Fosphenytoin Dilution Compatibility
Fosphenytoin can be diluted in both Normal Saline (NS) and 5% Dextrose solutions, but NOT in Dextrose in Normal Saline (DNS) combinations. 1
FDA-Approved Dilution Guidelines
The FDA label explicitly states that fosphenytoin should be diluted in either 5% Dextrose Injection OR 0.9% Sodium Chloride Injection (Normal Saline) to a concentration ranging from 1.5 to 25 mg PE/mL. 1
Specific Preparation Instructions
- Maximum concentration: 25 mg PE/mL in any solution 1
- Compatible diluents:
- Inspection required: Visually inspect for particulate matter and discoloration prior to administration 1
- Single-dose use only: Discard any unused product after opening 1
Critical Distinction from Phenytoin
Unlike phenytoin, which must be diluted ONLY in normal saline due to precipitation with glucose-containing solutions, fosphenytoin's superior water solubility allows compatibility with dextrose solutions. 2, 3, 4, 5
Why Fosphenytoin is Different
- Fosphenytoin has a pH of 8.6 compared to phenytoin's pH of 11-12, making it significantly more water-soluble 6, 5
- The formulation does not contain propylene glycol (40%) or ethanol (10%) that are present in phenytoin preparations 7, 5
- This improved solubility eliminates the precipitation issues seen with phenytoin in glucose-containing solutions 3, 4
Common Pitfall to Avoid
Do NOT confuse the vial concentration (50 mg PE/mL) with the total vial content. Fatal overdoses have occurred when clinicians misinterpreted the concentration to mean the total content was 50 mg PE, when vials actually contain 100 mg PE (2 mL vial) or 500 mg PE (10 mL vial). 1