An Adjusted phenytoin level will be calculated where serum albumin levels are less than 40g/L. This is because as albumin levels fall the proportion of active “free” drug is increased, and the reported level therefore requires adjustment to take this effect into account. This adjusted level is a more accurate reflection of pharmacologically active phenytoin than the measured level if albumin is less than 40g/L.

Other factors may also affect phenytoin binding – and caution should be used when interpreting levels in the following patient groups:

- Patients with impaired renal function (seek pharmacy advice where creatinine clearance is less than 25ml/min).
- Patients requiring renal replacement therapy need their adjusted phenytoin levels further corrected (seek advice from a specialist clinical pharmacist).
- Patients prescribed concomitant highly protein-bound medicines (eg sodium valproate).
- Pregnant women (seek specialist advice).
- Children (seek specialist advice).

Other factors to consider when interpreting phenytoin levels include:
- The time the sample was taken in relation to the dose (for routine sampling a trough level is recommended).
- Time for phenytoin levels to reach steady-state (takes 5 to 14 days for full effect of dose changes).
- Route of administration.