Rational development and validation of a new microbiological assay for linezolid and its measurement uncertainty

The aim of this work was to develop and validate a new microbiological assay to determine potency of linezolid in injectable solution. 24 factorial and central composite designs were used to optimize the microbiological assay conditions. In addition, we estimated the measurement uncertainty based on residual error of analysis of variance of inhibition zone diameters. Optimized conditions employed 4 mL of antibiotic 1 medium inoculated with 1% of Staphylococcus aureus suspension, and linezolid in concentrations from 25 to 100 µg mL−1. The method was specific, linear (Y=10.03X+5.00 and Y=9.20X+6.53, r2=0.9950 and 0.9987, for standard and sample curves, respectively), accurate (mean recovery=102.7%), precise (repeatability=2.0% and intermediate precision=1.9%) and robust. Microbiological assay׳s overall uncertainty (3.1%) was comparable to those obtained for other microbiological assays (1.7–7.1%) and for determination of linezolid by spectrophotometry (2.1%) and reverse-phase ultra-performance liquid chromatography (RP-UPLC) (2.5%). Therefore, it is an acceptable alternative method for the routine quality control of linezolid in injectable solution.