The creatinine clearance calculator finds the creatinine clearance (CrCl), which is an estimate of the glomerular filtration rate (GFR). A commonly used creatinine clearance formula is the Cockcroft-Gault equation. In the article, you can learn how the estimated CrCl calculation is conducted and how to find it directly using urine creatinine concentration. We are also presenting a step by step solution to help you understand how to calculate creatinine clearance.

What is creatinine clearance?

Creatinine clearance, or CrCl for short, is a measure of renal function. Kidneys play a significant role in human body, and are responsible for:

  • excreting metabolism waste products,
  • formation of urine,
  • regulation of osmolality,
  • maintaining the water and salt level of the body,
  • acid-base balance,
  • blood pressure regulation,
  • hormone secretion.

Creatinine clearance rate is the volume of blood plasma that is cleared of creatinine per unit time. It is a convenient method of estimating the patient's GFR.

CrCl or GFR are measured when renal disease is suspected, or when careful dosing of renally metabolized drugs is required (e.g. carboplatin).

Creatinine clearance equation (Cockcroft-Gault equation)

The Cockcroft-Gault equation is named after the scientists, dr. Donald William Cockcroft and dr. Matthew Henry Gault, who first published the formula in 1976. Their creatinine clearance formula uses serum creatinine (sCr) measurements and a patient's weight, age and sex to predict the creatinine clearance estimate in mL/min. The CrCl equation looks like this:

CrCl = (140 - age) * weight * sex / (72 * sCr)

The coefficient dependent on the patient's sex and equals 0.85 for females or 1 for males.

Creatinine clearance adjusted for body weight

Cockcroft-Gault equation is the most widely used method of quantifying renal function for drug dosing, however using this formula is associated with some inaccuracies. Abnormal body weight, especially obesity, is one of the problems in the estimation of renal function. Serum creatinine production is associated with the lean body mass, not fat, as the muscle tissue is responsible for creatine production. To account for this problem, clinicians have been discussing different methods of controlling for obesity.

The Cockcroft-Gault body weight controversy still remains unresolved, however according to the studies of Brown et al. and Winter et al., the following adjustments can be made:

  • For underweight patients (BMI <18.5) actual body weight should be used in the calculations.
  • Patients with normal weight (BMI 18.5-25) - unbiased CrCl can be calculated using ideal body weight (LINK!!!!!)
  • For patients who are overweight or obese (BMI ≥25), 40% adjustment factor should be used:

ABW = IBW + 0.4 × (TBW – IBW)

where:

ABW - adjusted body weight [kg]

IBW - ideal body weight [kg]

TBW - total body weight (current weight) [kg]

Other creatinine clearance formulas

Apart from the Cockcroft-Gault equation, there are four different ways of estimating GFR:

  1. CKD-EPI Creatinine Equation
  2. CKD-EPI Creatinine-Cystatin Equation
  3. CKD-EPI Cystatin C Equation
  4. MDRD Study Equation

There is a substantial difference in the way that GFR is calculated for adults and for pediatric use.

Creatinine clearance - interpretation

The normal range of GFR is presented below:

  • 110–150 mL/min for men
  • 100–130 ml/min for women

However, those values may not be applicable for older patients as after the age of 40, GFR decreases progressively by 0.4–1.2 mL/min per year.

How to calculate creatinine clearance - an example

Let's calculate CrCl for a theoretical patient named Bob. He is 67 years old, weighs 92 kilograms (203 lbs, you can switch between units when you use our CrCl calculator!) and is 173 cm tall. His serum creatinine equals 1.4 mg/dL.

  1. First, we need to check Bob's BMI:

BMI = weight [kg]/(height [m] x height [m])

BMI = 92/(1.73 x 1.73)

BMI = 30.74

  1. According to BMI, Bob is obese, so adjusted body weight needs to be calculated to use Cockcroft-Gault equation:

ABW = IBW + 0.4 × (TBW – IBW)

Where we need to calculate ideal body weight (IBW) first:

IBW = 50 kg + (0.9 kg × (height (cm) − 152))

IBW = 50 kg + (0.9 kg × (173 − 152))

IBW = 50 kg + 18.9 kg

IBW = 68.9 kg

After finding out what IBW is, we can come back to the equation:

ABW = IBW + 0.4 × (TBW – IBW)

ABW = 68.9 + 0.4 × (92 – 68.9)

ABW = 68.9 + 0.4 × (92 – 68.9)

ABW = 68.9 + 9.24

ABW = 78.14 kg

  1. Now we can use Cockcroft-Gault equation to calculate creatinine clearance:

    CrCl = (140 - age) * weight * sex / (72 * sCr)

    CrCl = (140 - 67) * 78.14 * 1 / (72 * 1.4)

    CrCl = 73 * 78.14 * 1 / 100.8

    CrCl = 5704.22 / 100.8

    CrCl = 56.6 ml/min

  2. Looking at this result, we can assume that patient's CrCl is below the normal range.

Direct CrCl calculation

Creatine clearance can also be calculated directly, using both serum (sCr) and urine creatinine (uCr), and urine volume. The result is reliable, but it's less popular because of the inconvenient 24-hour collection of urine. You can find this option in our creatinine clearance calculator by clicking advanced. It uses the following creatinine clearance formula:

dCrCl = uCr * urine volume / (sCr * urine collection duration)

Małgorzata Koperska, MD and Joanna Michałowska, PhD candidate