['To evaluate whether calcium adjusted for albumin can be used to monitor calcium homeostasis in critically ill patients.', 'Prospective single-single center observational study.', 'Clinical laboratory and critical care unit of a regional teaching hospital.', 'Fifty-three paired samples were from 36 patients requiring intensive care treatment.', 'None.', 'Total 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.'] Is albumin-adjusted calcium suitable for diagnosis of hyper- and hypocalcemia in the critically ill?