Kidney Function Decline: Aging and Test Criteria
Aging affects kidney function, but the most common causes of chronic kidney disease in adults are diabetes and high blood pressure. This article explains the limits of research on nephron loss, how to interpret blood and urine tests, and management principles based on the patient's condition.
Age-related kidney changes and damage caused by disease must be assessed separately.
The age-group averages in the 2017 nephron study cannot be interpreted as an individual's rate of decline.
Kidney health is assessed using both estimated glomerular filtration rate and a urine albumin test.
If muscle mass is low, creatinine-based tests may overestimate kidney function.
The decision to start dialysis takes symptoms, test results, and other factors into account, not nephron count.
Aging is one factor that contributes to declining kidney function. However, the most common causes of chronic kidney disease in adults are diabetes and high blood pressure. A decline in function should not be considered normal simply because someone is older.
The diagnostic criteria are based on the KDIGO 2024 guideline, and the nephron figures are based on a study published in 2017.
Is aging the biggest cause of declining kidney function?
It is difficult to say that aging is the biggest cause of every decline in kidney function. Age-related changes must be distinguished from the causes of disease. NIDDK, the U.S. National Institute of Diabetes and Digestive and Kidney Diseases, identifies diabetes and high blood pressure as leading causes. This is based on NIDDK’s guide to the causes of chronic kidney disease.
Persistently high blood sugar can damage the kidneys’ filtering structures. High blood pressure can damage kidney blood vessels and filtering tissue. As kidney function worsens, blood pressure can also become harder to control.
An evaluation of the cause also checks for:
· The possibility of glomerular disease or inherited kidney disease
· Any previous acute kidney injury
· Use of medications that could affect the kidneys
· The possibility of a condition that blocks urine flow
What do nephrons do?
Nephrons are the basic units in the kidneys that filter blood and make urine. A nephron’s glomerulus is a small cluster of blood vessels that filters blood. Its tubule reabsorbs needed substances from what has been filtered. Through this process, waste and excess water leave the body in urine.
The kidneys regulate the balance of water and electrolytes. They also help regulate the body’s acidity. Kidney functions also include hormonal roles needed for blood pressure, red blood cell production, and bone health. The effects of declining function therefore extend beyond waste buildup. This is based on NIDDK’s explanation of how the kidneys work.
Common misunderstandings about nephron loss figures
Average nephron counts by age cannot be used to calculate an individual’s kidney lifespan. A study published in 2017 compared healthy kidney donors. The researchers estimated nephron counts using CT scans and tissue examinations. The figures below are estimated averages for non-sclerotic glomeruli.
Age group in the study published in 2017 | Estimated average number of non-sclerotic glomeruli in one kidney
18~29 years | 990,661
70~75 years | 520,410
The older comparison group in this study was 70~75 years old, not in their 60s. The results also did not come from following the same people from youth into old age. Therefore, they cannot establish the early 20s as the age when nephron loss begins for everyone. The source is a study of aging and nephron numbers in healthy people.
Common interpretation | What to distinguish when reading the study
People in their 60s necessarily have half as many nephrons as people in their 20s | Check the study’s age groups and differences between individuals
Filtering function falls by the same percentage as nephron count | Nephron count and overall filtering function are different measures
Average nephron loss can be used to calculate when dialysis will be needed | A comparison between groups cannot predict an individual’s course of disease
Interpreting test results by circumstance
Blood and urine test results must be considered together to assess kidney health. Estimated glomerular filtration rate, or eGFR, estimates how well the kidneys filter blood. The urine albumin-to-creatinine ratio, or UACR, assesses albumin leakage. Abnormal test results can appear early, even without symptoms. This is based on NIDDK’s guide to kidney tests.
The KDIGO 2024 guideline calls for checking whether an abnormality persists for at least 3 months. Chronic kidney disease means an abnormality in kidney structure or function that affects health. The diagnostic criteria are in Table 1 of the official KDIGO guideline.
Test result or situation | Interpretation
eGFR less than 60 mL/min/1.73m² for at least 3 months | Meets a diagnostic criterion for chronic kidney disease
UACR at least 30 mg/g for at least 3 months | Is a marker of kidney damage
eGFR at least 60 mL/min/1.73m² | Other markers of damage, such as albuminuria, also need to be checked
An abnormality found on the first test | Assess whether it persists and what caused it using previous results and repeat testing
The 3 months are not a waiting period during which an abnormality should be left unaddressed. A sudden change should first be assessed for possible acute kidney injury. Even in older people, persistent abnormal test results should not be attributed to aging alone.
