Parameters:
(1) serum albumin in g/dL
(2) serum total phenytoin concentration in mg/L
estimated total serum phenytoin in mg/L if serum albumin were normal =
= (measured total serum phenytoin in mg/L) / ((0.2 * (serum albumin in g/dL)) + 0.1)
where:
• The reference serum albumin is 4.4 g/dL.
• 0.1 indicates that normally 10% of serum phenytoin is not protein bound (unbound or free).
• ((0.2 * (serum albumin in g/dL)) is derived from (1 - (unbound fraction)) * (serum albumin) / 4.4
Anderson et al modified the equation based on an analysis of elderly nursing home patients with hypoalbuminemia:
estimated total serum phenytoin in mg/L if serum albumin were normal =
= (measured total serum phenytoin in mg/L) / ((0.25 * (serum albumin in g/dL)) + 0.1)
Limitations -The prediction may be suboptimum if:
(1) the patients is taking one or more medications that would displace phenytoin from albumin
(2) the patient has renal failure
(3) the patient is in the hospital or has severe comorbid conditions
In these situations direct measurement of the free phenytoin concentration should be performed.
