What Is Creatinine in a Blood Test?

Creatinine is a waste product from muscle metabolism. Your kidneys filter it constantly, making it a reliable marker of kidney function.

Normal Range

• Men: 0.7–1.3 mg/dL
• Women: 0.6–1.1 mg/dL

High Creatinine Causes

• Chronic kidney disease — progressive loss of filtering capacity
• Acute kidney injury — sudden drop from dehydration, medications, infection
• High muscle mass — bodybuilders may have naturally higher creatinine
• High meat intake
• Rhabdomyolysis — massive muscle breakdown
• Medications — NSAIDs, ACE inhibitors

Low Creatinine Causes

• Low muscle mass — elderly, cachectic patients
• Pregnancy — increased GFR dilutes
• Liver disease
• Malnutrition

Creatinine and eGFR

• eGFR is calculated from creatinine + age + sex
• A muscular young man with creatinine 1.2 may have normal kidneys
• A frail elderly woman with creatinine 1.0 may have reduced function
• Always interpret with eGFR, not creatinine alone