['Case reports of catastrophic neurological sequelae during ESIs have questioned the safety of this procedure. A proposed mechanism is particulate steroid embolization resulting in neuralischemia. Previous reports have described steroid clumping in common epidural injection mixtures. We demonstrate that physiologic medium can also modify aggregation.', 'To inspect and compare aggregative properties of steroid preparations with and without human serum.', 'Academic tertiary care center.', 'Particulate steroids display different aggregation characteristics in serum compared to non-physiologic solutions.', 'Solutions were inspected under light microscopy: betamethasone sodium phosphate/betamethasone acetate, methylprednisolone, and dexamethasone were each mixed in lidocaine 1%, bupivacaine 0.5%, or sterile water in a 1:1 ratio. All preparations were inspected under light microscopy with 100x and 400x magnifications by a pathologist blinded to our expectations and hypothesis. Five random viewing fields were selected within each slide and the number of aggregates per field and the number of particles per aggregate was evaluated.', 'The addition of serum had a significant effect on steroid particle aggregation and number of particles per aggregate.'] Does human Serum modify Aggregation Properties of Commonly Used Epidural Steroids?