction: Ionised calcium is a good prognostic and diagnostic tool as opposed to total calcium in critical patients but is not available in most central laboratories and non-intensive care units. To date, four equations to calculate ionised calcium in critical patients have been published. Objectives: (1) Evaluate the four published equations’ performance in estimating ionised calcium; (2) Determine the accuracy of calculated ionised and adjusted total calcium in classifying patients according to calcium states; and (3) Identify factors associated with hypocalcaemia in the critically ill population. Materials and methods: This is a cross-sectional study involving 281 critically ill patients aged 18-80 years of both genders in a Malaysian tertiary intensive care unit. Performance of the four equations was analysed using Bland-Altman difference plot and Passing Bablok regression analysis. Crosstabulation was conducted to assess classification accuracy. Mann-Whitney U or Pearson Chi-Square tests were performed to identify variables associated with hypocalcaemia. Results: Calculated ionised calcium using all four equations significantly overestimated ionised calcium. Calculated ionised and adjusted total calcium had poor accuracies in classifying hypocalcaemic patients. pH was significantly higher in hypocalcaemics. Conclusion: Calculated ionised and adjusted total calcium significantly overestimate ionised calcium in the critically ill. In this specific population, calcium status should only be confirmed with ionised calcium measured by direct ion-selective electrode (ISE).
ABSTRACT
Background: Most of the laboratories use previously published regression equations for estimation of calcium which may not fit for their population. So deriving locally a regression equation for albumin-adjusted calcium (CaAd) is a mainstay to avoid population-based differences. Aims & Objective: To derive regression equation for albumin-adjusted calcium in our laboratory and validate it for the local population. Material and Methods: Total 575 normal healthy individuals of 35-65 years were included in the present study and were estimated for serum total calcium (CaT), ionized calcium (Ca2+), and albumin. The linear regression equation for the binding of calcium and albumin was derived in a cohort of 450 normal healthy individuals of 35-65 years, and the albumin-adjusted calcium equation was internally validated in a separate cohort of 125 subjects. The performance of this equation was compared with a previously published equation: CaAd (mmol/L) = CaT (mmol/L) + 0.02 (40 - [albumin] (g/L). Results: The local adjustment equation obtained from the derivation subset was expressed by the relationship; CaAd (mmol/L) = CaT (mmol/L) + 0.03 (37.33 – [albumin] (g/L)). The equation was internally validated with an adjusted r2 shrinkage value of 0.0009 in a validation subset. Bland-Altman plot showed statistically significant difference (Mean = 0.13 mmol/L) when both formulae were compared for the population. Conclusion: A locally derived and internally validated albumin-adjusted calcium equation differed significantly from previously published equations. Individual laboratories should determine their own linear albumin-adjusted regression equation for calcium rather than relying on published formulas.