ObjectiveTo evaluate whether calcium adjusted for albumin can be used to monitor calcium homeostasis in critically ill patients.DesignProspective single-single center observational study.SettingClinical laboratory and critical care unit of a regional teaching hospital.PatientsFifty-three paired samples were from 36 patients requiring intensive care treatment.InterventionsNone.Measurements and Main ResultsTotal calcium, albumin-adjusted calcium, and ionized calcium were measured in critically ill patients during an 8-wk period. Calcium was adjusted for albumin using the formula that is most frequently used in The Netherlands. Using ionized calcium as the gold standard, albumin-adjusted calcium overestimated hypercalcemia and totally missed hypocalcemia. The same seemed to be true for other formulas used for albumin or protein adjustment of calcium concentrations.ConclusionsAlbumin-adjusted calcium cannot be used in an intensive care setting to monitor reliably the calcium levels in critically ill patients and should be replaced by measurement of ionized calcium.