How to Read Your Kidney Function Test Results (eGFR, Creatinine, BUN Explained)

Your lab results come back and there it is: eGFR 68. The reference range says >60 is normal, so technically you're fine — but is 68 actually fine? What does eGFR even measure? And why is your creatinine 1.3 when the lab flags 1.2 as the upper limit?

Kidney function tests are among the most commonly ordered labs, yet most people leave the doctor's office with no real understanding of what the numbers mean. This guide explains every major kidney marker, what can push them in the wrong direction without any kidney disease at all, and the specific signals that actually warrant concern.

The 3 Key Kidney Markers

A standard metabolic panel includes three numbers that together give your doctor a picture of how well your kidneys are filtering your blood:

• Creatinine — a waste product filtered out by the kidneys. The raw number in your blood.
• eGFR (estimated Glomerular Filtration Rate) — a calculated estimate of how many milliliters of blood your kidneys filter per minute, adjusted for age, sex, and race.
• BUN (Blood Urea Nitrogen) — another waste product, from protein metabolism. Used alongside creatinine to distinguish different types of kidney stress.

No single number tells the whole story. A high creatinine with a normal eGFR means something different than a low eGFR with a normal creatinine. Understanding how they interact is what separates a false alarm from a real signal.

Creatinine: What It Is and What Affects It

Creatinine is a byproduct of creatine phosphate metabolism in muscle tissue. Your muscles produce it continuously at a rate roughly proportional to your muscle mass, and your kidneys filter it out of your blood.

Normal creatinine ranges are approximately:

• Women: 0.5–1.1 mg/dL
• Men: 0.7–1.2 mg/dL

But here's the critical nuance: creatinine is not a pure kidney signal. Multiple non-kidney factors can push it above the reference range:

• High muscle mass — a 200-pound competitive athlete will have naturally higher creatinine than a sedentary person of the same height. Both may have perfectly healthy kidneys.
• Creatine supplementation — creatine is converted to creatinine. Loading phases (20g/day) can raise creatinine by 0.2–0.5 mg/dL within days.
• Dehydration — concentrates all blood markers including creatinine. Even mild dehydration before a blood draw can falsely elevate results.
• Cooked meat before the test — dietary creatine from a large steak the night before a morning blood draw can temporarily raise creatinine. Raw or plant-based protein does not have the same effect.
• Intense exercise — heavy resistance training in the 24 hours before a test can transiently raise creatinine from muscle breakdown.

A 185-lb weightlifter with creatinine 1.4 mg/dL and eGFR 72 likely has perfectly healthy kidneys. The same value in a sedentary 130-lb woman warrants a follow-up.

eGFR: Estimating Your Filtration Rate

eGFR is not measured directly — it is calculated from your creatinine level using a formula that accounts for age and sex. The current standard is the 2021 CKD-EPI equation, which removed race as a variable (previous versions applied a correction factor for Black patients that is now considered scientifically unsupported).

An eGFR of 100 means your kidneys are filtering approximately 100 mL of blood per minute — roughly normal. The number naturally declines with age: an eGFR of 75 at age 70 is very different from an eGFR of 75 at age 35.

The CKD staging system is based on eGFR:

• Stage 1 (eGFR ≥90): Normal or high filtration. Kidney damage may be present (e.g., proteinuria) but filtration is intact.
• Stage 2 (eGFR 60–89): Mildly reduced. Often asymptomatic; may be normal for older adults.
• Stage 3a (eGFR 45–59): Mildly to moderately reduced. Monitoring and risk factor management begin here.
• Stage 3b (eGFR 30–44): Moderately to severely reduced. Nephrology referral is typically recommended.
• Stage 4 (eGFR 15–29): Severely reduced. Preparing for kidney replacement therapy.
• Stage 5 (eGFR <15): Kidney failure. Dialysis or transplant required.

A single eGFR reading below 60 does not diagnose CKD. CKD requires two readings below 60, at least 3 months apart. A one-time low reading from dehydration or illness is not the same as chronic kidney disease.

BUN and the BUN/Creatinine Ratio

BUN (Blood Urea Nitrogen) measures the nitrogen component of urea, a waste product from protein breakdown in the liver. Normal BUN is 7–20 mg/dL.

