Sampling schedule
| Serum concentration | Day 0 (day of enrolment) | Day 2 | Day 5 | Day of intensive care unit discharge |
| 25-hydroxyvitamin D | √ | – | √ | √ |
| 1,25-dihydroxyvitamin D | √ | √ | √ | √ |
| Calcium (Ca) | √ | √ | √ | √ |
| Phosphate (P) | √ | √ | √ | √ |
| Parathyroid hormone | √ | √ | √ | √ |
| Fibroblast growth factor 23 | √ | – | √ | √ |
| Vitamin D binding protein | √ | – | √ | √ |