Answer
Phenytoin is strongly protein-bound. With low albumin, renal or hepatic disease, the active unbound fraction may be higher than the total concentration suggests, making a free level more informative.
Sources for this answer
- Pro-Epanutin Concentrate for Infusion / Solution for Injection — Current selected SmPCemc · Summary of Product Characteristics
- NHS Specialist Pharmacy ServiceNHS Specialist Pharmacy Service · NHS medicine guidance
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.