Why a normal total level can hide toxicity
Phenytoin is highly bound to serum albumin, and only the small free (unbound) fraction crosses into the brain and produces both the therapeutic effect and the toxic effect. The usual laboratory test measures the total level, which counts bound and free drug together. The therapeutic range of 10–20 mcg/mL assumes a normal albumin. In this client, albumin is only 2.2 g/dL after a month of poor intake. With fewer albumin binding sites, a much larger share of the drug circulates free, so a total level of 12 mcg/mL can correspond to a free level that is already in the toxic range.
The clinical picture confirms it
The level alone would look reassuring, but the client's findings point the other way. Horizontal nystagmus, an unsteady (ataxic) gait, and slurred speech (dysarthria) are the classic cerebellar signs of phenytoin toxicity. They tend to appear as the free concentration rises: nystagmus first, then ataxia and slurred speech, and later lethargy and confusion. When the numbers and the client disagree, the nurse trusts the clinical signs and asks why the number might be misleading. Low albumin is the explanation here, and it is also common in malnutrition, liver disease, kidney disease, older adults, and critical illness.
| Finding | Value | Meaning |
|---|
| Total phenytoin | 12 mcg/mL | Looks therapeutic |
| Serum albumin | 2.2 g/dL (normal 3.5–5.0) | Fewer binding sites; free drug rises |
| Nystagmus, ataxia, slurred speech | New | Classic signs of phenytoin toxicity |
Why the other conclusions are wrong
Concluding that the level is therapeutic and that another cause must be found ignores the albumin result and delays recognition of toxicity. Raising the dose because the level is "barely in range" would increase the free drug further and worsen the toxicity he already shows. Missed doses would lower the drug level and could explain his breakthrough seizures, but they do not produce nystagmus and ataxia; those signs come from too much drug, not too little.
Nursing actions
The nurse reports the findings before the next dose so the prescriber can review it. A free phenytoin level or an albumin-corrected value is obtained, and the dose is adjusted. Meanwhile the nurse puts fall precautions in place because of his ataxia, assists with walking, and continues seizure precautions. Nutrition support is also part of the plan, since poor intake caused the low albumin. Other teaching points for phenytoin include good oral hygiene for gingival hyperplasia, consistent timing of doses, and avoiding abrupt discontinuation.
Exam takeaway
Watch out A drug level "within range" does not rule out toxicity for a highly protein-bound drug when albumin is low. Interpret phenytoin levels together with albumin and the client's signs.