Kidney Function: Complete Guide to eGFR, Creatinine & Kidney Health
Everything you need to know about kidney function tests and what they reveal about your kidney health. Learn how to interpret eGFR, creatinine, BUN, and understand the stages of chronic kidney disease.
Understanding Your Kidneys
Your kidneys are remarkable organs, filtering about 180 liters of blood daily—enough to fill a bathtub. These bean-shaped organs, each about the size of your fist, perform critical functions that keep your body in balance.
What Your Kidneys Do
- Filter waste – Remove metabolic waste products and excess substances from blood
- Balance fluids – Regulate the volume and composition of body fluids
- Control blood pressure – Regulate sodium, potassium, and fluid balance; produce renin
- Produce hormones – Make erythropoietin (for red blood cells) and activate vitamin D
- Balance electrolytes – Maintain proper levels of sodium, potassium, calcium, and phosphorus
- Regulate pH – Help maintain proper acid-base balance in the body
💡 The Silent Epidemic
Chronic kidney disease (CKD) affects 15% of US adults—37 million people—yet 90% don't know they have it. Unlike heart disease or diabetes, kidney disease often has no symptoms until advanced stages. Regular testing is the only way to detect it early.
eGFR: The Key Kidney Metric
Estimated Glomerular Filtration Rate (eGFR) is the best overall measure of kidney function. It estimates how much blood your kidneys filter per minute, measured in mL/min/1.73m². Higher numbers mean better kidney function.
How eGFR Is Calculated
eGFR is calculated from your blood creatinine level, age, and sex using the CKD-EPI equation (2021 update). The current equation does not use race as a factor, correcting previous disparities in kidney disease diagnosis.
eGFR Interpretation
| eGFR (mL/min) | Kidney Function | CKD Stage |
|---|---|---|
| ≥90 | Normal or High | G1 (if kidney damage present) |
| 60-89 | Mildly Decreased | G2 (if kidney damage present) |
| 45-59 | Mildly to Moderately Decreased | G3a |
| 30-44 | Moderately to Severely Decreased | G3b |
| 15-29 | Severely Decreased | G4 |
| <15 | Kidney Failure | G5 |
→ View detailed eGFR benchmarks by age
Important Notes About eGFR
- Age-related decline is normal: eGFR naturally decreases with age, typically 1 mL/min/year after age 40
- eGFR has limitations: It's less accurate at extremes (very high muscle mass, amputees, pregnancy)
- One test isn't enough: CKD diagnosis requires abnormal eGFR or markers for 3+ months
- Look at the trend: A stable eGFR of 55 is different from one declining rapidly
"The 2021 CKD-EPI equation removes race from the calculation, providing more equitable kidney disease staging. This means some previously classified as 'normal' may now be identified as having kidney disease—and can get earlier treatment."— National Kidney Foundation
Creatinine & BUN
Creatinine and Blood Urea Nitrogen (BUN) are waste products that your kidneys filter from blood. When kidney function declines, these substances accumulate—making them useful markers for kidney health.
Creatinine
Creatinine is a waste product from normal muscle metabolism. It's produced at a relatively constant rate and filtered entirely by the kidneys, making it a reliable marker of kidney function.
Creatinine Reference Ranges
| Group | Normal Range |
|---|---|
| Adult Males | 0.7-1.3 mg/dL |
| Adult Females | 0.6-1.1 mg/dL |
| Children | 0.3-0.7 mg/dL |
→ View detailed creatinine benchmarks
Blood Urea Nitrogen (BUN)
BUN measures urea, a waste product from protein metabolism. Unlike creatinine, BUN is affected by factors beyond kidney function:
- High protein diet increases BUN
- Dehydration increases BUN
- GI bleeding can raise BUN (blood is digested as protein)
- Liver disease can lower BUN (liver produces urea)
BUN Reference Ranges
| Age Group | Normal Range |
|---|---|
| Adults | 7-20 mg/dL |
| Adults 60+ | 8-23 mg/dL |
| Children | 5-18 mg/dL |
BUN/Creatinine Ratio
The ratio of BUN to creatinine provides additional diagnostic information:
- Normal: 10:1 to 20:1
- High ratio (>20:1): Suggests dehydration, GI bleeding, or high protein intake
- Low ratio (<10:1): May indicate liver disease or malnutrition
Chronic Kidney Disease Stages
Chronic Kidney Disease (CKD) is classified by both eGFR (G stages) and albuminuria/proteinuria (A stages). Both factors together determine prognosis and treatment approach.
