GFR Calculator

How does this GFR calculator work?

This health tool will evaluate and retrieve the overall index of kidney function for adults and children in two separate tabs by taking account of the serum creatinine level, age, sex and race.

In order to accomplish this task it returns the GFR value based on a few creatinine equations that will be described below. Once you enter the requested information the GFR calculator will be ready to offer the detailed information.

What is the glomerular filtration rate?

This is an index that represents the amount of liquid that gets filtered by the capillaries of the kidney glomerular into the Bowman's capsule during a specific time period.

GFR is one of the most important indicators that provides information on overall kidney function. Kidneys help clean blood of different waste products and toxins and transport them out of the body through urine. This value shows how much substance from blood is filtered by the kidneys and contained in urine.

How to determine GFR?

This can be measured through different methods for both children and adults. The usual methods determine GFR based on blood test results or by comparing the ingredient in a blood sample with the one in a sample of urine.

The most used method in practice involves measuring serum creatinine levels. Creatinine  is a break down product of the creatinine phosphate from muscles. Serum creatinine levels determine the clearance that is used further on to establish GFR.

This method has some limitations though because it is based on a prediction of the 24h creatinine excretion rate, which is itself determined by muscle mass. Therefore, people with different muscle mass and the same serum creatinine levels can receive different assessments of their renal function.

What are the most used GFR formulas?

In case of adults there are few methods to determine GFR as they are presented below:

■ The IDMS-traceable MDRD equation formula:

GFR = 175 * (S_Cr)^-1.154 * (Age)^-0.203 * (0.742 only if female) * (1.212 only if black).

■ The CKD-EPI formula is different for each of the cases presented below:

■ The Mayo Quadratic Formula:

-  The GFR = e^{(1.911 + 5.249/S_Cr - 2.114/S_Cr2 - 0.00686 * Age - (0.205 only if Female))}

Note that in any case where S_Cr < 0.8 mg/dL, the Mayo Quadratic Formula requires to use 0.8 mg/dL for S_Cr.

Where:

e = base of the e (2.71828182845905).

S_Cr = the serum creatinine, either expressed in mg/dL. When S_Cr  is expressed in mol/L, the conversion rate that should be used is 88.4 mol/L = 1 mg/dL.

GFR is measured in mL/min/1.73m².

In case of children it is used the Schwartz formula:

- The GFR = 0.41 * Height(in cm) / S_Cr

When the child’s height is expressed in inches, the height should be converted from inches to centimeters: 1 inch = 2.54 cm.

Related to the optimal GFR level, the 60mL/min/1.73m² level is adequate. Below there is a table describing the chronic kidney disease stages:

References

1) Guyton, Arthur; Hall, John (2006) Chapter 26: Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control. In Gruliow, Rebecca. Textbook of Medical Physiology (Book) (11th ed.) Philadelphia, Pennsylvania: Elsevier Inc.

2) Mathew TH, Johnson DW, Jones GR (2007) Chronic kidney disease and automatic reporting of estimated glomerular filtration rate: revised recommendations. The Medical Journal of Australia 187 (8): 459–63.