SFEBES2022 Poster Presentations Bone and Calcium (40 abstracts)
Comparison of newly derived and existing formulae for the estimation of biologically active calcium
Ahmed Salih 1 , Maria Phylactou 1,2 , Alexander Comninos 1,2 , Marina Labib 1 , Pei Chia Eng 1,2 , Sophie Clarke 1,2 , Tricia Tan 1,2,3 , Pope Moore 3 , Jaimini Cegla 1,2,3 , Waljit Dhillo 1,2 & Ali Abbara 1,2
1Section of Endocrinology and Investigative Medicine, Imperial College London, London, United Kingdom; 2Department of Endocrinology, Imperial College Healthcare NHS Trust, London, United Kingdom; 3Department of Clinical Chemistry, Imperial College Healthcare NHS Trust, London, United Kingdom
Introduction: Free ionised calcium is the biologically active component of total circulating calcium but is infrequently measured due to sampling requirements. Therefore, total calcium is ‘adjusted’ for albumin to provide a closer estimate of the biologically active ionised calcium level (e.g. James, Orell, Payne and Berry formulae). Here, we aim to derive a novel equation to estimate ionised calcium from readily available biochemical parameters and compare its performance to existing equations.
Material and Methods: Derivation cohort: 2806 serum samples (total calcium) taken with blood gas samples (ionised calcium) collected at Imperial College Healthcare NHS Trust were used to derive equations to estimate ionised calcium using multivariable linear regression. Validation cohort: The performance of newly derived and existing equations of calcium to predict PTH in a cohort of 5510 patients was compared using spearman correlation and multivariable linear regression, adjusted for vitamin D level.
Results: Derivation cohort: Adjusted calcium (r2=0.269) was less strongly associated with ionised calcium, than total calcium (r2=0.314). A newly derived equation incorporating total calcium, albumin, potassium and haematocrit levels had an r2 of 0.327, and 0.364 when all available laboratory variables were included, which were superior to the best performing existing formula (James: r2=0.27). Validation cohort: Adjustment of total calcium by the Berry resulted in a higher calcium level, whereas adjustment by Orell resulted in a lower level. The relationship between calcium and PTH was strongest in the setting of hypercalcemia, with the James formula having the highest correlation coefficient (+0.495).
Discussion: Accurate determination of calcium status has important implications for interpretation of PTH levels and reaching the correct diagnosis especially in borderline cases. Adjustment of total calcium levels for albumin using current equations does not always outperform unadjusted total calcium levels in reflecting ionised calcium, especially in scenarios of extreme perturbations of physiology.
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Endocrine Abstracts
ISSN 1470-3947 (print) | ISSN 1479-6848 (online)
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