Dilantin false high reading

I had a relative in the hospital who was malnourished, and his blood work gave a false reading that his dilantin levels were too high.  Is this a common phenomenon or did I misunderstand what i was told?

[Godofredo MD]

Hi.

I really am not sure whether you did misundertand what you were told or not. However, analytical errors happen and it would not be impossible for one to occur during measurements of plasma phenytoin concentrations.

Factors affecting serum albumin concentrations, such as malnutrition, renal failure, or burn patients, or concurrently administering medications with a high affinity for albumin, can have an extreme effect on the free phenytoin concentration, thus leading to levels which would be determined as high.

There are also many drugs which increase or decrease phenytoin levels or which phenytoin may affect. The most commonly occurring drug interactions are listed below (this information is from www.rxlist.com):

- Drugs which may increase phenytoin serum levels include: acute alcohol intake, amiodarone, chloramphenicol, chlordiazepoxide, cimetidine, diazepam, dicumarol, disulfiram, estrogens, ethosuximide, fluoxetine, H2-antagonists, halothane, isoniazid, methylphenidate, phenothiazines, phenylbutazone, salicylates, succinimides, sulfonamides, ticlopidine, tolbutamide, trazodone.

- Drugs which may decrease phenytoin levels include: carbamazepine, chronic alcohol abuse, reserpine, and sucralfate. Moban® brand of molindone hydrochloride contains calcium ions which interfere with the absorption of phenytoin. Ingestion times of phenytoin and antacid preparations containing calcium should be staggered in patients with low serum phenytoin levels to prevent absorption problems.

- Drugs which may either increase or decrease phenytoin serum levels include: phenobarbital, sodium valproate, and valproic acid. Similarly, the effect of phenytoin on phenobarbital, valproic acid, and sodium valproate serum levels is unpredictable.

- Although not a true drug interaction, tricyclic antidepressants may precipitate seizures in susceptible patients and phenytoin dosage may need to be adjusted.

- Drugs whose efficacy is impaired by phenytoin include: corticosteroids, coumarin anticoagulants, digitoxin, doxycycline, estrogens, furosemide, oral contraceptives, paroxetine, quinidine, rifampin, theophylline, vitamin D.

Hope this answers your query.

[sjwctncy]

Is there a reliable calculation to work out the free or unbound phenytoin levels if you have low albumin.  So, for example if a standard blood test showed Phenytoin 64.6umol/L and the albumin was 30g/L, can you predict accurately whether the unbound or free levels are in the therapeutic range?  Would other factors like creatinine be considered?
Many thanks