Kidney function – what is normal and what is not?

What are the functions of the kidneys?
The kidneys are the major player in our body’s waste disposal system. We each have two kidneys about the size of a fist located just below the ribcage. These mighty organs continuously filter the blood at a rate of about half a cup every minute. And just like a filter in a swimming pool system, the kidneys also filter out waste through about a million filtering units (nephrons) per kidney. This waste (e.g., toxins, urea, and extra salt) is removed along with extra water and it all becomes urine, flowing from the kidneys to the bladder, where it is stored until urination occurs (1).

The kidneys are not just powerful filters, as they also play several other essential roles in the body. The kidneys remove acid that is produced by our cells, and are important for maintaining the right balance of water, salts, and minerals. If this balance is off, lots of cells, tissues, and organs in the body will not function properly (1).

Kidneys are also little hormone factories, producing erythropoietin to stimulate the production of red blood cells, activating vitamin D for healthy bones and a good immune response, and producing prostaglandins (hormone-like substances) to help control blood pressure (2).

How is kidney function measured?
Kidney function is commonly determined by measuring blood creatinine levels and calculating the estimated glomerular function (eGFR).

Creatinine is mainly produced within muscles from the breakdown of a compound called creatine, which is part of the energy production cycle in muscles. Almost all creatinine is filtered from the blood by the kidneys and excreted from the body in the urine (3).

The eGFR value is calculated from creatinine levels and also takes into account the age and gender of the tested person. The eGFR value shows how well the kidneys are functioning (3).

Measurements of blood urea (or urea nitrogen) levels are also helpful for detecting kidney damage. Normally urea is filtered from the blood by the kidneys, but blood levels can increase when the kidneys are not functioning normally.

What is considered normal kidney function?
Normal eGFR is more than 90 for adults. eGFR declines with age, even in the absence of kidney disease, so younger adults should usually have an eGFR value considerably higher than 90. For example, the average eGFR for someone aged 20–29 years is 116 (3).

eGFR values between 60 and 89 may indicate kidney disease. Values less than 60 for more than 3 months occur during moderate-to-severe chronic kidney disease (3).

Normal urea nitrogen levels in the blood are between approximately 7 and 25 mg/dL for adults with the higher levels often occurring in older people (5).

What are the signs of chronic kidney disease?
Chronic kidney disease (CKD) occurs when the kidneys are damaged and cannot function as well they should. They are unable to efficiently filter the blood and other functions are also disrupted, including the stimulation of red blood cells production and activation of vitamin D (4).

CKD can contribute to heart disease and stroke due to the inefficient removal of wastes and excessive fluid (4). Other health consequences include:

  • Reduced number of red blood cells (anemia)
  • Increased infections (in part due to low vitamin D activation)
  • Mineral imbalances in the blood
  • Appetite loss
  • Depression

More than 1 in 7 US adults are estimated to have CKD, that’s about 37 million people! However, most of these people have not been diagnosed, including as many as 40% of people with severely reduced kidney function (4). This is because many people with CKD do not notice any symptoms, or they think the symptoms are due to something else, and they don’t get a simple blood test to determine their kidney function.

People with later stage CKD often do experience symptoms (3), including:

  • Change in urination frequency
  • Itchiness
  • Fatigue
  • Swelling in their arms, legs, or feet
  • Muscle cramps
  • Nausea and vomiting

What can cause chronic kidney disease?
The major risk factors for CKD are diabetes and high blood pressure, which are estimated to cause 3 out of 4 CKD cases (4). Other risk factors include heart disease, family history of CKD, and obesity.

Tips for reducing the risk of CKD include:

  • Keep a healthy blood pressure (below 140/90 mm Hg)
  • Stay in the target blood sugar range (for diabetics)
  • Get regular physical activity
  • Lose weight if overweight
  • Abstain from smoking (it can worsen CKD and interfere with medications)
  • Take medications as prescribed by your health care professional (e.g., to lower blood pressure)
  • Get tested regularly if you are at risk. Testing is quick and easy from a simple finger-prick blood sample. 

How can I check my kidney function?
We offer very informative lab tests to evaluate your kidney function. The Kidney Health Panel measures your urea nitrogen and creatinine levels and calculates your eGFR to determine your kidney function. Alternatively, you can select the Urea Nitrogen test or the Creatinine and eGFR test.

Each test just requires a very small blood sample collected from a simple finger-prick. No doctor’s visits or needles are required.

Of course, if you receive an abnormal test result, we recommend consulting your health care professional for follow-up treatment, medication, and any other analyses.

It is also important to note that various other factors can also result in elevated blood creatinine and/or urea nitrogen, including dehydration, kidney or other urinary tract obstruction, increased protein intake, shock and stress, and severe burns. Reduced creatinine in the blood can be a sign of muscle disease (e.g., muscular dystrophy), while both reduced creatinine and reduced urea can occur due to liver disease. 

1. Your Kidneys and How They Work. National Institute of Diabetes and Digestive and Kidney Diseases. NIH. Reviewed June 2018.
2. Kidneys. You and your hormones. Reviewed April 2019.
3. Estimated Glomerular Filtration Rate (eGFR). National Kidney Foundation. Reviewed September 2021.
4. Chronic Kidney Disease Basics. Chronic Kidney Disease Initiative. CDC. Reviewed August 2021.
5. Thomas L. Clinical Laboratory Diagnostics: Use and Assessment of Clinical Laboratory Results. Frankfurt/Main, Germany: TH-Books Verlagsgesellschaft mbH; 1998:374–377.