A kidney function test measures how well your kidneys are filtering waste from your blood. Two key numbers tell this story: your eGFR (Estimated Glomerular Filtration Rate) and your creatinine level. eGFR shows the percentage of kidney function you have remaining, measured in mL/min/1.73m² of body surface area. If your eGFR is above 90 and your creatinine falls within normal range (0.74–1.35 mg/dL for men, 0.59–1.04 mg/dL for women), your kidneys are working well.
If eGFR drops below 60 or creatinine rises above normal, it’s a sign your kidneys may not be filtering efficiently. Understanding these numbers matters more than you might think, especially as we age. Kidney function naturally declines with age, and even mild kidney disease can go unnoticed until significant damage occurs. For people concerned about brain health and dementia risk, kidney function is particularly relevant—emerging research suggests that poor kidney function may increase the risk of cognitive decline. This article walks you through what these tests mean, how to read your results, and what steps to take if your numbers are off.
Table of Contents
- What Are eGFR and Creatinine Tests, and How Do They Work?
- Understanding eGFR Results and the Five Stages of Kidney Function
- What Normal Creatinine Levels Tell You About Kidney Health
- How to Read and Interpret Your Kidney Function Test Results
- Factors That Affect Your Test Results and When to Retest
- Why Trends Over Time Matter More Than Single Test Results
- Kidney Function, Brain Health, and Why This Matters for Dementia Prevention
- Conclusion
What Are eGFR and Creatinine Tests, and How Do They Work?
Creatinine is a waste product your muscles produce during normal metabolism. Every day, your muscles break down protein and create creatinine as a byproduct. Healthy kidneys filter creatinine out of your blood and send it into your urine for elimination. If your kidneys aren’t working properly, creatinine builds up in your bloodstream instead of being cleared. A simple blood test measures your serum creatinine level—this is the foundation of kidney function assessment.
eGFR is a calculated number derived from your creatinine level (sometimes using cystatin C as well), along with your age, sex, and race. It estimates how many milliliters of blood your kidneys filter each minute per 1.73 square meters of body surface area. Think of it as a percentage-like score: an eGFR of 90 or higher means your kidneys are working at near-normal capacity, while an eGFR of 30 means they’re working at roughly 30% of normal function. The test doesn’t directly measure filtration—it’s an estimate based on how much creatinine your kidneys are failing to remove. For this reason, trends in eGFR over time are more informative than a single reading.
Understanding eGFR Results and the Five Stages of Kidney Function
Your eGFR result falls into one of five categories that define your kidney function stage. An eGFR greater than 90 mL/min/1.73m² is considered normal kidney function, though values as low as 60 can still be normal if there’s no other evidence of kidney disease. Between eGFR 60 and 89, mild to moderate kidney function is present but usually without symptoms—many people at this stage don’t realize they have any kidney issue at all. When eGFR drops to 30–59, you have moderate to severe decreased kidney function, and symptoms may begin to develop.
You might notice fatigue, difficulty concentrating, or changes in urination patterns. Below 30 (specifically 15–29), kidney function is severely compromised, and below 15, you’re in kidney failure requiring dialysis or a kidney transplant. However, it’s important to remember that a single test result doesn’t define your kidney status. Dehydration, recent illness, or certain medications can temporarily lower your eGFR. Your doctor looks at trends over time to determine whether kidney function is actually declining or whether the result was an anomaly.
What Normal Creatinine Levels Tell You About Kidney Health
Normal creatinine ranges differ between men and women because men typically have more muscle mass than women, and muscle is what produces creatinine. For men, the normal range is 0.74–1.35 mg/dL. For women, it’s typically lower at 0.59–1.04 mg/dL. A creatinine level above these ranges suggests your kidneys may be retaining creatinine that should have been filtered out—a sign of reduced kidney function.
What complicates matters is that creatinine levels can vary based on body composition, diet, and hydration status. Two people with the same actual kidney function might have slightly different creatinine values if one has more muscle mass or eats more protein. This is why eGFR was developed—it adjusts creatinine for these variables to give a better estimate of actual kidney function. That said, if your creatinine is progressively rising over months or years, even if it remains within the “normal” range, that upward trend warrants investigation. For example, if a patient’s creatinine rises from 0.8 to 1.2 over two years, that’s a significant change even though 1.2 is technically normal for men.
