Creatinine is a waste product made by your muscles and cleared almost entirely by your kidneys. Its concentration in blood reflects how well the kidneys are filtering — when filtration drops, creatinine rises.
The number that matters more than creatinine itself is eGFR (estimated glomerular filtration rate), which converts creatinine into milliliters per minute of filtration, accounting for age and sex. Modern guidelines (KDIGO 2024, NKF-ASN 2021) use the race-free CKD-EPI 2021 equation, which removed the historical race adjustment that systematically under-estimated kidney disease in Black patients.
What creatinine measures
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.
Creatinine and eGFR ranges
| Demographic | Low | High | Unit |
|---|---|---|---|
| Men — typical | 0.74 | 1.35 | mg/dL |
| Women — typical | 0.59 | 1.04 | mg/dL |
| eGFR — normal (G1) | 90 | 200 | mL/min/1.73m² |
| eGFR — mildly decreased (G2) | 60 | 89 | mL/min/1.73m² |
| eGFR — moderate CKD (G3) | 30 | 59 | mL/min/1.73m² |
| eGFR — severe CKD / failure | 0 | 29 | mL/min/1.73m² |
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.
What high creatinine means
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.
What low creatinine means
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.
Reading kidney function over time
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.
Track this biomarker over time in AskAnything.health — upload your lab results and see trends at a glance.
When to act on kidney labs
- 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.
This information is for educational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider about your lab results.
Tests that complete the kidney picture
- 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.