M.T., a 75-year-old, white 60kg IBW woman with a serum creatinine of 1.8 mg/dL,
ID: 122551 • Letter: M
Question
M.T., a 75-year-old, white 60kg IBW woman with a serum creatinine of 1.8 mg/dL, has atrial fibrillation. A decision has been made to use digoxin for heart failure. The target concentration of digoxin for the treatment of atrial fibrillation is 0.5 ng/mL to less than 1 ng/mL, but her current level is high at 2 ng/mL.
What is her estimated creatinine clearance using the Cockcroft/Gault method? GFR using the MDRD method?
Assuming a half-life of four days, how long will it take for M.T. to achieve a 1 ng/mL level? A 0.5 ng/mL level?
Explanation / Answer
Cockcroft/Gault method:
Creatinine clearance = Sex * ((140 - Age) / (SerumCreat)) * (Weight / 72)
= 0.85 X ((140-75)/1.8) X (60/72)
= 25.58 mL/min
MDRD Equation:
GFR (mL/min/1.73 m2) = 186 × (Scr)-1.154 × (Age)-0.203 × (0.742 if female) × (1.212 if African American)
= 186 X 0.507 X 0.416 X 0.742
= 29.1 mL/min/1.73 m2
Elapsed time= half life X [(log (beginning concentration/end concentration)) / log2]
1. For 1ng/mL:
Elapsed time= 4 X [(log (2/1)) / log2]
= 4 days
2. For 0.5 ng/mL:
Elapsed time= 4 X [(log (2/0.5)) / log2]
= 8days
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