Patients with kidney disease may present with a variety of symptoms or no symptoms at all. Some have renal signs such as gross hematuria or flank pain; others present with edema, hypertension, or uremia; and for many, lab tests will show an elevated serum creatinine concentration or an abnormal urinalysis (Inker & Perrone, 2021). Once diagnosed, the degree of kidney disease is assessed, and the underlying disorder is determined. Estimating the glomerular filtration rate (GFR) is a key indicator of kidney function. What is GFR and what does it tell us about kidney disease?

Pathophysiology

To understand GFR we must first review the anatomy and physiology of the kidney. The kidneys perform many critical functions within the body (Inker & Perrone, 2021):

• Excrete waste products of metabolism such as urea, creatinine, and uric acid
• Regulate the excretion of water and electrolytes
• Secrete hormones such as renin, prostaglandins, and bradykinin
• Participate in red blood cell production (erythropoietin)
• Play a role in calcium, phosphorus, and bone metabolism

The nephrons are the functioning units of the kidney that produce urine. There are about one million nephrons in each human kidney. Each nephron contains a glomerulus within the Bowman’s capsule, a proximal tubule (convoluted and straight components), an intermediate tubule (loop of Henle), a distal convoluted tubule, a connecting tubule, and cortical, outer medullary, and inner medullary collecting ducts.

Glomerular Filtration Rate (Inker & Perrone, 2021)

The glomerulus within each nephron filters the blood of toxins and waste products. Collectively, the glomeruli filter about 180 liters of plasma per day. The GFR is a rough indication of the volume of blood (mL) filtered per minute (min) per body surface area (1.73 m²) in all the functioning nephrons. The normal value for GFR depends on age, sex, and body size and is approximately 120 (in women) and 130 (in men) mL/min/1.73m². A decreased GFR may signify worsening kidney disease, progression of an underlying condition, or the development of a reversible issue such as reduced kidney perfusion due to volume depletion. A GFR test may be needed if an individual has risk factors for kidney disease such as diabetes, hypertension, or a family history of kidney failure.

Stages of Chronic Kidney Disease

GFR is an important value used to stage chronic kidney disease (CKD). CKD is diagnosed when the eGFR remains below 60 mL/min/1.73m² for three consecutive months. There are five stages of CKD.
 

Stages of Chronic Kidney Disease (Levey & Inker, 2018)
Stage	Description	GFR mL/min/1.73m2	% Kidney Function
1	Kidney damage with normal kidney function	90 or higher	90 – 100%
2	Kidney damage with mild loss of kidney function	60 – 89	60 – 89%
3a	Mild to Moderate loss of kidney function	45 – 59	45 – 59%
3b	Moderate to severe loss of kidney function	30 - 44	30 – 44%
4	Severe loss of kidney function	15 – 29	15 – 29%
5	Kidney failure	Less than 15	Less than 15%

Calculating exact GFR is a complex process and too burdensome to complete at the patient bedside. Instead, GFR is typically estimated from serum markers. Use the GFR as a tool to help you recognize and manage kidney disease.

References:
Inker, L.A. & Perrone, R.D. (2021, October 4). Assessment of kidney function. UpToDate. https://www.uptodate.com/contents/assessment-of-kidney-function
 
Levey, A. S. and Inker, L. A. (2018). Definition and staging of chronic kidney disease in adults. UpToDate. Retrieved from https://www.uptodate.com/contents/definition-and-staging-of-chronic-kidney-disease-in-adults