Several studies point at a potential relationship between vitamin D deficiency and worse outcome in critically ill patients admitted to the intensive care unit. It is linked with the lack of vitamin D pleiotropic effects in the state of hypovitaminosis D. The pleiotropism of vitamin D is dependent on a specific feature of vitamin D receptor (VDR) namely polymorphism and its universal existence in the human body. Vitamin D pleiotropism is linked with cancer cells inhibition, a modulation of the immune system, an influence on cardiovascular system and neuroprotection.

In 35-65% critically ill patients hospitalized in the intensive care unit the acute kidney injury (AKI) is diagnosed. Acute kidney injury increases significantly the probability of death. The standard therapy of a severe AKI in many intensive care units is the regional citrate anticoagulation continuous renal replacement therapy by means of continuous veno-venous hemodiafiltration (CVVHDF). The specificity of the regional citrate anticoagulation by means of precise ionized calcium and citrate dosing evokes questions regarding its influence on vitamin D and entire calcium-phosphate metabolism in the state of a severe AKI treated with regional citrate anticoagulation continuous renal replacement therapy.

The intention of that trial is to measure vitamin D plasma levels and other parameters (parathormone, ionized and total calcium, magnesium, phosphate, albumin, globulin) linked with calcium-phosphate metabolism in the human body. We would like to assess potential relationships between the regional citrate anticoagulation continuous renal replacement therapy and these parameters.

Inclusion Criteria:

• Respiratory, circulatory failure and acute kidney injury mechanically ventilated critically ill patients admitted to the critical care unit undergoing regional citrate anticoagulation continuous renal replacement therapy by means of continuous veno-venous hemodiafiltration (CVVHDF)

Exclusion Criteria:

• age less than 18 years
• acute liver failure
• hypercalcemia at admission (total calcium plasma level > 10.6 mg/dL; total ionized calcium plasma level > 1.35 mmol/L)
• parathyroid glands disease at admission
• serum vitamin D level < 10 ng/ml at admission
• end stage renal disease at admission
• lack of relatives consent

Acute Kidney Injury - CRRT group: Multi-organ failure with acute kidney injury critically ill patients admitted to the critical care unit undergoing regional citrate anticoagulation continuous renal replacement therapy by means of continuous veno-venous hemodiafiltration (CVVHDF). Multi-organ failure is defined as a respiratory, circulatory and renal failure.

Control group: Multi-organ failure non acute kidney injury critically ill patients admitted to the critical care unit. Multi-organ failure is defined as a respiratory and circulatory failure.

All patients: machanically ventilated.