Creatinine, Blood Biomarker

Overview

Blood creatinine, usually reported as serum creatinine, measures the concentration of creatinine, a waste product generated from muscle metabolism and cleared almost entirely by the kidneys.
Because creatinine production is relatively constant and its excretion depends on glomerular filtration, serum creatinine is a key indicator of kidney function.
High serum creatinine levels typically suggest impaired kidney function, reduced renal blood flow, urinary tract obstruction, or systemic conditions that affect the kidneys.
Low levels are less common and may reflect reduced muscle mass or certain neuromuscular conditions.
Clinically, serum creatinine is widely used to screen for and monitor chronic kidney disease, estimate glomerular filtration rate (eGFR), and guide dosing of renally cleared medications.

Clinical Use Cases

Assess overall kidney function and detect chronic kidney disease and acute kidney injury.
Estimate glomerular filtration rate (eGFR) using creatinine-based equations to stage kidney disease.
Monitor progression of known kidney disorders and response to treatments.
Evaluate potential nephrotoxicity from medications or contrast agents and guide drug dosing adjustments.
Aid diagnosis of conditions affecting renal perfusion or causing urinary tract obstruction (for example, heart failure, dehydration, obstructive uropathy).
Contribute to comprehensive metabolic or basic metabolic panels for general health assessment.
Support evaluation of systemic diseases such as diabetes and hypertension that can impair kidney function.

Specimen Types

Venous blood (serum), collected by standard venipuncture, is the primary specimen for serum creatinine.
Plasma (for example, lithium heparin plasma) in some clinical laboratories.
Paired serum and 24‑hour urine specimens when creatinine clearance is measured to estimate GFR.

Measurement Methods

Enzymatic creatinine assays (specific enzymatic methods recommended for clinical use, especially for estimating GFR).
Jaffe method (reaction of creatinine with alkaline picrate, an older colorimetric method with more interferences).
Photometric / spectrophotometric determination of absorbance change after the creatinine reaction.
IDMS‑traceable methods (isotope dilution mass spectrometry–standardized creatinine measurements used as reference to improve accuracy of eGFR calculations).

Test Preparation and Influencing Factors

Fasting: Some clinicians may request overnight fasting, especially when serum creatinine is part of a metabolic panel, though fasting is not universally required.
Diet: Recent large meals of cooked red meat can transiently increase serum creatinine; some protocols advise avoiding meat for a period before testing to improve accuracy.
Supplements: Creatine supplements and other muscle‑building compounds can raise serum creatinine independently of kidney function; stopping creatine use before testing is often recommended.
Physical activity: Heavy exercise in the preceding 24 hours can modestly increase creatinine.
Muscle mass and body composition: Higher muscle mass increases baseline creatinine, while low muscle mass, malnutrition, amputation, or neuromuscular disease lower it, affecting interpretation.
Age, sex, and race/ethnicity: Reference ranges vary with age, sex, and sometimes race, largely due to differences in average muscle mass.
Medications affecting tubular secretion or assay interference:
- Trimethoprim‑sulfamethoxazole and cimetidine can increase serum creatinine by inhibiting tubular secretion without true GFR change.
- Certain drugs and substances (for example, cephalosporins, nitromethane, bilirubin, glucose, ketones) can interfere with Jaffe assays and alter reported creatinine.
Hydration status and acute illness: Dehydration, heart failure, and acute changes in renal perfusion can acutely raise creatinine.
Pregnancy: Pregnancy‑related disorders such as preeclampsia and eclampsia can be associated with abnormal creatinine values reflecting kidney involvement.