Creatinine is a breakdown product of creatine phosphate in muscle. It is produced at a steady rate and cleared by the kidneys, so its blood level is used to judge kidney function.
But there is a crucial caveat: creatinine depends on muscle mass. In an athlete it can look "high" with perfect kidneys, while in a slim elderly woman it can stay "normal" even when function is already reduced. That is why creatinine is almost never interpreted in isolation — GFR is calculated instead.
Reference ranges
Approximate reference for adults (venous blood):
| Men | 62 – 106 µmol/L |
| Women | 44 – 80 µmol/L |
| Bottom line | interpret only together with eGFR |
What an elevated level means
A high creatinine most often means a decline in the kidneys' filtration function. But before you panic, rule out reversible and "false" causes.
Not kidney-related: large muscle mass, an intense strength workout the day before, a heavy meat meal, creatine supplements, dehydration, some medications.
Kidney causes: chronic kidney disease, acute kidney injury, glomerulonephritis, diabetic and hypertensive nephropathy.
What to do: recalculate eGFR, repeat the test under calm conditions, and if confirmed — see a nephrologist.
What a low level means
A low creatinine is usually not worrying and almost always reflects low muscle mass: in the elderly, in sarcopenia, in vegans, during prolonged bed rest, or in pregnancy (blood volume and filtration rise).
The danger of low creatinine is not the value itself but that it masks a decline in kidney function: in a person with low muscle mass, creatinine can look "normal" even though eGFR is already low.
How the test is done
Venous blood. Preferably fasting, in the morning, at rest.
For 24–48 hours, avoid: intense strength training, large amounts of meat, creatine supplements. Do not test while dehydrated — the result will be inflated. Always ask the lab to calculate eGFR rather than looking at bare creatinine.
What affects the value
- Muscle mass — the main non-kidney factor: the more muscle, the higher the creatinine.
- Strength training the day before — temporarily raises the level.
- Meat and creatine — raise it.
- Hydration — dehydration inflates creatinine.
- Age — over the years both muscle and filtration decline.
- Medications — NSAIDs, some antibiotics and ACE inhibitors affect the level.
Calculate using this marker
Calculators where Creatinine is used directly:
Frequently asked questions
Creatinine is high — is it definitely the kidneys?
No. First rule out muscle mass, a strength workout the day before, a meat meal, creatine supplements and dehydration — these are the most common causes of a "false" rise. Recalculate eGFR and repeat the test under calm conditions.
Why do doctors look at GFR rather than creatinine?
Because creatinine depends on muscle mass, age and sex. The CKD-EPI formula converts it into a glomerular filtration rate that accounts for these factors — and only eGFR lets you stage chronic kidney disease.
Is a low creatinine a good thing?
Not necessarily. Most often it simply reflects low muscle mass. The problem is that it can mask a genuine decline in kidney function — eGFR turns out to be low despite a "normal" creatinine.