Testing kidney function: what do eGFR and creatinine tell you?

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.

What is a normal creatinine or eGFR value?

For creatinine the same applies as for other blood values: the reference range differs per laboratory and relates among other things to sex and muscle mass. The range on your own report is therefore leading. A muscular person naturally sits higher than someone with little muscle mass, without the kidneys working any less well.

The eGFR is calculated by the laboratory and delivered with the result, and already accounts for your age and sex. Here too, interpretation belongs to the whole picture: some decline of eGFR with age is common. Whether a result fits you is explained by our doctor in the report.

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.

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. Cystatin C can be requested as an individual test, but it comes into its own in combination. Creatinine and cystatin C capture the same filtration function along two independent routes, so muscle mass, creatine use or a meat-rich meal cannot skew both results in the same direction. If the two point the same way, the picture is solid. If they diverge, you know it is your creatinine value that needs further interpretation. It therefore makes sense to include both in a single blood draw, for example alongside the Kidney Function package, which measures the remaining kidney values from the same tube.

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 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.