A 73-yr-old woman with atrial fibrillation presented to the emergency room complaining of chest discomfort, dyspnea, and agitation. The patient had suffered from atrial fibrillation for the last 2 yr. Her medications for the previous 2 yr included diltiazem 90 mg, metoprorol 25 mg, and aspirin 100 mg twice a day. In addition, she had been taking mirtazapine 30 mg, alprazolam 0.25 mg, and zolpidem 10 mg daily for a 6 yr history of dysthymia.
Physical examination showed a blood pressure of 100/83 mmHg, a heart rate of 100 beats/min, and a normal body temperature. Her body weight was 62.3 kg, and she was 158 cm in height. Initial laboratory results were unremarkable with a normal PT of 12.3 sec (INR 1.0). Routine chemical studies indicated normal values for total protein (6.6 g/dL), albumin (3.9 g/dL), aspartate aminotransferase (17 U/L), alanine aminotransferase (10 U/L), alkaline phosphatase (79 U/L), total bilirubin (0.3 mg/dL) blood urea nitrogen (16.4 mg/dL), creatinine (0.6 mg/dL), and electrolytes. Results of hematological studies including complete blood counts were also normal. Mild cardiomegaly was found on the chest radiography. Electrocardiogram showed atrial fibrillation. Echocardiogram revealed mild aortic, mitral, and tricuspid regurgitation and left atrial enlargement (45 mm).
The patient underwent heparinization with an initial bolus administration of 3,600 IU heparin followed by continuous infusion for 6 days. Then the patient was started on 2 or 3 mg of warfarin daily (16.3 mg/week) to prevent thromboembolism, and was discharged after 3 days with a PT of 37.4 sec (INR 2.81). When she revisited the outpatient clinic after 3 days, her PT was found to be 50.0 sec (INR 3.68). After normalization of the patient's PT with warfarin withdrawal for 3 days, 1 or 2 mg/day (10.5 mg/week) of warfarin was prescribed. The PT after 3 days of treatment was 41.6 sec (INR 3.11). The administration of warfarin dose was changed (6.5-10.5 mg/week) continuously, which resulted in a PT of 20.9-43.1 sec (INR 1.64-3.21). She had to discontinue warfarin therapy temporarily 3 times. The difficulty in finding the optimal treatment dose during the induction period of anticoagulation continued for more than 2 month.
The patient continues to receive warfarin therapy at the outpatient clinic monthly and her PT remains within the recommended target therapeutic range (INR 2.0-3.0), even though her warfarin dose has been reduced to 6.5 mg/week.
The patient was genotyped for CYP2C9 after informed consent. DNA was isolated from peripheral blood, and all 9 exons of the CYP2C9 gene were amplified by PCR (-). The PCR products were sequenced using the ABI PRISIM BigDye terminator Cycle sequencing kit and an ABI Prism 3100 Genetic Analyzer (Applied Biosystems, Foster City, CA, U.S.A.). The patient was found to be an intermediate metabolizer with the genotype of CYP2C9*3/*4 ().
The formula for computing Cockcroft-Gault is given by CrCl = ((140 - age) * adjusted weight * (gender_coefficient)) / (serum creatinine * 72), where the gender_coefficient is 1 if male, and 0.85 if female. The serum creatinine concentration is in mg/dL.
The patient's gender is female, which means that the gender coefficient is 0.85.
The patient is 73 years old. 
The concentration of creatinine is 0.6 mg/dL. 
The formula for computing the patient's BMI is (weight)/(height * height), where weight is the patient's weight in kg and height is the patient's height in m.
The patient's height is 158.0 cm, which is 158.0 cm * 1 m / 100 cm = 1.58 m. The patient's weight is 62.3 kg. The patient's bmi is therefore 62.3 kg / (1.58 m * 1.58 m) = 24.956 kg/m^2.The patient's BMI is 25.0, indicating they are overweight/obese.
Because the patient is overweight/obese, we use the adjusted body weight formula to get the adjusted weight used for Cockroft-Gault Equation. Shown below is the computation for IBW (ideal body weight).
The patient's gender is Female.
The patient's height is 158.0 cm, which is 158.0 cm * 0.393701 in/cm = 62.205 in. 
For females, the ideal body weight (IBW) is calculated as follows:
IBW = 45.5 kg + 2.3 kg * (height (in inches) - 60)
Plugging in the values gives us 45.5 kg + 2.3 kg * (62.205 (in inches) - 60) = 50.571 kg.
Hence, the patient's IBW is 50.571 kg.Shown below is the computation for ABW (adjusted body weight).
To compute the ABW value, apply the following formula: ABW = IBW + 0.4 * (weight (in kg) - IBW (in kg)). ABW = 50.571 kg + 0.4 * (62.3 kg  - 50.571 kg) = 55.263 kg. The patient's adjusted body weight is 55.263 kg.

Using the Cockcroft-Gault equation:
CrCl = ((140 - age) * adjusted weight * gender_coefficient) / (serum creatinine * 72).
Plugging the patient's values gives us ((140 - 73) * 55.263 * 0.85) / (0.6 * 72) = 72.852 mL/min. Hence, the patient's creatinine clearance is 72.852 mL/min.