estimated creatinine clearance =
= A + B/C
where:
• A = 2 * R / ((creatinine at time t1) + (creatinine at time t2))
• B = 2 * V * ((creatinine at time t1) - (creatinine at time t2))
• C = ((time t2) - (time t1)) * ((creatinine at time t1) + (creatinine at time t2))
• V (expressed in liters) = total body water = 0.6 * (body weight)
where:
• V is the volume of distribution for creatinine in liters
R = production rate of creatinine in mg/day =
= ((140 - (age in years)) * (body weight in kg)) / K
where:
• K = 5 for males and 5.9 for females; this is equivalent to female = 0.85 * (male value)
• production rate per hour = (production rate in mg/day) / (24 hours per day)
Note: Be careful with units.
(1) in equations, creatinine values should be converted to mg per liter (mg/dL * 10) to maintain units
(2) clearance from equation will be in liters per hour; multiply by 16.67 to get mL per minute
Limitations:
• Estimates of V and R are difficult in obese or cachectic patients.
• If the 2 determinations are too close together, then the estimate for the creatinine clearance may be poor if not at steady state. For creatinine levels not obviously at steady state, about 2 half-lives is necessary for steady state to be reached and reflected in the second determination.
• This assumes that an event to affect renal function occurs followed by a new steady state. If renal function is unstable, then estimates of creatinine clearance will be inaccurate.
