Linezolid Serum Concentration Variability Among Critically Ill Patients Based on Renal Function and Continuous Renal Replacement Therapy Administration

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Background: Linezolid standard dosing is typically applied in ICU without adjustments, even in renal impairment. This study examines serum concentration variability by renal function or CRRT administration in patients receiving 1200 mg/day of linezolid. Methods: This retrospective, single-center, non-randomized observational study was conducted at Niguarda Hospital (Milan, Italy) on data from the two-year period 2023–2024. ICU patients receiving linezolid, with a renal function determination and trough TDM performed at steady-state were included and stratified by renal function or CRRT status. Results: 54 patients were included, with 18 (33%) undergoing CRRT (CVVH). CRRT patients presented higher median linezolid concentrations (4.6 mg/L) than non-CRRT patients (3.2 mg/L), and a lower risk of underdosing (17% vs. 39%). CRRT patients showed significantly lower concentrations (4.6 mg/L vs. 10 mg/L, p = 0.007) than non-CRRT patients with renal function ≤ 30 mL/min, with fewer out-of-range levels (39% vs. 91%, p = 0.008) and overdosing (22% vs. 73%, p = 0.018). A significant inverse correlation was found between renal function and linezolid levels (Spearman’s rho = −0.61, p < 0.001), with CRRT patients exhibiting concentrations comparable to those of individuals with moderately impaired renal function. Continuous infusion resulted in significantly higher median concentrations (7.2 mg/L) than extended infusion (2.7 mg/L), with an increased risk of overdosing (47% vs. 17%; p = 0.018). Conclusions: After standard-dosing administration, linezolid levels vary widely in critically ill patients. Renal function significantly affects pharmacokinetics: severe impairment increases overdose risk, while ARC may cause underdosing. Standard-dosing appears adequate in CRRT patients, with levels similar to moderate-impairment. Continuous infusion aids target attainment in normal or ARC patients but raises overdose risk in severe impairment. TDM-based personalized dosing seems crucial to optimize therapy and reduce toxicity or failure.

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