Unit Converter
Creatinine
(Marker of Kidney Function, GFR Estimation & Muscle Metabolism)
Synonyms
- Creatinine
- Serum creatinine
- Plasma creatinine
- Blood creatinine
- Cr
- Creatinine concentration
Units of Measurement
- mmol/L
- µmol/L
- mg/dL
- mg/100 mL
- mg%
- mg/L
- µg/mL
1 mg/dL = 88.4 µmol/L
1 mg/L = 0.1 mg/dL
µg/mL = mg/L
Description
Creatinine is a breakdown product of creatine phosphate in muscle.
It is produced at a constant rate proportional to muscle mass and is excreted almost entirely by the kidneys (glomerular filtration).
Because of this:
- Serum creatinine is a key marker of kidney function.
- Creatinine is essential for estimation of GFR using eGFR formulas (CKD-EPI, MDRD).
Creatinine rises only after substantial loss of GFR, making it a late but reliable marker of chronic or acute kidney injury.
Physiological Role
Creatinine itself has no metabolic function.
It reflects:
- Muscle mass
- Renal filtration efficiency
Not significantly affected by diet except high meat intake.
Clinical Significance
High Creatinine (Most Important)
Indicates reduced kidney function or increased production.
1. Acute Kidney Injury (AKI)
- Pre-renal (dehydration, shock)
- Intrinsic renal (ATN, glomerulonephritis)
- Post-renal (obstruction)
2. Chronic Kidney Disease (CKD)
Progressive decline in GFR.
Creatinine is used for:
- Staging CKD
- Monitoring progression
- Treatment decisions
3. Increased Muscle Breakdown
- Rhabdomyolysis
- Intense exercise
- Muscular dystrophies
- Crush injuries
4. Drugs Affecting Tubular Secretion
- Trimethoprim
- Cimetidine
- DTG/RTG
Cause pseudo-elevation.
5. Large Muscle Mass
- Athletes
- Bodybuilders
Higher baseline creatinine.
Low Creatinine
Usually benign.
Seen in:
- Low muscle mass (elderly, cachexia)
- Pregnancy (↑ GFR)
- Severe liver disease
Low levels rarely indicate pathology.
Reference Intervals
(Tietz 8E + KDIGO + Mayo + ARUP)
Adults
- Men: 0.74 – 1.35 mg/dL (65 – 120 µmol/L)
- Women: 0.59 – 1.04 mg/dL (52 – 92 µmol/L)
Children
Lower values; varies with age and muscle mass.
Critical Values
- > 4 mg/dL → significant renal impairment
- > 10 mg/dL → severe renal failure
Creatinine & GFR (Key Clinical Use)
eGFR Formulas
- CKD-EPI 2021 (current standard)
- MDRD
- Cockcroft–Gault (for drug dosing)
eGFR <60 mL/min/1.73m² for ≥3 months = CKD.
Diagnostic Uses
1. Kidney Function Assessment
- AKI detection (KDIGO criteria)
- CKD staging
- Monitoring renal recovery
- Drug dose adjustment
2. eGFR Calculation
Creatinine is essential for computerized eGFR reporting.
3. Muscle Injury
Monitoring CK + creatinine in:
- Rhabdomyolysis
- Crush injury
- Myopathies
4. Dialysis Decisions
Rising creatinine helps determine timing of:
- Hemodialysis
- Peritoneal dialysis
5. Renal Toxicity from Drugs
- Aminoglycosides
- Amphotericin
- Contrast agents
- Calcineurin inhibitors
Analytical Notes
- Serum preferred
- Avoid hemolysis
- Meat intake & exercise may transiently raise creatinine
- Jaffe method may give false results (bilirubin, hemolysis, cephalosporins)
- Enzymatic methods more accurate
- eGFR should always accompany creatinine on reports
Clinical Pearls
- A small rise in creatinine may reflect a large drop in GFR.
- Creatinine lags by 24–48 hours in AKI - cystatin C rises earlier.
- High creatinine with normal urine output can still be serious (non-oliguric AKI).
- Elderly may have “normal” creatinine but low GFR due to low muscle mass.
- Always interpret creatinine with BUN, urinalysis, and electrolytes.
Interesting Fact
Despite being one of the oldest lab tests, serum creatinine remains the backbone of kidney medicine and is used in almost every diagnostic scoring system for renal function.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Renal Function Tests.
- KDIGO Clinical Practice Guidelines for AKI & CKD.
- National Kidney Foundation - GFR & Creatinine.
- IFCC Reference Methods for Creatinine.
- Mayo Clinic Laboratories - Serum Creatinine.
- ARUP Consult - Kidney Function Testing.