We are all too well aware of the adverse effects of particulate matter contamination within parenteral injectable pharmaceutical products and the consequences for patient safety. Here, particulate matter refers to the small, sub visible particles. The United States Pharmacopoeia, USP <788> provides two tests for detecting such particulates: light obscuration and microscopic assay. Both are generally accepted for use in testing large volume parenteral (LVP) and small volume parenteral (SVP) for the determination of sub visible particulate matter. Normally, samples are first tested by the light obscuration method. If the sample fails the specified limits, the microscopic assay method can then be used.