Does lower muscle mass change how test results are interpreted?
When muscle mass is low, kidney function estimated from creatinine may appear higher than it really is. Creatinine is a waste product made by muscles. As muscle mass decreases with age, creatinine production may also decrease. Creatinine levels alone can therefore make kidney health difficult to assess.
A cystatin C test may help in this situation. Cystatin C is another blood marker used to estimate kidney filtering function. However, factors such as thyroid disease and steroid use can also affect it. Consult a healthcare professional about test selection, using National Kidney Foundation’s guide to cystatin C as a reference.
Tell your healthcare professional if you have recently lost a substantial amount of weight or muscle. Bring previous test results as well. Explaining changes in your results alongside changes in how you feel can help with interpretation.
Managing your current condition
A management plan should be based on the underlying condition and test results rather than age. If you have diabetes, check your blood sugar target. If you have high blood pressure, review how well it is controlled. The interval between routine tests varies with kidney health.
Current situation | What to discuss at an appointment
Diabetes | Blood sugar management and a kidney testing plan
High blood pressure | Blood pressure target, medications, and kidney test results
A family history of heart or kidney disease | Whether kidney tests are needed and how often
A diagnosis of chronic kidney disease | Changes in eGFR and albuminuria, and response to treatment
Weight loss or reduced food intake | Nutritional status and whether dietary restrictions are appropriate
Prepare for your appointment in this order:
· Gather previous blood and urine test results and the dates of the tests.
· Make a list of medications and dietary supplements you take.
· Organize your blood pressure records and note recent changes in how you feel.
· Check when your next tests should be and what dietary changes you need to make.
Not smoking and getting physical activity suited to your condition are also part of management. Take prescribed medications as directed. If your test results have changed, tell your healthcare professional rather than stopping a medication on your own. This is based on NIDDK’s guide to managing chronic kidney disease.
Mistakes to avoid with food and medications
A kidney diet should be adjusted to your test results and nutritional status. The same dietary restrictions do not apply to everyone. Cutting protein too much can lead to poor nutrition. Protein needs may differ during dialysis.
Common choice | What to check
Drinking a lot of water to clean the kidneys | Whether fluid intake needs to be adjusted based on swelling and kidney function
Restricting all vegetables and fruit | Blood potassium levels and whether restrictions are actually needed
Replacing regular salt with low-sodium salt | Whether it contains potassium chloride
Greatly reducing protein foods such as meat and beans | An appropriate intake based on nutritional status and whether you are on dialysis
Frequently eating processed foods | Sodium content and phosphate additives
The dietary advice is based on NIDDK’s nutrition guide for adults with chronic kidney disease.
Nonsteroidal anti-inflammatory drugs such as ibuprofen can damage the kidneys. The risk may be greater if you are dehydrated. Ask a pharmacist to check the ingredients when choosing a pain reliever or cold medicine. This is based on NIDDK’s medication safety guide.
When is dialysis needed?
Dialysis is not started at the point when all nephrons are gone. Symptoms, abnormal test results, and filtering function are assessed together. Quality of life and treatment preferences are also considered. This principle appears in the KDIGO 2024 guideline’s criteria for starting dialysis.
When kidney function falls substantially, waste and fluid can build up. The condition in which this causes symptoms is called uremia. Treatments that replace kidney function include hemodialysis, peritoneal dialysis, and kidney transplantation. Consult a healthcare professional about treatment options, using NIDDK’s guide to kidney failure as a reference.
Is a sudden decline also due to aging?
A sudden decline in kidney function should not be attributed to aging alone. Dehydration, infection, or a blockage in the urinary tract can cause acute kidney injury. Function may recover after the cause is treated. In some cases, damage may remain.
Seek prompt medical care if any of these symptoms appear:
· A marked decrease in urine output compared with usual
· Persistent vomiting with suspected dehydration
· Leg swelling or shortness of breath
· Confusion or severe drowsiness
Acute kidney injury may also occur without obvious symptoms. Symptoms alone cannot establish the condition of your kidneys. If an abnormality is suspected, testing is needed to check it. This is based on NHS’s guide to acute kidney injury.
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