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# GFR Calculator

The GFR (glomerular filtration rate) Calculator estimates GFR values based on factors including serum creatinine value, age, race, and gender. For comparison purposes, the results for three prevalent formulas used for estimating GFR are provided.

This calculator is for people aged 18 years or older.

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## GFR Calculator for Children

This calculator is for people aged 18 years or younger.

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### What is Glomerular filtration rate (GFR)?

Glomerular filtration rate describes the rate of flow of filtered fluids through the kidney and is used as a measure of kidney function. Generally, a higher value of GFR indicates better kidney function. A significant decrease in GFR indicates that the kidney is not properly functioning, resulting in lower volumes of fluids being filtered over a given time . The normal range of GFR adjusted for body surface area is 100 to 130 mL/min/1.73m2 in men and 90 to 120mL/min/1.73m2 in women below the age of 40. After age 40, GFR decreases progressively with age. Refer to the table below for population mean estimated GFR for a number of age ranges.

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### Chronic Kidney Disease Stages

A GFR above 60 mL/min/1.73m2 is usually adequate for most patients and alone is not necessarily indicative of a patient having chronic kidney disease (CKD).1 If however there has been a significant decline of GFR since a previous test, this could be an early indicator of kidney disease. Note that the sooner that kidney disease is diagnosed and treated, the higher the probability that the need for dialysis can be prevented. Below is a table of GFR ranges for the different stages of CKD. The different stages are dependent on other factors in conjunction with GFR, as noted in the table.

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### Measuring GFR

The most accurate way to determine GFR involves the measurement of the rate at which an external substance such as inulin is removed from the bloodstream, termed inulin clearance. Inulin is freely filtered by the kidney and does not undergo metabolism, tubular secretion or absorption, making it an ideal candidate for measuring GFR.2 This measurement method requires continuous intravenous infusion, a number of blood and urine collections, and proper timing of blood samples, making it impractical for routine clinical use.3 As such, many formulas have been developed to estimate GFR, many of which are based on creatinine clearance, or estimates of creatinine clearance based on serum creatinine levels. Creatinine clearance does however have the limitation of being affected by muscle mass. Higher muscle mass increases serum creatinine for any given rate of clearance. This means that the same serum creatinine values can signify different levels of renal function for different people based on muscle mass. Furthermore, creatinine is actively secreted in small amounts in the body resulting in GFR determined through creatinine clearance typically being overestimated by 10-20%. Despite its limitations, creatinine clearance still provides an acceptable estimation of GFR due to its ease of measurement when compared to inulin clearance.4 Below are three formulas often used for estimating GFR through serum creatinine.

The IDMS-traceable MDRD study equation
GFR = 175 × (SCr)-1.154 × (age)-0.203 × (0.742 if female) × (1.212 if Black)
where SCr = serum creatinine in mg/dL

The CKD-EPI formula
Black female
If SCr< 0.7
GFR = 166 × (SCr/0.7)-0.329 × 0.993age
If SCr > 0.7
GFR = 166 × (SCr/0.7)-1.209 × 0.993age
Black male
If SCr< 0.9
GFR = 163 × (SCr/0.9)-0.411 × 0.993age
If SCr > 0.9
GFR = 163 × (SCr/0.9)-1.209 × 0.993age
Non-Black female
If SCr< 0.7
GFR = 144 × (SCr/0.7)-0.329 × 0.993age
If SCr > 0.7
GFR = 144 × (SCr/0.7)-1.209 × 0.993age
Non-Black male
If SCr< 0.9
GFR = 141 × (SCr/0.9)-0.411 × 0.993age
If SCr > 0.9
GFR = 141 × (SCr/0.9)-1.209 × 0.993age