Creatinine is the breakdown product of muscle creatine, filtered freely by the kidneys and barely reabsorbed. Its blood level is the everyday yardstick of kidney function and the raw material of every eGFR equation.
Standard range versus optimal range
| Framework | Creatinine value | Source |
|---|---|---|
| Standard lab reference, men | about 0.7-1.3 mg/dL | Common laboratory range |
| Standard lab reference, women | about 0.6-1.1 mg/dL | Common laboratory range |
| Always interpret as eGFR | ml per min per 1.73 m² | KDIGO guideline |
When it runs high
High creatinine means the kidneys are clearing less, or muscle mass and creatine intake are high. Because the relationship to filtration is curved, a rise from 1.0 to 2.0 signals roughly half the kidney function lost, which is why eGFR tells the truer story.
- Chronic kidney disease from diabetes or hypertension
- Acute kidney injury from dehydration, infection or medication
- Large muscle mass, creatine supplements or a heavy cooked meat meal before the draw
- Drugs that block tubular secretion such as trimethoprim and cimetidine
- Urinary tract obstruction
When it runs low
Low creatinine reflects low muscle mass more often than superb kidney function. In older adults a deceptively low value can hide meaningful kidney impairment, one reason cystatin C is added when the picture is unclear.
- Low muscle mass, frailty or sarcopenia
- Pregnancy, through dilution and higher filtration
- Liver failure reducing creatine synthesis
- Malnutrition
A muscle byproduct doing kidney duty
Creatine powers short bursts of muscle work, and about two percent of it degrades to creatinine each day. The kidneys filter creatinine freely and secrete a little extra, so its blood level rises as filtration falls. The catch is that production scales with muscle mass, so a bodybuilder and a frail grandmother can share an eGFR with very different creatinine values. This is why guidelines insist on reporting eGFR, which mathematically adjusts for age and sex, rather than the raw number.
The curved relationship that fools people
Creatinine does not rise linearly with kidney decline. Early losses of filtration barely move it, and by the time it clearly exceeds the reference range, substantial function is already gone. A rise from 0.6 to 1.2 mg/dL looks small on a report yet represents losing roughly half of filtration capacity. Trends matter more than single values, and any rise of 0.3 mg/dL within days meets the definition of acute kidney injury.
Getting a clean reading
Avoid creatine supplements and unusually large cooked meat meals for a day or two before testing, since both transiently raise the value. When the eGFR from creatinine does not fit the clinical picture, the 2012 CKD-EPI combined equation using both creatinine and cystatin C gives the most accurate estimate and is the recommended confirmatory strategy.
Related calculators
Related biomarkers Cystatin C · Urea · Albumin