In short
- Creatinine is a waste product from your muscles that is measured in the blood.
- The eGFR is not a measurement but a calculation based on creatinine, age and sex.
- Your muscle mass influences the creatinine value, which can make the result misleading.
- Kidney damage causes no complaints for a long time, because the kidneys have plenty of reserve capacity.
What your kidneys do all day
Your two kidneys filter your blood day and night. Together they contain around two million tiny filtering units that remove waste products and surplus fluid from the blood and pass them out as urine. Filtering is their best-known task, but it is not their only one. Your kidneys also do this:
- Keep the balance of water and salts such as sodium and potassium in order
- Play a part in regulating your blood pressure
- Produce a hormone that drives the production of red blood cells
- Convert vitamin D into the active form your bones need
How well the kidneys filter is expressed as the filtration rate, known as the glomerular filtration rate or GFR. Measuring that rate directly is possible, but it is laborious and happens mainly in hospital.
In practice the filtration rate is therefore estimated on the basis of a substance in your blood. That is why a kidney function test is usually just a blood draw, supplemented by urine testing when there is reason for it.
Creatinine: a waste product as a yardstick
Creatinine is a waste product formed by the normal breakdown of creatine in your muscles. Your muscles produce it at a fairly constant rate, after which the kidneys filter it out of the blood and pass it out with the urine. If the kidneys filter less well, more creatinine remains in the blood.
That is precisely why creatinine has been the standard marker for decades: it is cheap, widely available and usually reliable.
The weak spot lies at the start of that story. The amount of creatinine you produce depends on how much muscle mass you have. A strength athlete naturally produces more and can have an elevated value while the kidneys work perfectly well. Someone with little muscle mass, for example at an older age or with a low body weight, may have a reassuring value while filtration is already reduced.
These circumstances can also lift the value temporarily:
- A large portion of meat shortly before the draw
- Creatine supplements
- A heavy training session
- Dehydration
eGFR: an estimate, not a measurement
The eGFR on your result is not measured but calculated. A formula combines your creatinine value with your age and sex to arrive at an estimate of the filtration rate. The letter e stands for estimated.
That estimate works well for most people, but it inherits all the limitations of creatinine. With high or low muscle mass, an unusual build or a markedly different diet, the eGFR can be off.
On top of that, a single result is rarely the whole story. For kidney function the trend over time usually says more than one measurement, and a one-off deviation mainly calls for a repeat under settled conditions.
Why kidney damage stays silent for so long
Kidneys have considerable reserve capacity. If part of the filtering units becomes damaged, the rest take over the work. As a result, filtration can already have declined substantially before you notice anything.
And when complaints do appear, they are vague:
- Tiredness
- Less appetite
- Itching
- Swollen ankles
- Passing urine at night
- Poor concentration
Each of those you easily attribute to something else. That is exactly why reduced kidney function is often discovered by chance, during blood testing done for an entirely different reason. Early insight into the trend is valuable, because at that stage there is often still a lot to be done about the underlying causes.
Who is at higher risk?
The two best-known causes of chronic kidney damage are high blood pressure and diabetes. In both, the small blood vessels in the kidneys are under strain for years. You are also at higher risk in these situations:
- Cardiovascular disease
- Kidney disease in the family
- Excess weight
- Smoking
- Repeated urinary tract infections or kidney stones
- An older age
Medication use is a category of its own. Anti-inflammatory painkillers such as ibuprofen, diclofenac and naproxen, together the NSAIDs, place a load on the kidneys when you use them long term or in high doses. Some blood pressure medicines, water tablets and antibiotics also call for kidney function monitoring.
If you take medicines long term, ask your GP or pharmacist how often testing is sensible in your case.
Have your kidney function measured
The Kidney Function package maps your kidney values in a single blood draw, including doctor interpretation and a personal PDF report. Price excludes €29 blood draw fee, which is waived from €225.
Cystatin C: more accurate when creatinine falls short
Cystatin C is a small protein that virtually all cells in your body produce at a constant rate. That protein is also filtered out of the blood by the kidneys, but its production barely depends on your muscle mass, sex or diet.
As a result cystatin C gives a more stable picture of filtration function, precisely in the groups where creatinine is hardest to interpret:
- Strength athletes and bodybuilders
- Older people
- People with a low body weight
- People taking creatine
When a creatinine result falls just outside the range and you want to know whether something is really behind it, cystatin C also helps to confirm or dismiss that. As an individual test, cystatin C costs €89, including the blood draw fee and doctor interpretation. The combination of creatinine and cystatin C usually gives the most reliable picture.
| Test | What is it? | Strength and limitation |
|---|---|---|
| Creatinine | A waste product from your muscles, measured in the blood | Cheap and widely available, but depends on your muscle mass |
| eGFR | A calculation from creatinine, age and sex | Easy to read, but inherits the limitations of creatinine |
| Cystatin C | A protein that virtually all cells produce at a constant rate | Barely dependent on muscle mass, sex or diet |
When a blood test is not the answer
A blood test is a snapshot of filtration, not a complete kidney examination. Protein or albumin in the urine is often the earliest sign of kidney damage, and you will not see that in your blood. With a raised risk, urine testing therefore belongs alongside it, and your GP arranges that.
If you suddenly pass much less urine, see blood in your urine, have a fever with pain in your flank, or if your legs and eyelids swell quickly, do not wait for a blood result but contact your GP straight away.
If you are already being treated for a kidney condition, keep to the monitoring schedule set by your doctor. A one-off test on your own initiative does not replace those checks.
Frequently asked questions
What is the difference between creatinine and eGFR?
Creatinine is a waste product that is actually measured in your blood. The eGFR is not a measurement but a calculation: a formula converts your creatinine, together with your age and sex, into an estimate of the filtration rate. That makes the eGFR easier to interpret, but it inherits the limitations of creatinine.
Do I need to fast for a kidney function test?
Usually not. It is wise to have drunk enough, to avoid heavy strength training in the 24 hours before the draw and not to eat a large portion of meat shortly beforehand. All three can temporarily influence your creatinine value. Always follow the instruction you receive with your appointment.
Can my kidney function recover?
That depends on the cause. A temporary decline caused by dehydration, an infection or medication can recover completely. With long-standing damage from, for example, high blood pressure or diabetes, preserving what is there is the aim, helped by blood pressure and blood sugar treatment, drinking enough and being cautious with anti-inflammatory painkillers. Your doctor decides what suits your situation.
How often should I have my kidney function checked?
Without risk factors and without complaints, periodic checks are rarely necessary. If you have high blood pressure, diabetes or cardiovascular disease, or if you take medicines long term that load the kidneys, a fixed testing interval is customary. Your GP tailors that to your situation.
Do I need a referral?
No. At Bloodworks you have your kidney values tested without a referral, at one of the more than 750 affiliated collection points in the Netherlands. Your result is reviewed by a doctor and you receive a personal PDF report, typically within 2 working days.