Low Phenytoin Level in a Patient Receiving Nasogastric Tube Feeding
A 58-year-old man with generalized tonic-clonic seizures is admitted with aspiration pneumonia. A nasogastric tube is placed for feeding and continuous enteral formula is running at 60 mL/h. His home phenytoin dose, 300 mg by mouth daily as an oral suspension, is continued through the tube. He has had no seizure activity in hospital. On day 4 the team asks whether the phenytoin dose should be increased. Labs: total phenytoin 6.2 mg/dL (reference 10–20 mg/dL), albumin 24 g/L, creatinine 88 micromol/L, ALT 30 U/L, normal electrolytes. He weighs 78 kg. He has no history of liver disease, and no interacting drugs have been started other than the antibiotics.
1. Calculate the albumin-corrected phenytoin concentration and interpret it. Is the level truly subtherapeutic?
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- Correction formula (Sheiner–Tozer): corrected phenytoin (mg/L) = measured total concentration ÷ [(0.2 × albumin in g/dL) + 0.1].
- Convert units first: albumin 24 g/L = 2.4 g/dL.
- Calculation: 0.2 × 2.4 = 0.48; 0.48 + 0.1 = 0.58; 6.2 ÷ 0.58 = about 10.7 mg/L (10.7 mg/dL).
- Interpretation: the corrected level is at the very bottom of the reference range, not clearly subtherapeutic. His hypoalbuminemia (common in acute illness) lowers total drug without necessarily lowering free drug, so the raw value of 6.2 overstates the deficit and invites an inappropriate dose increase.
- Best next step: measure free (unbound) phenytoin where available. Free phenytoin is the pharmacologically active fraction and is the most reliable guide in hypoalbuminemia and uremia. A free level in the 1–2 mg/L range is therapeutic.
- Why the distinction matters: phenytoin follows saturable (Michaelis–Menten) kinetics. If the true free level is already adequate, a dose increase can produce a disproportionate, nonlinear jump in concentration and toxicity (nystagmus, ataxia, sedation).
2. Identify the non-pharmacokinetic reason the level may be low and describe how you would modify administration. What would you do if the patient cannot take enteral medication?
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- The likely cause is an absorption interaction, not simply an inadequate dose. Enteral formulas bind phenytoin to protein and to the feeding tube, reducing oral bioavailability. Continuous feeding at 60 mL/h is the classic scenario. Phenytoin suspension also settles and adsorbs to plastic tubing.
- Practical modification in adults: hold enteral feeding for 1–2 hours before and 1–2 hours after each dose, flush the tube with at least 30–60 mL of water before and after the dose, and shake the suspension well. Some centres switch to a twice-daily schedule to concentrate the dosing windows. Recheck a free (or corrected total) level 5–7 days after the change, since phenytoin has a long, dose-dependent half-life.
- Check tube placement and residual volume to exclude malabsorption or a displaced tube.
- If enteral therapy is unreliable, or the patient is NPO with an ileus: switch to IV fosphenytoin, dosed in phenytoin sodium equivalents (PE). The oral-to-IV conversion is approximately 1:1 in PE, but because bioavailability of the enteral dose was likely poor, do not blindly assume the full 300 mg is needed intravenously - use a loading dose if a level is low with clinical concern, then maintenance guided by levels.
- Avoid IM phenytoin (erratic absorption, tissue damage) and avoid rapid IV push (hypotension, arrhythmia); maximum IV rate is 50 mg PE/min for fosphenytoin with cardiac monitoring.
3. Write the monitoring and safety plan you would propose to the team, including what you would tell the patient about signs of toxicity.
Read the worked answer
- Levels: obtain a free phenytoin level now if available; otherwise use the corrected total (~10.7 mg/dL). Recheck 5–7 days after any change in administration technique or dose, and after discharge until stable on oral therapy.
- Clinical monitoring: seizure diary, and each shift assess for nystagmus, ataxia, slurred speech, sedation, and confusion - the earliest signs of phenytoin toxicity. Check a CBC and liver enzymes periodically; phenytoin is a known cause of hepatotoxicity, rash, and (rarely) agranulocytosis and Stevens–Johnson syndrome.
- Patient counselling (with teach-back, since he is recovering from illness): take the dose at the same times each day, shake the bottle, pause the feed as instructed, do not double up on missed doses, and report rash, fever, sore throat, unsteadiness, or return of seizures. Reinforce that the feeding schedule is part of the treatment, not just nutrition.
- Discharge planning: the enteral feeding interaction resolves when he resumes oral intake - recalculate the dose and recheck a level after the transition, because the same 300 mg may now produce higher concentrations. Also confirm he is not resuming any interacting drug at home (e.g., isoniazid, fluconazole, carbamazepine) and recheck albumin as his illness resolves, since the corrected level shifts with albumin.