On almost every blood result with kidney values are two things: a creatinine and an eGFR. They belong together, because the eGFR is calculated from your creatinine. Together they give a doctor a first impression of how well your kidneys filter.
The nice thing is that with a little explanation you can place these values much better yourself. Then you do not needlessly take fright at a number that may simply fit your body.
What is creatinine?
Creatinine is a waste product formed in the normal working of your muscles. Your body makes it continuously, and your kidneys filter it from your blood. If your kidneys work less well, more creatinine stays in your blood and the value rises.
Because creatinine comes from your muscles, your value partly depends on your muscle mass. A muscular man naturally often has a higher creatinine than a slight person, without anything being wrong. The broader picture is in our pillar on kidney function in men.
What is the eGFR?
The eGFR stands for the estimated filtration rate of your kidneys. The lab calculates it from your creatinine, your age and your sex. In a single number, the eGFR gives an estimate of how well your kidneys filter: the higher, the better.
| Value | What it is | How you read it |
|---|---|---|
| Creatinine | Measured waste from your muscles | Higher can point to less filtering, but also to a lot of muscle mass |
| eGFR | Calculated filtration rate | Higher is better; a drop over time is telling |
You will find both as individual markers: creatinine and eGFR. Often there is also urea, another waste product.
Why do they belong together?
Creatinine alone does not say much, because it depends on your muscle mass, age and sex. The eGFR partly corrects for that, and is therefore often more usable for a doctor to estimate kidney function. That is why you best read them together.
Still, the eGFR remains an estimate. In very muscular men or with little muscle mass, it can over- or underestimate reality somewhat. A doctor takes that into account.
Why can muscle and meat colour my value?
This surprises many people. A protein-rich meal, especially a lot of meat, just before a blood test can temporarily raise your creatinine. Heavy training just before can do that too. Neither means your kidneys work worse.
If you want a clean picture, better avoid heavy training and a big meat meal just before the test. If you do, mention it, so a doctor reads your creatinine in the right context. If you train intensely and use creatine, also read our piece on protein, creatine and your kidneys.
What if my eGFR is lowered?
A one-off slightly lowered eGFR without symptoms is often no reason for panic, but it is a reason to repeat and check the cause. According to Thuisarts.nl, the trend over time says more than a single measurement, and a doctor also looks at your blood pressure and your urine.
A markedly lowered eGFR or a falling trend calls for further checks with a doctor. The Kidney Function panel maps your values, but the explanation and follow-up belong with your doctor.
Frequently asked questions
Is a high creatinine always bad?
No. A lot of muscle mass, a meat-rich meal or creatine use can raise your creatinine without anything being wrong. That is why a doctor mainly looks at the eGFR and the trend, and places your value in the context of your body and lifestyle.
Do I need to fast for a kidney test?
For creatinine and eGFR you usually do not need to fast strictly, but a big meat meal just before is better skipped. If in doubt, follow the advice with your test or ask your doctor. That way you avoid a distorted picture.
Does my eGFR change with age?
Yes, a slight drop in the eGFR with ageing is normal and need not mean disease. That is why a doctor looks at your age and at the trend, not only at a threshold value. A stable, slightly lower eGFR at an older age is different from a rapidly falling one.
Sources
- Thuisarts.nl. I have a reduced kidney function.
- Nierstichting. What your kidney values say.
- NHG (Dutch College of General Practitioners). NHG guideline Chronic kidney damage.
Disclaimer
Caliberhealth works with BIG-registered doctors who review your blood results. This article provides general information and is not a substitute for medical advice from a GP or specialist. A blood test is a tool, not a diagnosis in itself. For serious or persistent symptoms, contact your GP, or call 112 in an emergency.
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