Background and Importance Both prospective and retrospective trials and case reports suggest that therapeutic drug monitoring (TDM) of linezolid may be useful, especially in situations when there’s a potential alteration of its pharmacokinetics or an increased risk of adverse events (AE); obesity, renal failure, drug interactions or prolonged treatments.

Aim and Objectives To assess effectiveness and safety of linezolid in SOI regarding linezolid serum concentrations (LSC) and analyse the influence of glomerular filtration rate (GFR) and body mass index (BMI) in LSC.

Material and Methods Observational retrospective study including patients with SOI treated with linezolid between January/2019 and December/2021.

Demographic, prescription and clinical data were collected from hospital´s medical records. Creatinine clearance was estimated by the Cockcroft-Gault formula.

Quantification of linezolid was performed by HPLC-UV. Therapeutic target trough concentrations were settled at 2–8 mg/L.

We studied the relationship among GFR and BMI with LSC using a multivariate regression analysis with IBM SPSS® Statistics program

Results Forty-two patients (mean age 58.7 ± 16.1, 69.1% male) were included. All patients received linezolid 600mg q12h orally as initial dose. The median duration of treatment was 34.2 ± 17.4 days. No relevant drug interactions were observed.

Twenty-two patients (52.4%) had LSC outside therapeutic range (TR): 10(45.5%) above and 12(54.5%) below TR. In only 3(18.7%) patients with supratherapeutical LSC posology was modified. All infections (including ones in patients with LSC below TR) were resolved.

AE occurred in 16(38.1%) patients, 7(43.8%) over TR. Eight of them (50%) discontinued treatment due to AE (50% diarrhea, 62.5% glossitis, 25% thrombocytopenia, 12.5% anaemia).

Seven (16.6%) patients had GFR<60 ml/min, of which 4(57.1%) were over TR. Seventeen (40%) patients had a BMI>30, of which 5(29.4%) had linezolid determinations outside the TR: 3(60%) below TR. It was not found a significant correlation between BMI and LSC (p=0.34), whereas a significant inverse correlation was found between GFR and LSC (p=0.01).

Conclusion and Relevance A great proportion of patients were outside the TR, and the variable that seems to affect the most is GFR (p=0.01), so TDM would be specially recommended in patients with a lower GFR to decrease AE, which occur frequently with high LSC. Effectiveness was demonstrated in all patients including the ones with LSC below TR.

Conflict of Interest No conflict of interest