Low serum total calcium concentration as a marker of low serum ionized calcium concentration in critically ill patients receiving specialized nutrition support.

Background: The intent of this study was to ascertain to what extent serum total calcium concentration (tCa) <7 mg/dL reflects hypocalcemia (defined by ionized calcium concentration [iCa] of < or = 1.12 mmol/L) in critically ill patients receiving specialized nutrition support.

Methods: Adult patients (> or = 18 years) admitted to the trauma, surgical, medical, burn, or neurosurgical intensive care units, trauma stepdown unit, or progressive care unit and referred to the nutrition support service were retrospectively identified for potential inclusion into the study. Serum chemistries, arterial blood gas measurements, nutrition markers, and serum iCa were simultaneously obtained from each patient approximately 1 day after initiation of specialized nutrition support. Patients with a serum creatinine > or = 2 mg/dL, hyperphosphatemia (> or = 6 mg/dL), severe hypomagnesemia (< or = 1.12 mg/dL), history of metabolic bone disease, or parathyroid disease were excluded from the analysis.

Results: One hundred ninety-five patients (91% who had multiple trauma, with a mean Injury Severity Score 31 +/- 13) were enrolled into the study. Specialized nutrition support was initiated 2.8 +/- 1.8 days and calcium status was studied 4.2 +/- 3.1 days after hospital admission, respectively. The majority (28 of 33, or 85%) of patients with a tCa <7 mg/dL were hypocalcemic compared with 33% (22 out of 66) of patients with a tCa of 7-7.4 mg/dL, and 11% (11 of 96) of those with a tCa of 7.5-7.9 mg/dL (p < .001).

Conclusions: Critically ill patients with a serum total calcium concentration of <7 mg/dL have a high rate of hypocalcemia (iCa < or = 1.12 mmol/L). Hypocalcemia, defined as a serum iCa of < or = 1.12 mmol/L, occurs in 85% of acutely ill patients with a serum tCa <7 mg/dL.