Biochemistry · Molecule
Creatinine !
Creatinine is the waste product of muscle creatine. It is made at a steady rate and cleared by the kidneys, so it is the main blood marker of filtration.
Updated 2026-11-01
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Key facts
- Formed non-enzymatically from creatine and phosphocreatine (about 2 % of the pool per day), in proportion to muscle mass.
- Freely filtered by the glomerulus, not reabsorbed and slightly secreted by the tubules, so creatinine clearance slightly overestimates GFR.
- Serum creatinine is used in equations (such as CKD-EPI) that estimate GFR from age and sex.
- Serum creatinine and GFR have an inverse, curved relationship: halving GFR roughly doubles creatinine.
- Formula
- C₄H₇N₃O
- PubChem
- CID 588
Structure
- The cyclic anhydride (lactam) of creatine, formed by loss of water.
Clinical notes
- Creatinine can be low in people with little muscle (older adults, amputees, malnutrition), hiding kidney disease.
- Trimethoprim and cimetidine block tubular secretion and raise creatinine without changing GFR.
- The colorimetric Jaffe method reacts with ketones (acetoacetate) and some cephalosporins, falsely raising the result; enzymatic methods avoid this.
Related conditions
Related
- Creatine
- Creatine, phosphocreatine and creatinine
- Urea
- Spectrophotometry and colorimetry (Beer–Lambert law)
- Creatinine and estimated glomerular filtration rate (eGFR)
- Creatine kinase (CK) and CK-MB as lab tests
Test yourself
Think of the answer, then tap the question to see it.
Why does creatinine clearance slightly overestimate GFR?
Because a small amount of creatinine is secreted by the tubules in addition to being filtered.
Why can a frail older woman have kidney disease with a 'normal' creatinine?
Her low muscle mass produces little creatinine, so its level stays low even when GFR has fallen.
Which drugs raise serum creatinine by blocking its tubular secretion?
Trimethoprim and cimetidine (also some antiretrovirals).
What happens to serum creatinine when GFR falls by half?
It roughly doubles, because production is constant and excretion depends on filtration.
Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Creatinine (CID 588) (PubChem, NCBI, 2026); Creatinine blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Creatinine Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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