Creatinine and eGFR: How to Read Your Kidney Function Test

Creatinine is generated by skeletal muscle at a roughly constant rate. The kidneys filter it out continuously, so its blood level depends on the balance between production (muscle mass) and clearance (kidney function).

Two key implications:

• Muscle mass matters. A muscular bodybuilder can have a "high" creatinine on paper with completely normal kidneys. A frail older adult can have a "normal" creatinine with substantial kidney impairment because they are producing less.
• Recent meat intake raises creatinine for several hours. A heavy steak dinner the night before a fasting blood draw can shift the result modestly upward.

This is why eGFR — which adjusts for these factors as best it can — is the more useful number. eGFR is calculated automatically from creatinine, age, and sex on most lab panels.

Serum creatinine reference ranges (typical adult):

• Men: 0.74–1.35 mg/dL (65–119 µmol/L)
• Women: 0.59–1.04 mg/dL (52–92 µmol/L)

eGFR categories (CKD staging):

• Above 90 mL/min/1.73m²: normal kidney function (G1).
• 60–89: mildly decreased — only meaningful if albuminuria or other kidney damage is present (G2).
• 45–59: mild-to-moderate decrease (G3a) — chronic kidney disease.
• 30–44: moderate-to-severe decrease (G3b).
• 15–29: severe decrease (G4) — pre-dialysis preparation typically begins.
• Below 15: kidney failure (G5) — dialysis or transplant territory.

CKD diagnosis requires either an eGFR below 60 sustained for at least 3 months, or evidence of kidney damage (albuminuria, abnormal imaging, or biopsy findings). A single low eGFR is not CKD.

High creatinine — and correspondingly low eGFR — can be acute or chronic.

Acute kidney injury (AKI):

• Dehydration, vomiting, diarrhea, blood loss.
• Heart failure or sepsis reducing kidney perfusion.
• Medications: NSAIDs (ibuprofen, naproxen), ACE inhibitors and ARBs in setting of dehydration, contrast dye, certain antibiotics (gentamicin, vancomycin).
• Obstruction — kidney stone, prostate enlargement, retroperitoneal fibrosis.
• Rhabdomyolysis — severe muscle injury releases creatinine and myoglobin.

Chronic kidney disease causes:

• Diabetes — the leading cause globally.
• Hypertension — the second leading cause.
• Glomerulonephritis, polycystic kidney disease, autoimmune kidney disease.
• Long-term NSAID use.
• Recurrent kidney infections or kidney stones.

Causes that look like CKD but are not:

• High muscle mass — bodybuilders, very lean and muscular individuals.
• Creatine supplementation raises serum creatinine modestly.
• Recent intense exercise can raise creatinine for 24–48 hours.
• Trimethoprim, cimetidine, fenofibrate block tubular secretion of creatinine and raise the lab number without affecting actual kidney function.

For these reasons, a single elevated creatinine is rarely sufficient to diagnose kidney disease. Repeat in 2–4 weeks. If still elevated, urine albumin/creatinine ratio (UACR) and renal ultrasound are usually next.

Low creatinine usually reflects low muscle mass — older adults, malnutrition, prolonged bed rest, severe weight loss, advanced liver disease, or pregnancy. It is rarely a problem in itself but can mask kidney disease, since the eGFR formulas assume average muscle mass.

In a thin older adult, a "normal" creatinine of 0.9 mg/dL may correspond to an eGFR substantially worse than the formula suggests. Cystatin C — a separate filtration marker independent of muscle mass — is the right second test in these patients.

The single most useful thing you can do with kidney labs is track them. Stable eGFR over years is reassuring even at 70–80. A creeping decline of 3–5 mL/min/year is the signature of progressive kidney disease and is what drives interventions like ACE inhibitors, SGLT2 inhibitors, and tighter blood pressure control.

The 2026 KDIGO guidelines emphasize:

• Annual screening with eGFR plus urine albumin/creatinine ratio (UACR) for all adults with diabetes, hypertension, cardiovascular disease, or family history of kidney disease.
• SGLT2 inhibitors (empagliflozin, dapagliflozin) for kidney protection in CKD with proteinuria, even in non-diabetic patients.
• Finerenone for diabetic kidney disease with persistent albuminuria.
• The race-free CKD-EPI 2021 equation — older calculations that included a race coefficient under-estimated CKD in Black patients and have been retired.

• eGFR below 60 on two tests at least 3 months apart — diagnostic for CKD; nephrology referral usually appropriate at G3b and below.
• Acute drop in eGFR or rise in creatinine of more than 50% — investigate immediately. Dehydration, medications, obstruction.
• UACR above 30 mg/g — kidney damage marker; treat with ACE inhibitor or ARB regardless of eGFR.
• UACR above 300 mg/g — significant proteinuria; SGLT2 inhibitor and specialist referral.
• Any eGFR below 30 — pre-dialysis preparation; nephrology mandatory.

• eGFR — calculated from creatinine, age, and sex on the same panel.
• Urine albumin/creatinine ratio (UACR) — detects kidney damage before eGFR drops; the right early-warning test for CKD.
• BUN (blood urea nitrogen) — second filtration marker; the BUN/creatinine ratio gives clues about hydration and prerenal vs intrinsic causes.
• Cystatin C — alternative filtration marker independent of muscle mass; useful when creatinine is unreliable.
• Electrolytes (sodium, potassium, bicarbonate, phosphorus, calcium) — often disturbed in advanced CKD.
• Urinalysis — protein, blood, casts; gives clues about glomerular vs tubular disease.