What is known and objective Thrombocytopenia is one of the typical adverse events caused by linezolid (LZD). Recently, some cases of severe hyponatraemia occurring while receiving LZD have been reported. This study investigated a possible relationship between LZD‐induced hyponatraemia and thrombocytopenia and identified the risk factors for hyponatraemia and/or thrombocytopenia. Methods In this retrospective, single‐centre, observational cohort study, 63 hospitalized patients aged over 18 years who received intravenous injection of LZD for more than seven consecutive days in Oita University Hospital between April 2015 and March 2018 were analysed. Results Thrombocytopenia occurred in 25 (39.7%) patients and hyponatraemia in 11 (17.5%) patients. Seven of 11 patients with hyponatraemia had concurrent thrombocytopenia. Although both serum sodium level and platelet count declined in most patients who developed hyponatraemia, no significant association between thrombocytopenia and hyponatraemia was found. Creatinine clearance level (Ccr) was significantly lower not only in the thrombocytopenia (vs no‐thrombocytopenia) but also in the hyponatraemia group (vs no‐hyponatraemia group). Univariate and multivariate logistic regression analyses identified different risk factors for thrombocytopenia and/or hyponatraemia (thrombocytopenia: Ccr and administration period; hyponatraemia: serum albumin; thrombocytopenia and hyponatraemia: administration period and serum albumin). What is new and conclusion In conclusion, this study found no significant relationship between LZD‐induced thrombocytopenia and hyponatraemia and identified some possible risk factors associated with onset of the two adverse events. These require further validation.

Linezolid is an oxazolidinone antibiotic against Gram-positive bacteria. Although thrombocytopenia is a major adverse effect of linezolid, hyponatremia also often develops after linezolid administration. This study examined the frequency of hyponatremia that developed during linezolid treatment and identified its risk factors. In this retrospective, single-center, observational cohort study, 61 hospitalized patients treated with linezolid between January 2013 and January 2015 were analyzed. Hyponatremia was defined as a sodium level of ≤134 mEq/L for the duration of linezolid treatment. Its risk factors were identified via a logistic regression analysis. Hyponatremia occurred in 11 (18.0%) patients, and it was severe in a case (a sodium level of ≤128 mEq/L). Univariate and multiple logistic regression analyses identified the plasma C-reactive protein (CRP) level before the initial administration of linezolid and the concomitant use of a potassium-sparing diuretic as the independent variables associated with the development of hyponatremia. The odds ratios were 1.081 (95% confidence interval [CI]; 1.008-1.158) (p 0.028) and 11.017 (95% CI; 1.869-64.939) (p 0.008), respectively. Before linezolid treatment, the CRP levels of the hyponatremia group were significantly higher than those of the no-hyponatremia group (p<0.001). The frequency of hyponatremia development was significantly higher in the patients who received both the potassium-sparing diuretic and linezolid ( p 0.016). These results suggest that the plasma sodium levels of patients with severe inflammation who are treated with linezolid and those of linezolid-treated patients co-administered a potassium-sparing diuretic should be continuously monitored.