How to Read and Interpret Your Kidney Function Test Results
When you receive test results, you’ll see two numbers: serum creatinine (in mg/dL) and eGFR (in mL/min/1.73m²). Start by comparing your creatinine to the normal range for your sex. Then look at your eGFR—this is the number your doctor will use to determine your kidney function stage. Compare your eGFR to the five stages: above 90 is normal, 60–89 is mild function loss, 30–59 is moderate to severe loss, 15–29 is severe, and below 15 is kidney failure. The key is looking at the direction of change over time.
If your eGFR was 85 last year and 78 this year, your kidneys are declining in function—even though both numbers are technically in the “mild to moderate loss” range. Conversely, if your eGFR is 45 but it was 44 last year and 43 the year before, this gradual steady decline is different from an eGFR that suddenly drops from 60 to 40. Your doctor should discuss trends with you, not just react to a single number. Ask your doctor for copies of your past results so you can see your personal pattern. Many online patient portals now make this easy to track over years.
Factors That Affect Your Test Results and When to Retest
Your eGFR and creatinine can fluctuate based on several factors that don’t reflect true kidney disease. Dehydration is the most common culprit—if you haven’t drunk enough water before your blood test, your creatinine may appear falsely elevated because the concentration is higher in less blood volume. This can cause an eGFR to appear lower than it really is. Illness, fever, and intense exercise can also temporarily raise creatinine. Certain medications, including some blood pressure drugs and NSAIDs (like ibuprofen), can affect kidney function tests.
Another important factor: what you eat the day before your test. Eating a large amount of meat increases serum creatinine concentration in the blood, which is why some doctors recommend avoiding meat for 12 hours before blood work for kidney function. If your results come back lower than expected, mention to your doctor whether you fasted, how much water you drank, whether you were sick, or what medications you’re taking. If your doctor suspects a temporary change rather than true kidney disease, they may recommend retesting after a few weeks. A single abnormal result, especially if accompanied by normal kidney function signs, shouldn’t trigger immediate alarm—but it does warrant follow-up.
Why Trends Over Time Matter More Than Single Test Results
One kidney function test is just a snapshot. To understand whether your kidneys are truly declining or whether you have stable chronic kidney disease, your doctor needs to see the pattern across multiple tests over months and years. eGFR naturally decreases with age—a 60-year-old with an eGFR of 70 may have normal age-appropriate kidney function, while a 40-year-old with the same score likely has true kidney disease. Some people have stable eGFR in the 45–50 range for 10 years without progression, while others see their eGFR drop 5 points per year.
For this reason, most kidney disease is detected through regular screening during routine checkups, not through urgent discovery of a critically low result. If you have risk factors like diabetes, high blood pressure, or a family history of kidney disease, your doctor will monitor your kidney function annually or semi-annually. If your numbers are stable, you may switch to less frequent testing. The goal isn’t to obsess over each individual result but to watch for troubling trends that suggest intervention is needed—whether that’s adjusting medications, modifying diet, or referring to a kidney specialist (nephrologist).
Kidney Function, Brain Health, and Why This Matters for Dementia Prevention
Why does a dementia care website focus on kidney function? Because emerging research suggests a link between poor kidney function and cognitive decline. The kidneys aren’t just waste filters—they also regulate blood pressure, produce hormones that stimulate red blood cell production, and maintain electrolyte balance. When kidneys don’t work well, blood pressure rises, inflammation increases, and toxic compounds build up in the blood. These changes can damage blood vessels in the brain, potentially accelerating cognitive aging.
People with moderate to severe kidney disease (eGFR below 45) have higher rates of cognitive impairment compared to those with normal kidney function. While kidney disease doesn’t cause dementia in a simple one-to-one relationship, it’s part of the bigger picture of vascular and metabolic health. Maintaining normal kidney function through blood pressure control, healthy diet, regular exercise, and diabetes management may help protect cognitive function as you age. If your kidney function tests show decline, getting prompt medical evaluation and treatment isn’t just about preventing kidney failure—it’s also about protecting your brain.
Conclusion
Reading kidney function tests isn’t complicated once you understand what the numbers mean. eGFR tells you what percentage of kidney function remains, normal is above 60 (ideally above 90), and creatinine tells you whether waste is building up in your blood. Knowing your numbers and trends over time gives you and your doctor the information needed to catch kidney disease early, before serious damage occurs.
If your results are abnormal, don’t panic—ask your doctor whether the result is truly concerning or possibly temporary, and request follow-up testing if recommended. Taking charge of your kidney health is taking charge of your overall health, including your brain health. Monitor your kidney function regularly if you have risk factors, maintain a healthy lifestyle, manage blood pressure and blood sugar, and stay hydrated. If you notice changes in your test results or have concerns, talk to your doctor about what the trends mean for your long-term health.
