Background and importance Vancomycin is used as fortified eye drops for the treatment of bacterial keratitis. Given the absence of an available equivalent speciality, the hospital pharmacy prepares these eye drops under aseptic conditions. Content uniformity is required before each batch is released.

Aim and objectives The objective of this study was the development and validation of a method for vancomycin eye drops dosage by ultraviolet-visible spectrophotometry.

Material and methods Analyses were performed at 280 nm. The method was validated according to the International Conference on Harmonisation (ICH) Q2(R1): specificity, linearity, repeatability, intermediate precision, accuracy, limit of detection, limit of quantification. Memory effect, vial equivalence and background noise were studied. Five standard solutions were performed from 0.1 to 0.4 mg/mL and a 200th dilution to analyse the samples. A relative standard deviation (RSD) of 5% was accepted for each of the criteria.

Results The method was specific. The equivalence of vials was demonstrated with a variation of 0.58%. The background noise measured variations up to 0.00097 mg/mL. Linearity was established with the equation y=4.6489x–0.0256 and R²=0.9975. RSD were 1.91% for repeatability and 4.65% for intermediate precision. The recovery rates varied between 98.5% and 101.9%. The limit of detection was 0.007 mg/mL and the limit of quantification was 0.021 mg/mL.

Conclusion and relevance When measuring accuracy, the prepared eye drops had a vancomycin concentration of 45 mg/mL and not 50 mg/mL as expected. After questioning our manufacturing protocol we questioned our supplier. The powder vials contained a quantity of vancomycin base equivalent to an antimicrobiological activity of 1 000 000 UI/vial. Legislation requires presentation in milligrams and UI. The amount of vancomycin base in a vial therefore varied between 850 mg and 950 mg depending on the initial content of active ingredient and was not 1 g as indicated.

The analytical method was validated. The method is suitable for routine use due to its speed and accuracy allowing a control before release of each batch of our eye drops.

Conflict of interest No conflict of interest