CKD Stages and Management
| Stage | eGFR | Description | Management Focus |
|---|---|---|---|
| G1 | ≥90 | Kidney damage with normal function | Treat underlying cause, reduce risk factors |
| G2 | 60-89 | Mild decrease | Estimate progression, reduce risk factors |
| G3a | 45-59 | Mild-moderate decrease | Monitor, manage complications |
| G3b | 30-44 | Moderate-severe decrease | Nephrology referral, manage complications |
| G4 | 15-29 | Severe decrease | Prepare for renal replacement therapy |
| G5 | <15 | Kidney failure | Dialysis or transplant needed |
Albuminuria Categories
Albumin in urine (albuminuria) indicates kidney damage and increases cardiovascular risk:
- A1 (Normal): Albumin/creatinine ratio <30 mg/g
- A2 (Moderately increased): 30-300 mg/g
- A3 (Severely increased): >300 mg/g
💡 Albuminuria Matters
Even with normal eGFR, the presence of albuminuria indicates kidney damage and increased cardiovascular risk. This is why comprehensive kidney assessment includes both eGFR and urine albumin testing.
Risk Factors for Kidney Disease
Understanding risk factors helps identify who should be screened for kidney disease and what can be done to prevent it.
Major Risk Factors
- Diabetes – The #1 cause of kidney disease, responsible for ~44% of new kidney failure cases
- High blood pressure – The #2 cause, damages blood vessels in kidneys
- Heart disease – Kidney and heart disease often occur together
- Family history – CKD runs in families
- Age over 60 – Kidney function naturally declines with age
- Obesity – Independently increases CKD risk
Other Contributing Factors
- Smoking
- Frequent use of NSAIDs (ibuprofen, naproxen)
- Autoimmune diseases (lupus, etc.)
- Urinary tract obstructions
- Recurrent kidney infections
- Low birth weight
⚠️ Medication Warning
- NSAIDs (ibuprofen, naproxen) can damage kidneys with long-term use
- Some antibiotics, PPIs, and contrast dyes can affect kidney function
- Always inform doctors about your kidney function before taking new medications
Protecting Your Kidneys
Whether you have healthy kidneys or early CKD, the same strategies help preserve kidney function and prevent progression.
Key Strategies
1. Control Blood Pressure
High blood pressure is both a cause and consequence of kidney disease. Target <130/80 mmHg for most people with CKD.
→ See our Complete Blood Pressure Guide
2. Manage Blood Sugar
For people with diabetes, tight glucose control (A1c <7% for most) significantly reduces kidney disease risk and progression.
→ See our Metabolic Health Guide
3. Healthy Diet
- Reduce sodium intake (<2,300 mg/day)
- Limit processed foods
- In advanced CKD, may need to limit protein, potassium, and phosphorus
- Stay well-hydrated (unless fluid-restricted)
4. Exercise Regularly
Physical activity helps control blood pressure, blood sugar, and weight—all of which protect kidneys.
5. Don't Smoke
Smoking accelerates kidney function decline and increases cardiovascular risk in CKD.
6. Medication Management
- Avoid NSAIDs when possible
- SGLT2 inhibitors and certain BP medications provide kidney protection
- Adjust medication doses for reduced kidney function
Frequently Asked Questions
What is a good eGFR number?
An eGFR of 90 or higher is considered normal. Values between 60-89 may be normal for older adults but warrant monitoring. Below 60 indicates chronic kidney disease. Below 15 indicates kidney failure requiring dialysis or transplant.
Can kidney function improve?
In some cases, yes. Acute kidney injury can fully recover. In CKD, removing the cause (stopping harmful medications, controlling diabetes/blood pressure) can stabilize or modestly improve function. However, significant damage is often irreversible—making early detection crucial.
What causes high creatinine?
High creatinine usually indicates reduced kidney function—the kidneys aren't filtering waste effectively. Other causes include dehydration, high muscle mass, intense exercise, high protein diet, or certain medications. Persistently elevated creatinine warrants medical evaluation.
How often should I check kidney function?
For healthy adults, annual screening is recommended starting at age 60, or earlier with risk factors (diabetes, hypertension, family history, obesity). With CKD, monitoring frequency increases with disease severity—from annually in early stages to monthly in advanced disease.
What are symptoms of kidney disease?
Early kidney disease typically has no symptoms—which is why testing is important. Advanced disease may cause fatigue, swelling (especially in legs/ankles), decreased urination, nausea, difficulty concentrating, muscle cramps, and itching. These symptoms often don't appear until kidneys are severely damaged.
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