Creatinine Clearance Calculation - ASK DIS

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Tuesday, March 1, 2016

Creatinine Clearance Calculation

Cockcroft and Gault eq utilizing BW(Total body weight)
Cockcroft and Gault equation: CrCl = [(140 - age) x TBW] / (Scr x 72) (x 0.85 for females) http://www.globalrph.com/crcl.htm
Cockcroft and Gault eq utilizing IBW (Ideal Body Weight)
Cockcroft and Gault equation: CrCl = [(140 - age) x IBW] / (Scr x 72) (x 0.85 for females) Note: if the ABW (actual body weight) is less than the IBW use the actual body weight for calculating the CRCL. Estimate Ideal body weight in (kg) Males: IBW = 50 kg + 2.3 kg for each inch over 5 feet. Females: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet. 5feet ~ 152 cm, 1 inch ~ 2.5 cm http://www.globalrph.com/ibw_calc.htm
Cockcroft and Gault eq utilizing AjBW(Adjusted Body Weight)
CrCl = [(140 - age) x AjBW] / (Scr x 72) Note: (Multiply result by 0.85 for females) AjBW = adjusted body weight: AjBW = IBW + 0.4( ABW - IBW) Estimated IBW: Males: IBW = 50 kg + 2.3 kg for each inch over 5 feet. Females: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet. 5feet ~ 152 cm, 1 inch ~ 2.5 cm http://www.globalrph.com/ibw_calc.htm Some studies have shown that utilizing the adjusted body weight improves accuracy compared to other commonly used equations in estimating the creatinine clearance in the elderly population.
Other adjustments: Adjustment for Obesity
  • creatinine production is approximated based on lean body weight because muscle tissue (not fat) is responsible for creatine production.
  • Furthermore, a change in total body mass does not increase the size of the kidney (or GFR) proportionally.
  • Equations that do not correct or adjust for obesity risk overestimation of true renal function.
  • While there is still significant debate regarding the optimal method of controlling for obesity, it appears that using the Cockcroft-Gault equation with a 40% adjustment is the most appropriate method. In one of the largest study on the topic to date of nearly 3000 overweight and obese patients, the following conclusions can be drawn:
    • Actual body weight will significantly overestimate renal function
    • Ideal body weight will significantly underestimate renal function
    • The LBW2005 equation, while initially very promising,15,16 significantly underestimates renal function.
    • For all classes of obesity (overweight, obese, and morbid obesity), the Cockcroft-Gault equation with a 40% adjustment proved to consistently offer the most accurate estimate of creatinine clearance (often within about 5 mL/min of accuracy)
Rounding Creatinine in Underweight Patients
  • In underweight patients, a low serum creatinine may be more reflective of a decrease in production rather than an increased rate of renal elimination.
  • The most accurate method to control for underweight patients is to multiply the patient's Cockcroft-Gault value by an adjustment factor of 0.69 (regardless of whether the patient's serum creatinine is above or below 1 mg/dL).
  • This correction factor was shown to be more precise and less bias than rounding or making no adjustment
Medications that Modify Serum Creatinine
  • Because serum creatinine undergoes tubular secretion, any medications that interfere with this process will falsely elevate the patient's serum creatinine; however, this will not impact the patient's true GFR.
    • Cefoxitin
    • Cimetidine
    • Cisplatin
    • Flucytosine
    • Trimethoprim
Populations who are Difficult to Estimate
  • Certain patient groups have dramatically different serum creatinine production or elimination compared to the normal patient population. The following groups are notoriously difficult to estimate true renal function:
    • Amputation - Falsely low serum creatinine due to less production from muscle mass
    • Burn injury - Increased GFR
    • Cirrhosis - Falsely low serum creatinine due to less muscle mass and reduced hepatic conversion of creatine to creatinine
    • Cystic fibrosis - Increased GFR
    • Muscle disorders - Muscular dystrophy and other muscle disorders that can cause cachexia
    • Pregnancy - Difficult to estimate lean body mass, increased GFR
    • Unstable renal function - Equations used to estimate unstable renal function are very old and not validated in a large patient population
References:
  1. http://www.globalrph.com/multiple_crcl_2012.htm
  2. Demirovic JA, Pai AB, Pai MP. Estimation of creatinine clearance in morbidly obese patients. Am J Health Syst Pharm. 2009 Apr 1;66(7):642-8.
  3. http://clincalc.com/kinetics/crcl.aspx#ObesityAdj40
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