Glomerular Filtration Rate Test

Introduction

There are many clinical tests available which are useful in evaluating kidney function; the most practical test to assess renal function is to get an estimate of the glomerular filtration rate. The glomerular filtration rate test is a test used to measure how well your kidneys are functioning. The concept of glomerular filtration was developed by Carl Ludwig in 1816, wherein the glomerulus (the filtering unit of the kidney) was described as a filter where urine formation begins. It is important to speak with your healthcare provider about any concerns regarding kidney health and to follow their recommendations for testing and monitoring. We will discuss in-depth what this test is, its normal range, its importance, how is it calculated and the risks associated.

What Is The Glomerular Filtration Rate Test?

Glomerular filtration rate (GFR) test measures the glomerular filtration rate which is the rate (millilitres per minute) at which substances in the blood are filtered via the glomerulus. In simpler terms, this test measures the clearance of a substance from the blood. A substance that helps to carry out this test is called a marker. The markers used in the testing will show a colour reaction with certain chemical agents or dyes used in the test, and the formation of a coloured complex confirms the presence of these markers.

The following are the properties of an ideal marker for measuring GFR:

• It should be filtered freely at the glomerulus.
• It should not be cleared via any non-renal route.
• It should not be reabsorbed nor secreted by the kidneys.
• It should be available in the plasma at a constant rate.

The most common marker used to measure GFR is creatinine. Other markers include inulin, radioisotopes like technetium-99-labelled diethylene-triamine-pentaacetate (99 Tc-DTPA) and chromium-51 ethylene-diamine-tetra-acetic acid (51 Cr-EDTA). Especially in children, a non-radioactive contrast agent, iohexol is widely used.

Once the concentrations of these markers are known from the blood reports, mathematical equations are used to calculate GFR.¹ 

Why Is The Glomerular Filtration Rate Test Important?²

• GFR test represents the excretory capacity of the kidneys and is important due to the following reasons.
• It is the best indicator of kidney function, both in healthy and diseased patients.
• It helps correlate kidney functioning mass and may help in classifying chronic kidney disease into different stages.
• It may help in deciding the medication dose for kidney disorder patients.

How To Prepare For The Glomerular Filtration Rate Test

Before the test, you should ask your healthcare physician about the instructions that you need to follow. Your physician may ask you to avoid eating or drinking (except water) anything a few hours before the test. Eating any cooked meat the night before the test may alter the results of the test.³

Following are the conditions in which your doctor will recommend a GFR test:¹

• To identify kidney disease
• To prevent deterioration of kidney function
• To classify the stage of kidney disease
• To monitor the progression of kidney disease
• To assess donor suitability in kidney donors, this test will help in detecting any deterioration of kidney function after donation.

Did you know the glomeruli filter less blood when you have kidney disease? As a consequence, your blood may get contaminated with harmful chemicals. It might take up to 40% of kidney function loss before you experience kidney disease symptoms, including frequent urination and muscle spasms. estimated glomerular filtration rate (eGFR) may aid healthcare professionals in early illness detection so you may stave off kidney deterioration.

Dr. Siddharth Gupta, B.A.M.S, M.D (Ayu)

What Do The Results Of The Glomerular Filtration Rate Test Mean?¹

A GFR test will help your doctor know about the functioning of your kidneys. Low GFR values are suggestive of reduced kidney functions and are indicative of reduced filtering capacity of kidneys. 

The normal value for glomerular filtration is a value 90-120 ml/min. For young men, GFR>110ml/min and for young women, GFR>100ml/min is considered normal. Your physician will use the results obtained to evaluate your kidney function. If your results show a value which is less than 90ml/min, it is suggestive of impaired renal function. 

Next, your doctor will utilize the GFR value and categorize kidney disease in the following stages.^4,5

• Stage I GFR >90 ml/min/1.73 m²  
• Stage  II GFR 60-89 ml/min/1.73 m²
• Stage III a GFR 45-59 ml/min/1.73 m²
• Stage III b GFR 30-44 ml/min/1.73 m²
• Stage IV GFR 15-29 ml/min/1.73 m²
• Stage V GFR <15 ml/min/1.73 m² (end-stage renal disease)

To my knowledge, as you become older and lose muscular mass, the GFR inevitably decreases. An individual in their 20s has an eGFR of around 116 mL/min/1.73m2. In adults over 60, it falls to 85 mL/min/1.73 m2. If your eGFR is 60 or more, your kidney function is at least 60%. In general, your kidney function will be better as shown by a higher percentage.

Dr. Rajeev Singh, BAMS

What Happens After The Glomerular Filtration Rate Test?

After the test is done, your doctor will evaluate and assess the test results and will determine the need for medical therapy management. He/she will instruct you about the dietary changes and fluid intake restrictions that may be needed to improve or decrease the load on the kidneys. 

For taking a blood sample, a syringe with a small needle will be injected into one of the veins of either hand. This may cause a little stinging sensation and pain and you may feel lightheaded for a few minutes after blood collection. Do not panic and rest for a couple of minutes before heading home.

Are There Any Risks To The Test? ³

Risks to this test are rare. Measurement of the glomerular filtration rate using isotopes may expose you to minimal radiation. It is best to avoid repeated exposures to the isotopes over short periods. Some patients may develop allergic reactions to iodine-containing radio-contrast agents.

Contraindications: As per general recommendations, special categories like pregnant women, children and elderly; people with an illness or acute infection or people who recently had a procedure involving the use of a contrast dye should talk with their doctors before going for the test. 

Conclusion

The most practical test to assess renal function is to get an estimate of the glomerular filtration rate. The glomerular filtration rate test measures the glomerular filtration rate which is the rate (millilitres per minute) at which substances in the blood are filtered via the glomerulus. In simpler terms, this test measures the clearance of a substance from the blood. It is the best indicator of kidney function, both in healthy and diseased patients. It may help in classifying chronic kidney disease into different stages and helps in dosage adjustments in patients with decreased renal function. After the test is done, your doctor will evaluate and assess the test results and will determine the need for medical therapy management.

Frequently Asked Questions (FAQs)

References:

1. Gounden V, Bhatt H, Jialal I. Renal function tests. StatPearls, 2022. Available at: https://www.ncbi.nlm.nih.gov/books/NBK507821/
2. Cusumano AM, Tzanno-Martins C, Rosa-Diez GJ. The glomerular filtration rate: from the diagnosis of kidney function to a public health tool. Frontiers in Medicine. 2021 Nov 25; 8:769335. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8675900/
3. Kaufman DP, Basit H, Knohl SJ. Physiology, glomerular filtration rate. InStatPearls [Internet] 2021 Jul 22. StatPearls Publishing. Available at: https://www.ncbi.nlm.nih.gov/books/NBK500032/
4. Tarwater K. Estimated glomerular filtration rate explained. Missouri medicine. 2011 Jan; 108(1):29. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6188456/

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