Critical evaluation and guidance for using the Coulter method for counting subvisible particles in protein solutions.

In this paper, we evaluate the suitability of Coulter method for detecting and quantifying subvisible particles in protein solutions and compare results with other particle-counting technologies. The effects of key instrument and operational parameters such as aperture diameter, solution conductivity, and cleaning procedures are demonstrated. Degraded and nondegraded intravenous immunoglobulin and human serum albumin were chosen as model proteins and sample types for this evaluation. Multisizer™4 was able to obtain reproducible and linear particle counts; however, customized analysis and cleaning procedures are needed depending on the protein analyzed and the sample type (degraded or nondegraded). The Coulter method consistently detected more particles than micro-flow imaging and light obscuration. The presence of translucent particles likely accounts for this observation because detection by the Coulter method does not depend on the optical properties of the particles or solution.