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[Biochemistry](https://durgical.com/en/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](https://pubchem.ncbi.nlm.nih.gov/compound/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

- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Duchenne muscular dystrophy](https://durgical.com/en/conditions/duchenne-muscular-dystrophy/)

## Related

- [Creatine](https://durgical.com/en/biochemistry/molecules/creatine/)
- [Creatine, phosphocreatine and creatinine](https://durgical.com/en/biochemistry/topics/creatine-phosphocreatine-and-creatinine/)
- [Urea](https://durgical.com/en/biochemistry/molecules/urea/)
- [Spectrophotometry and colorimetry (Beer–Lambert law)](https://durgical.com/en/biochemistry/topics/spectrophotometry-and-colorimetry/)
- [Creatinine and estimated glomerular filtration rate (eGFR)](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/)
- [Creatine kinase (CK) and CK-MB as lab tests](https://durgical.com/en/biochemistry/topics/creatine-kinase-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.

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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)](https://pubchem.ncbi.nlm.nih.gov/compound/588) (PubChem, NCBI, 2026); [Creatinine blood test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003475.htm) (MedlinePlus, US National Library of Medicine, 2026); [Creatinine Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/creatinine-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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