This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Why total phenytoin can mislead
Phenytoin is an anti-seizure drug with a narrow window between too little and too much. In the blood, about 90% of it is attached to albumin and only about 10% is free. Only the free drug crosses into the brain, prevents seizures and causes side effects. Laboratories usually measure the total level, which assumes the normal 90/10 split.
When albumin is low, for example in malnutrition, liver disease, burns, pregnancy or critical illness, a larger share of the drug is free. A total level of 8 µg/mL may then carry as much free drug as a level of 13 in a patient with normal albumin. Severe kidney failure also reduces binding.
The Sheiner-Tozer equation
Corrected level = measured level ÷ (0.2 × albumin + 0.1)
with albumin in g/dL. In patients with a creatinine clearance below 10 mL/min or on dialysis:
Corrected level = measured level ÷ (0.1 × albumin + 0.1)
At a normal albumin of 4.5 g/dL, the divisor is 1.0 and no correction is made.
Example
A measured total phenytoin of 8 µg/mL with albumin of 2.5 g/dL:
- Divisor = 0.2 × 2.5 + 0.1 = 0.6
- Corrected level = 8 ÷ 0.6 = 13.3 µg/mL
The uncorrected value looks low, but the corrected value is within the usual 10 to 20 range. Increasing the dose based on the total level alone could cause toxicity.
Reading the result
| Corrected level (µg/mL) | Usual meaning |
|---|---|
| Below 10 | Below the usual range; may be enough for some patients |
| 10 to 20 | Usual therapeutic range |
| 20 to 30 | Nystagmus and unsteadiness are common |
| Above 30 | Toxic range; drowsiness and confusion above 40 |
Limits
The equation was derived in 1978 from a small number of patients. Later studies, particularly in intensive care and in older adults, found that it can be quite inaccurate. Other drugs, such as valproate, also displace phenytoin from albumin. A directly measured free phenytoin level is the most reliable test; the usual free range is 1 to 2 µg/mL.