Study group

We included critically ill patients with vitamin D levels above 10 ng mL^-1 at admission, with respiratory and circulatory failure and acute kidney injury treated with continuous renal replacement therapy by continuous veno-venous haemodiafiltration (CVVHDF), which was started no later than 48 hours after admission. CVVHDF was performed in each patient using regional citrate anticoagulation, Prismaflex system, and an ST150 set (Prismaflex, Gambro Lundia AB, Lund, Sweden). The CVVHDF dose was set between 30 and 40 mL kg^-1 h^-1.

Patients who met any of the following criteria were excluded: acute liver failure, hypercalcaemia at admission (total calcium plasma level > 10.6 mg dL^-1, total ionised calcium plasma level > 1.35 mmoL L^-1), parathyroid gland disease at admission, serum vitamin D level < 10 ng mL^-1 at admission, end-stage renal disease, admission or readmission from another ICU, age under 18 years, or lack of consent from relatives. As in the control group, we established a cut-off value for serum vitamin D level of 10 ng mL^-1 as extremely low.

Vitamin D (25-hydroxy-vitamin D) was measured in exactly the same way for both groups. Blood samples were taken from an arterial line or central venous line, or by direct peripheral venous puncture, and were collected in ethylenediaminetetraacetic acid (EDTA) tubes. Blood samples were protected from exposure to light, transported to the hospital laboratory within 30 minutes, centrifuged at 3500 rpm for 10 minutes, and processed by laboratory technicians. The vitamin D serum level was measured using an electrochemiluminescence binding assay on Cobas e411 or Cobas 6000 immunoassay analysers (Roche Diagnostics GmbH, Mannheim, Germany). The coefficient of variation (the amount of variability relative to the mean) of the method was estimated to be 0.8–5.8%.

Consecutive patients admitted to the ICU were assessed in terms of study participation (inclusion and exclusion criteria). In the majority of patients, the first vitamin D serum level was measured at the time of admission to the ICU. If the first vitamin D serum level was higher than 10 ng mL^-1, the patient was included in the study. The first vitamin D measurements in the study group were performed before starting renal replacement therapies. The next set of vitamin D serum levels were taken in 12-hour time intervals (twice daily, at 6 a.m. and 6 p.m.). The minimum number of vitamin D measurements was four and the maximum was eight per patient [9].

All demographic data (date, name, hospital documentation number, sex, age, diagnosis at admission, comorbidities, Sequential Organ Failure Assessment Score [SOFA], and additional laboratory tests) were recorded in the hospital’s electronic database. After the recruitment process, patient data were extracted from the electronic database, the patient’s identification was blinded, and the data were transferred to the statistician for analysis [9].