Why Kidney Function Tests Matter
Your kidneys filter approximately 200 liters of blood every day, removing waste products, balancing electrolytes, regulating blood pressure, and producing hormones that control red blood cell production and bone health. Chronic kidney disease (CKD) affects more than 37 million Americans — and 90% of them don't know they have it, because early stages often cause no symptoms.
Kidney function tests are your early warning system. They're included in the standard Comprehensive Metabolic Panel (CMP) and can be ordered separately as a Basic Metabolic Panel (BMP) or renal function panel. Here's what each marker means.
Creatinine: The Primary Waste Marker
Creatinine is a waste product produced by your muscles as they break down creatine (an energy source). Your kidneys filter creatinine out of the blood continuously. When kidney function declines, creatinine accumulates in the blood.
Normal ranges:
- Men: 0.74–1.35 mg/dL
- Women: 0.59–1.04 mg/dL
Creatinine levels are affected by muscle mass — athletes and larger, more muscular individuals naturally have higher creatinine than smaller or less muscular people. This is why sex-specific reference ranges exist, and why creatinine alone isn't the best standalone marker of kidney function.
A single elevated creatinine should prompt repeat testing. Acute causes include dehydration, intense exercise, or a recent high-protein meal (particularly red meat or creatine supplements, which temporarily boost creatinine).
BUN: Blood Urea Nitrogen
BUN measures urea nitrogen, a waste product formed when the liver breaks down protein. Urea is filtered by the kidneys and excreted in urine.
Normal range: 7–20 mg/dL.
BUN is less specific to kidney function than creatinine because it's influenced by many non-renal factors:
- High BUN with normal creatinine often indicates dehydration, a high-protein diet, or upper GI bleeding
- High BUN with high creatinine suggests genuine kidney impairment
- Low BUN can occur with low-protein intake, liver disease (reduced urea production), or overhydration
BUN/Creatinine Ratio
The BUN/Creatinine ratio is calculated to help distinguish between kidney damage and pre-renal causes (problems with blood flow to the kidneys):
- Ratio > 20:1 suggests pre-renal azotemia — dehydration, heart failure, or reduced blood flow to the kidneys
- Ratio 10–20:1 is generally normal
- Ratio < 10:1 suggests low protein intake, liver disease, or conditions that reduce urea production without affecting creatinine
eGFR: The Most Important Kidney Number
eGFR (Estimated Glomerular Filtration Rate) is the single most important number for assessing kidney function. It estimates how many milliliters of blood your kidneys filter per minute per 1.73 m² of body surface area.
eGFR is calculated from your creatinine, age, sex, and sometimes race using validated equations. It's automatically included on most lab reports that measure creatinine.
eGFR Stages of Kidney Disease:
| eGFR (mL/min/1.73 m²) | Stage | Interpretation |
|---|---|---|
| ≥ 90 | Normal or High | Normal kidney function |
| 60–89 | Stage G2 | Mildly decreased |
| 45–59 | Stage G3a | Mildly to moderately decreased |
| 30–44 | Stage G3b | Moderately to severely decreased |
| 15–29 | Stage G4 | Severely decreased |
| < 15 | Stage G5 | Kidney failure |
CKD is defined as eGFR below 60 mL/min/1.73 m² for three months or longer, or evidence of kidney damage (protein in urine) regardless of eGFR. A single low eGFR reading isn't a diagnosis — persistence over time is what defines the condition.
eGFR naturally declines with age. The average eGFR at age 70 is approximately 65–75 mL/min/1.73 m², compared to 100–120 in a healthy young adult.
Urinalysis and Urine Albumin: The Missing Piece
The blood tests above don't tell the whole story. Protein in the urine (proteinuria) — particularly albumin — is an early sign of kidney damage that can appear years before creatinine or eGFR become abnormal. A urine albumin-to-creatinine ratio (uACR) greater than 30 mg/g indicates significant proteinuria and is a key diagnostic criterion for CKD.
If your eGFR is borderline or your doctor suspects early kidney disease, a urine test is a critical companion to blood tests.
What Can Damage Your Kidneys
The two leading causes of CKD are:
- Diabetes — chronic high blood sugar damages the kidneys' filtering units over time (diabetic nephropathy). This is why A1C monitoring matters — see our A1C results guide. People managing diabetes or other ongoing conditions can learn more about tracking multiple lab values in our guide to managing chronic conditions with digital health tools.
- Hypertension — chronically elevated blood pressure stiffens and narrows the arteries supplying the kidneys
Other causes include:
- Autoimmune conditions (lupus, IgA nephropathy)
- Certain medications (long-term NSAID use, some antibiotics)
- Recurrent kidney infections or stones
- Inherited conditions (polycystic kidney disease)
Protecting Your Kidney Health
Even in the early stages of CKD, several interventions can slow progression:
- Blood pressure control (target below 130/80 mmHg for people with CKD)
- Blood sugar management for people with diabetes
- Avoiding NSAIDs like ibuprofen and naproxen, particularly at high doses
- Staying well hydrated (water, not sugary drinks)
- Moderating protein intake — very high protein diets add load to damaged kidneys
- Medications (ACE inhibitors, ARBs) that specifically protect kidney function in people with diabetes or protein in the urine
Tracking Kidney Function Over Time
A kidney function panel taken once is a snapshot. The trajectory over 6–12 months tells you whether function is stable, slowly declining, or rapidly changing. Tracking your lab trends is especially critical for kidney health — small month-to-month changes accumulate into meaningful disease progression.
MediSphere™ stores your kidney function history and uses AI to visualize your eGFR trend, flag declining values, and explain what your specific numbers mean in context. For the full metabolic picture, see our CMP results guide — kidney markers appear there as part of the panel. For a complete overview of blood test interpretation, see our guide to understanding your lab results. Visit How It Works to see how MediSphere™ brings your health data together.
This article is for educational purposes only. Always discuss your specific results with your healthcare provider.