BUN alone is a weak signal — it rises with high protein intake, dehydration, and kidney disease alike. Its diagnostic power comes from the BUN/creatinine ratio:

• Normal ratio: 10–20:1
• High ratio (>20:1): Suggests dehydration, heart failure, or upper GI bleeding (blood in the gut is digested as protein, driving BUN up disproportionately).
• Low ratio (<10:1): Suggests liver disease (impaired urea production) or malnutrition (insufficient protein intake to generate BUN).

If your BUN is 30 and creatinine is 1.0, your ratio is 30:1 — that pattern points toward dehydration or GI bleeding rather than primary kidney disease. If BUN is 12 and creatinine is 2.0, the ratio is 6:1 — more consistent with a liver or nutrition issue alongside whatever is causing the elevated creatinine.

Common False Alarms

Most people who search their kidney results in a panic after a routine blood draw fall into one of these categories:

• Muscular people with elevated creatinine. If you lift weights regularly or have above-average muscle mass, a creatinine of 1.3–1.5 mg/dL may be your personal normal. Your eGFR may be calculated as mildly reduced even though your kidneys are filtering at full capacity — the formula does not fully account for high muscle mass.
• Dehydration-lowered eGFR. A single blood draw taken first thing in the morning after a night of poor hydration can drop eGFR by 10–15 points. Retest after a well-hydrated day and the number often returns to baseline.
• Acute illness. Fever, vomiting, diarrhea, and any serious illness temporarily impair kidney filtration. eGFR during a hospitalization or illness is not a reliable baseline — always retest when recovered.

When to Actually Worry

The signals that warrant genuine concern are different from a one-time abnormal result:

• Persistent eGFR <60 on two tests at least 3 months apart. This is the threshold for a CKD diagnosis. One low reading is not enough.
• Proteinuria (protein in urine). Healthy kidneys do not let significant protein through. A urine albumin-to-creatinine ratio (uACR) above 30 mg/g is an early and sensitive marker of kidney damage — often more informative than eGFR alone.
• Rapid decline. An eGFR drop of more than 5 points per year, or more than 10 points in any 12-month period, warrants nephrology referral regardless of the absolute level.
• eGFR <45 on any reading. Even a single result this low should prompt repeat testing promptly (within weeks, not months) and consideration of nephrology referral.

If any of these apply, a nephrology referral is reasonable. Most primary care physicians will initiate this automatically once the threshold criteria are met.

Medications That Affect Kidney Markers

Several common medications alter kidney markers independently of any change in actual kidney function — knowing this prevents unnecessary alarm:

• NSAIDs (ibuprofen, naproxen): Chronic use constricts blood flow to the kidneys, genuinely reducing filtration and raising creatinine. Short-term use has minimal effect in healthy kidneys, but is dangerous in those with pre-existing CKD.
• ACE inhibitors and ARBs (lisinopril, losartan): These drugs intentionally reduce filtration pressure in the kidneys as a protective mechanism. A modest eGFR drop of 10–15% after starting these medications is expected and does not indicate harm — it is actually the desired therapeutic effect.
• Metformin: Does not harm kidneys, but is contraindicated at eGFR <30 because impaired kidneys cannot clear it fast enough, raising the (rare) risk of lactic acidosis. Many guidelines now caution use below eGFR 45.
• IV contrast dye (used in CT scans): Can cause contrast-induced nephropathy, a transient rise in creatinine peaking 48–72 hours post-procedure. In patients with eGFR >45 and normal hydration, the risk is low. Pre-hydration protocols are used in higher-risk patients.

Always tell your doctor about any recent medication changes, contrast procedures, or supplement use when reviewing kidney labs. Context changes the interpretation entirely.

For ongoing monitoring, Vitalix lets you track kidney markers over time alongside your other labs — so you can see trends, not just snapshots. Use our eGFR calculator to estimate your filtration rate, or check your iron status if your doctor is investigating anemia alongside kidney disease. And if you want to understand the full picture of what your labs mean, start with how to read lab results and the tests worth adding to your annual panel.