Background: Supplementation of calcium during continuous venovenous hemofiltration (CVVH) with citrate anticoagulation is usually titrated using a target blood ionized calcium concentration. Plasma calcium concentrations may be normal despite substantial calcium loss, by mobilization of calcium from the skeleton. Aim of our study is to develop an equation to calculate CVVH calcium and to retrospectively calculate CVVH calcium balance in a cohort of ICU-patients. Methods: This is a single-center retrospective observational cohort study. In a subcohort of patients, all calcium excretion measurements in patients treated with citrate CVVH were randomly divided into a development set (n = 324 in 42 patients) and a validation set (n = 441 in 42 different patients). Using mixed linear models, we developed an equation to calculate calcium excretion from routinely available parameters. We retrospectively calculated calcium balance in 788 patients treated with citrate CVVH between 2014 and 2021. Results: Calcium excretion (mmol/24 h) was - 1.2877 + 0.646*[Ca](blood,total) * ultrafiltrate (l/24 h) + 0.107*blood flow (ml/h). The mean error of the estimation was - 1.0 +/- 6.7 mmol/24 h, the mean absolute error was 4.8 +/- 4.8 mmol/24 h. Calculated calcium excretion was 105.8 +/- 19.3 mmol/24 h. Mean daily CVVH calcium balance was - 12.0 +/- 20.0 mmol/24 h. Mean cumulative calcium balance ranged from - 3687 to 448 mmol. Conclusion: During citrate CVVH, calcium balance was negative in most patients, despite supplementation of calcium based on plasma ionized calcium levels. This may contribute to demineralization of the skeleton. We propose that calcium supplementation should be based on both plasma ionized calcium and a simple calculation of calcium excretion by CVVH.[GRAPHICS].Afdeling Klinische Chemie en Laboratoriumgeneeskunde (AKCL
