Which laboratory value should be monitored during phenytoin therapy?

• A. Albumin levels
• B. White blood cell count
• C. Platelet count
• D. Sodium

Answer: (A)

Phenytoin binds very strongly to albumin, so the amount of active, free drug depends on how much albumin is circulating. Because of this, the total phenytoin level can be misleading if albumin levels are abnormal. In conditions that lower albumin (like liver disease, malnutrition, or certain illnesses), the free, active fraction of phenytoin can rise even if the total level looks in range, increasing the risk of toxicity. For this reason, albumin levels are monitored to accurately interpret phenytoin therapy and, when needed, clinicians assess the free phenytoin concentration or use a correction method to estimate the active drug.

The other listed labs aren’t the primary parameter for guiding phenytoin dosing. White blood cell count and platelet count can be affected by phenytoin in some patients, but they’re not used to monitor drug exposure routinely. Sodium can be influenced by various factors, but it doesn’t specifically reflect phenytoin activity or toxicity risk the way albumin does.