The kidneys are two bean-shaped organs located on either side of the spine behind the abdomen. Each kidney is about the size of a fist. The kidneys filter waste products, excess water, and other impurities from the blood. They help regulate blood pressure and the levels of electrolytes. They also produce hormones that regulate various bodily functions.
Healthy kidneys filter about 120-150 quarts of blood to produce 1-2 quarts of urine per day. If the kidneys are not working properly, waste builds up in the blood and may cause symptoms like nausea, weakness, poor appetite, trouble concentrating, swelling, among others. Understanding kidney function involves looking at some key numbers and ranges.
What do kidney function numbers mean?
Kidney function can be assessed by different tests that measure the levels of certain substances in the blood and urine. Common tests include:
Blood urea nitrogen (BUN)
BUN measures the amount of nitrogen from the waste product urea in the blood. Urea is formed when protein is broken down in the body. Normal BUN levels are:
– 7-20 mg/dL for adults
– 8-18 mg/dL for children
Higher than normal BUN indicates that the kidneys are not filtering properly. Levels may be temporarily elevated due to dehydration or a high protein diet. Chronically high BUN indicates impaired kidney function.
Creatinine
Creatinine is a waste product formed by the normal breakdown of muscle tissue. It is filtered out of the blood by the kidneys. Normal creatinine levels are:
– For adult males: 0.6 to 1.2 mg/dL
– For adult females: 0.5 to 1.1 mg/dL
Higher than normal creatinine indicates impaired filtering ability of the kidneys. Levels may be temporarily increased after strenuous exercise which breaks down muscle.
Estimated glomerular filtration rate (eGFR)
The glomeruli are tiny filters inside the kidneys that filter the blood. GFR measures how much blood passes through these filters each minute. eGFR is an estimate of the GFR based on the creatinine level. Normal eGFR is 90 or above.
An eGFR below 60 for over 3 months indicates chronic kidney disease. An eGFR below 15 indicates kidney failure. The stages of chronic kidney disease based on eGFR are:
– Stage 1: Kidney damage with normal eGFR (>90)
– Stage 2: Mild loss of kidney function (60-89)
– Stage 3: Moderate loss (30-59)
– Stage 4: Severe loss (15-29)
– Stage 5: Kidney failure (<15)
Albumin and protein in urine
Healthy kidneys do not allow albumin or protein to pass from the blood into the urine. Albumin is a protein made by the liver. The presence of albumin in urine, called albuminuria or proteinuria, indicates damaged filters in the kidney.
Normal urine albumin levels should be:
– <30 mg/day - Albumin-to-creatinine ratio: <30 mg/g Higher levels indicate kidney damage. Levels above 300 mg/day indicate severe damage.
What is a normal GFR level?
The normal glomerular filtration rate (GFR) is 90 mL/min or higher. This means healthy kidneys can filter over 50 gallons of blood per day to generate 1-2 quarts of urine.
As kidney function declines, the GFR decreases. Anything lower than 60 for over 3 months indicates chronic kidney disease. However, a normal GFR does not always mean the kidneys are damage-free. Other tests like albumin in urine are also important.
Some key points about normal GFR:
– The normal range is 90 or higher
– GFR naturally decreases with age
– African Americans typically have higher GFRs
– GFR varies throughout the day and night
– Certain drugs and medical tests can temporarily lower GFR
Normal GFR by Age
While 90+ is considered normal, GFR does gradually decline naturally with age:
| Age | Normal GFR Range |
|---|---|
| 20-29 years | 116-130 mL/min |
| 30-39 years | 107-118 mL/min |
| 40-49 years | 99-108 mL/min |
| 50-59 years | 90-99 mL/min |
| 60-69 years | 81-90 mL/min |
| 70-79 years | 72-81 mL/min |
| 80+ years | 63-72 mL/min |
So while 90+ is normal, a GFR of 80 in a 70-year-old may be normal for their age. Changes in GFR are more significant than single measurements.
What GFR indicates kidney disease?
A GFR below 60 for over 3 months indicates chronic kidney disease. The stages based on GFR are:
| Stage | GFR Range (mL/min) |
|---|---|
| 1 | 90+ (kidney damage but normal GFR) |
| 2 | 60-89 (mild loss of function) |
| 3 | 30-59 (moderate loss) |
| 4 | 15-29 (severe loss) |
| 5 | Below 15 (kidney failure) |
The lower the GFR, the more advanced the kidney disease. Treatment focuses on slowing progression and managing complications. Kidney failure requires dialysis or transplant.
However, GFR is not the only indicator of kidney health. Albumin in urine and imaging tests also help assess kidney damage and function.
What causes abnormally low or high GFR?
Some causes of abnormal GFR include:
Low GFR
– Chronic kidney disease
– Blockages or damage to the urinary tract
– Dehydration
– Heart failure
– Liver disease
– Lupus
– Diabetes
– Hypertension
High GFR
– Early stages of diabetic kidney disease
– Pregnancy
– Anemia
– High protein diet
– Use of certain drugs like nonsteroidal anti-inflammatories
Diseases that damage the kidney filters like diabetes and hypertension are the most common causes of chronically low GFR.
Kidney function tests
Some common tests to measure kidney function are:
Blood tests
– BUN
– Creatinine
– eGFR
– Electrolytes like sodium and potassium
Urine tests
– Albumin/protein
– Examining urine under a microscope
– Measuring urine volume and concentration
Imaging tests
– Ultrasound
– CT scan
– MRI
These tests help determine the underlying cause of abnormal kidney function. They also monitor progression in chronic kidney disease and guide treatment decisions.
Protecting kidney function
Some tips to maintain healthy kidney function include:
– Drinking plenty of fluids
– Controlling blood pressure and blood sugars
– Eating a kidney-friendly diet low in sodium
– Maintaining a healthy weight
– Not smoking
– Taking medications as prescribed
– Avoiding excessive use of NSAIDs like ibuprofen
– Getting regular medical check-ups
Detecting and treating kidney disease early helps prevent further damage and preserve kidney function. Blood and urine tests to assess kidney health should be part of routine health screening, especially for those at increased risk like people with diabetes or hypertension.
Conclusion
The kidneys filter the blood to remove wastes and maintain fluid and electrolyte balance. Key numbers that reflect kidney function include GFR, BUN, creatinine, and urine albumin. Normal GFR is 90 ml/min or above. Lower numbers indicate kidney damage. Other tests besides GFR are needed to fully evaluate kidney health. Kidney function normally declines with age. Various conditions can also damage the kidneys and cause abnormal GFR levels. Getting regular check-ups and protecting kidney health through healthy lifestyles helps preserve function. Monitoring numbers through simple blood and urine tests allows early detection and treatment of kidney disease.