Effect of Protein Concentration and Binding on Antibiotic Assays

Assay curves, using a disk diffusion method for the antibiotics gentamicin and cefazolin, were prepared with: saline, saline plus 10% serum, and ascitic, synovial, cerebrospinal, and pleural fluids. The curves were compared with a standard curve prepared with pooled human serum. The pH, total protein, glucose, blood urea nitrogen, sodium, potassium, calcium, phosphorus, chloride, CO₂ content, uric acid, cholesterol, bilirubin, serum glutamic oxalacetic transaminase, CPK, LDH, and alkaline phosphatase were determined and compared for all fluids. Measurements for cefazolin levels were falsely elevated in those fluids with low protein content when serum was used as a reference standard. There was a linear inverse relationship between the protein content of the fluids and the cefazolin level with serum as the standard for the assay of this highly protein-bound antibiotic. No discrepancies were observed in the assay curves for gentamicin, an antibiotic known not to be bound by serum proteins.