Patient selection: serum albumin >= 480 µmol/L (>= 3.2 g/dL)
free phenytoin in µmol/L =
= (83.1 * (total phenytoin in µmol/L) / (serum albumin in µmol/L)) - 0.6
where:
• The molecular weight of phenytoin is 252.27 grams. The conversion from mg/L or µg/mL to µmol/L is 3.96.
• The molecular weight of albumin is 66,460 grams. The conversion for g/dL to µmol/L is 150.466.
• The authors gave the equation in conventional units. The ratio in this equation is multiplied by 0.55; this can be reached by dividing 83.1 by 150.466 for albumin. The conversion factors for phenytoin cancel out. The coefficient in the equation is 0.14, which is approximately 0.6 divided by 3.96 (0.6 divided by 3.96 is 0.15; 0.6 may be 0.55 rounded up).
free phenytoin in mgl/L =
= (0.55 * (total phenytoin in mg/L) / (serum albumin in g/dL)) - 0.14
Limitations:
• The estimate is inaccurate when a patient is taking other drugs that may displace phenytoin from serum albumin.
• Hypoalbuminemia (serum albumin < 480 µmol/L or < 3.2 g/dL) results in inaccuracies in the estimate.
