Glomerular filtration rate describes the rate of fluid filtration through the kidney. There are many methods of calculating the glomerular filtration rate of you kidney. These methods include, measurement using insulin, creatinine-based approximations and from the creatinine clearance. This article on how to calculate glomerular filtration rate will help you in calculating the filtration rate using all these methods.
Calculating glomerular filtration rate
- GFR is the volume of fluid that is filtered from the glomerular capillaries in the kidney into the Bowman’s capsule per unit time.
- To calculate glomerular filtration rate we need to calculate or measure the concentration of any chemical present within the body or inject such a chemical into the body which is not harmful to the body and is neither reabsorbed nor secreted by the kidneys.
Glomerular filtration rate is given by the formula
Hence, glomerular filtration rate is the quantity of the substance in the urine that came from the calculable volume of blood.
GFR is calculated in the units of volume per time.
Calculating GFR using insulin
In this method, insulin is injected into the plasma. Insulin is neither secreted nor reabsorbed by the kidney. Therefore the rate of excretion of insulin is directly proportional to the rate of filtration of solutes from the glomerular capillaries.
While collecting urine make sure that you are collecting the urine completely or it may cause errors in your calculations.
Calculating GFR using creatinine clearance
Creatinine clearance or the estimates that are based upon the creatinine level of serum are used to measure GFR.
Creatinine is present naturally in the body; we need not inject any substances into the body. Hence, measuring the creatinine clearance is easier than the method of using insulin as creatinine is readily available in the body. Creatinine concentration is steady in the blood.
These calculations are based upon the calculation or prediction of twenty four hour creatinine excretion rate.
Formula for using creatinine clearance in calculation of GFR is:
Ccr = (Ucr × V)/ Pcr
In the above formula
- Ucr = creatinine concentration
- V = urine flow rate
- Pcr = plasma creatinine concentration
To calculate the creatinine concentration, urine is collected from the empty bladder in the morning on two consecutive days with the comparative blood test.
To correct the creatinine clearance we use BSA. The formula for correcting the creatinine clearance is
Ccr-corrected = Ccr × 1.73 / BSA
MDRD formula
You can also use the formula devised by modification of diet and renal disease study group to calculate the GFR. This formula is largely used in the calculation of GFR where the creatinine concentrations are used in the measurement of the glomerular filtration rate.
This formula is given as:
GFR = 186 × Serum Creatinine -1.154 × Age -0.203 × [1.210 if black] × [0.742 if female]
The above formula is called as a 4-variable MDRD formula. This formula uses four values such as serum creatinine, race, age and gender. The original formula also used the values such as blood urea nitrogen and albumin.
To use the above formula for calculating the glomerular filtration rate, you need to get the values of creatinine concentration in mg/dL. If you have the concentrations in any other units then convert them into the units compatible with this formula and use.
Some of the other formulas used for calculating GFR are:
CKD-EPI formula
GFR = 141 × min (SCr/k, 1) a × max (SCr/k, 1)-1.209 × 0.993age × [1.018 if female] × [1.159 if Black]
In the above formula,
- SCr = serum creatinine in mg/dL
- “min” indicates the minimum of SCr per k
- “max” indicates maximum of SCr per k
- The value of “k” is 0.7 for females and 0.9 for males
- The value of “a” is -0.329 for females and 0.411 for males
Mayo Quadratic formula
GFR = exp (1.911 + 5.249/SCr – 2.114/Scr2 – 0.00686 * Age -0.205 (if female))
The value for SCr in this formula is 0.8
Schwartz formula for calculating GFR in children
GFR = k × height / SCr
“k” is the muscle mass which varies with the child’s age
- K =0.33 for pre term babies
- K=0.45 for full term infants
- K = 0.55 for infants between 1 to 12 years of age
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