Kidney function tests assess how well the kidneys filter waste from blood. The standard measures include serum creatinine, blood urea nitrogen (BUN), estimated glomerular filtration rate (eGFR), and urine albumin. Kidney function tests are part of routine metabolic panels and are essential for diagnosing chronic kidney disease, monitoring patients with kidney disease, and adjusting medications that depend on kidney clearance. This guide covers what each test measures, normal ranges, and how to interpret results.
Standard kidney function measures
- Creatinine: Muscle breakdown product cleared by kidneys; primary kidney function marker
- BUN (blood urea nitrogen): Protein metabolism waste cleared by kidneys
- eGFR (estimated glomerular filtration rate): Calculated from creatinine, age, sex, and weight
- Cystatin C: Alternative kidney function marker
- Urine albumin/creatinine ratio (UACR): Protein leakage marker
Reference ranges
- Creatinine: 0.7-1.3 mg/dL (men), 0.6-1.1 mg/dL (women)
- BUN: 7-20 mg/dL
- eGFR: 90+ mL/min/1.73m² is normal
- UACR: under 30 mg/g is normal
How eGFR works
eGFR estimates the rate at which kidneys filter blood. The 2021 CKD-EPI equation is the current standard, calculated from creatinine, age, and sex. Race-based modifications were eliminated in the 2021 update. eGFR provides a more clinically useful kidney function measure than creatinine alone, since the same creatinine can mean different things in patients with different ages and body sizes.
Per National Kidney Foundation CKD staging:
- Stage 1: eGFR 90+ with kidney damage markers
- Stage 2: eGFR 60-89 with kidney damage markers
- Stage 3a: eGFR 45-59
- Stage 3b: eGFR 30-44
- Stage 4: eGFR 15-29
- Stage 5: eGFR under 15 (kidney failure)
Creatinine elevations
Elevated creatinine indicates decreased kidney filtration. Common causes include chronic kidney disease, acute kidney injury (dehydration, NSAIDs, medication effects, blockage), and rarely muscle disease (which increases creatinine without kidney dysfunction).
The trend over time matters more than single values. Stable elevation suggests chronic kidney disease; rapidly rising creatinine suggests acute kidney injury and warrants urgent evaluation.
BUN-to-creatinine ratio
The BUN/creatinine ratio provides clues:
- Ratio above 20:1: dehydration, upper GI bleeding, or high protein intake
- Ratio 10-20:1: normal
- Ratio below 10:1: liver disease, low protein intake, or overhydration
Urine albumin testing
Urine albumin/creatinine ratio (UACR) detects protein leakage from kidneys before significant function loss occurs. It’s particularly important for diabetics and patients with hypertension, who are at high risk for kidney disease. Persistent UACR above 30 mg/g indicates kidney damage even with normal eGFR.
Microalbuminuria (UACR 30-300 mg/g) and macroalbuminuria (UACR >300 mg/g) are both significant. Treatment with ACE inhibitors or ARBs reduces albumin loss and slows progression.
SGLT-2 inhibitors and kidney disease
SGLT-2 inhibitors (originally diabetes drugs) have shown substantial kidney benefits in trials, slowing eGFR decline in chronic kidney disease whether or not patients have diabetes. They’ve become standard care for many CKD patients. Discuss with your nephrologist or primary care physician if you have CKD.
Frequently Asked Questions
Do I need to fast for kidney function tests?
Not strictly required. Some labs prefer fasting for the fuller metabolic panel context.
What’s a normal eGFR?
90+ mL/min/1.73m² is normal. Mild reduction (60-89) without other kidney damage markers may be normal aging.
Can muscle exercise affect creatinine?
Yes. Heavy exercise before testing can transiently elevate creatinine. Avoid intense workouts the day before testing.
What does CKD stage 3 mean?
Moderate kidney function reduction. Stage 3a (eGFR 45-59) is often watched; Stage 3b (30-44) requires more aggressive intervention. Treatment focuses on slowing progression.
Should I see a nephrologist?
Generally recommended for eGFR below 30, rapid decline, significant proteinuria, or unclear cause of kidney disease. Earlier referral may be appropriate for specific situations.
The bottom line on kidney function tests
Kidney function tests provide essential information about kidney health and disease progression. eGFR is the primary functional measure; UACR detects early kidney damage. Treatment depends on the cause and stage of kidney disease. Discuss results with your healthcare provider and consider nephrology referral for significant kidney disease.