The prevalence of Vitamin D deficiency in the critically ill is substantial. Vitamin D deficiency is associated with infection severity and increased cost in patients admitted to ICUs. Observational data in critically ill cohorts support the concept that Vitamin D deficiency can serve as a biomarker of illness severity and outcomes in critically ill patients.

Vitamin D deficiency may be a major contributor to common morbidities seen among critically ill patients, including the systemic inflammatory response syndrome, development and outcome of sepsis, multiorgan failure, and metabolic dysfunction. Importantly, Vitamin D deficiency has been associated with increased mortality in the critically ill.

A single random measurement may not be reflective of the vitamin D status in ICU patients because of changes with fluid administration.

Due to a decrease in the concentration of the Vitamin D Binding Protein during critical illness the measurement of total 25OH Vitamin D is no longer a good representation of the concentration of the free 25OH Vitamin D.

The total 25OH Vitamin D test underestimates the free fraction, with the risk of Vitamin D status misclassification.

Recent research has shown that the measurement of free 25OH Vitamin D during critical illness seems to be a better marker of Vitamin D deficiency than the current total 25OH Vitamin D test. Additional clinical studies are currently initiated in order to bring more evidences.