© 2022 American Society for Parenteral and Enteral Nutrition.Background: Acute kidney injury (AKI) treated with continuous renal replacement therapy (CRRT) may deplete micronutrient levels. Patients are also at risk for micronutrient depletion due to underlying illness(s), poor nutrient intake prior to intensive care unit (ICU) admission and/or increased requirements. We determined vitamin and trace element status before, during and after CRRT in critically ill patients. Methods: This prospective observational study performed in mixed medical and surgical ICU patients. Serial serum vitamin B6 and vitamin C concentrations were measured by HPLC and folic acid by ECLIA. Serum chromium, copper, selenium, and zinc were measured using ICP-MS. Serum ceruloplasmin was measured by the Erel method. Results: Fifty adult ICU patients with AKI were recruited. The median APACHE II score on ICU admission was high at 24.0 (6.0-33.0). The median days on CRRT was 2.0 (2.0-4.0) days. At baseline (within 10-15 minutes of CRRT initiation), serum vitamin C, selenium and zinc were below normal. Serum vitamin B6 levels at 72 hours on CRRT were significantly lower than at 24 hours (p = 0.011). Serum vitamin C values fell significantly at 24 and 72 hours during CRRT (p = 0.030 and p = 0.001), respectively, and remained low 24 and 48 hours after CRRT was stopped (p = 0.021). At baseline and during CRRT, 96% of participants had at least two or more micronutrient levels below the normal range. Conclusion: Serum vitamin C, selenium and zinc concentrations were below the normal range at baseline. CRRT was associated with a significant further decrease in levels of vitamin C, selenium and zinc.