Question

A 65 kg, 34-year-old epileptic patient is receiving 300 mg phenytoin daily for epilepsy. Her albumin is 2.4 g/dL and creatinine clearance is 79 ml/min. After three weeks of therapy, the total phenytoin concentration is 8.5 mg/L. What is the recommended new phenytoin dose to achieve a therapeutic concentration of 10 - 20 mg/L?

Answer 1

Answer :)

Albumin to Creatinine ratio normally is around 30.

Calculate creatinine amount in the urine by the following formula:

Creatinine Clearance = ((140 – age) x weight)/ (72 x serumCr)) x 0.85

Age = 34 years

Weight = 65 Kg

Creatinine Clearance = 79 ml/min

Creatinine Clearance = 79/100 dl/min

Creatinine Clearance = 0.79 dl/min

0.79 = ((140 – 34) x 65)/ (72 x serumCr)) x 0.85

0.79 = ((6890)/ (72 x serumCr)) x 0.85

0.79 = ((95.7)/ (serumCr)) x 0.85

serumCr = (95.7/ 0.79) x 0.85

serumCr = 103 mg/dL

The normal range of serum creatinine is 1.2 mg/dL.

The respected proportion to reduce the creatinine content = 103/1.2

The respected proportion to reduce the creatinine content = 85.8 times

The required phenytoin dose = 300 x 85.8

The required phenytoin dose = 25740 mg for three weeks

Daily dose = 25740 /21

Daily dose = 1226 mg approximately.

However, the maximum therapeutic concentration of phenytoin in the blood is 20 mg/dL.

Initial concentration of phenytoin in blood = 8.5 mg/dL

The proportion of phenytoin to increased = 20/8.5

The proportion of phenytoin to increased = 2.35

Therefore, maximum recommended dose = 300 x 2.35 mg

Therefore, maximum recommended dose = 705 mg

Therefore, the patient should be given a 705 mg phenytoin dose.