Kidney function: creatinine, eGFR and what your kidneys say about your health

Your kidneys filter about 180 litres of blood a day, and you barely notice a thing. They remove waste, manage your fluid balance and help keep your blood pressure in check. When filtering slowly declines, it often causes no symptoms for a long time. We often see people caught off guard when a blood value is off, because they felt fine. A Lancet estimate suggests that around 1 in 10 adults worldwide has some form of reduced kidney function, and much of it stays unnoticed (Webster 2017).

This overview explains what your kidneys do, what creatinine and eGFR measure, and which values such as urea and uric acid give extra insight. Think of it as the hub: the separate topics are worked out in their own articles below.

What do your kidneys do?

You have two kidneys, each about the size of a fist, in your lower back. Their main job is filtering: they sieve waste and excess fluid from your blood, which you then pass as urine. But they also help regulate blood pressure, your salt and fluid balance and the acidity of your blood, and they make hormones involved in bone health and red blood cell production.

Filtering happens in tiny units called nephrons, roughly a million per kidney. A useful thing to know: your kidneys have spare capacity, so you can function normally for a long time with reduced filtering. That is why blood values often signal earlier than your body does.

Which blood values say something about your kidneys?

A kidney work-up usually revolves around a small set of values. Creatinine and eGFR are central, while urea and uric acid round out the picture.

For what a specific number means, separate articles go deeper: read on about high creatinine, about eGFR explained, or about urea in your blood.

Creatinine and eGFR: the duo that estimates kidney function

Creatinine is a waste product your muscles make all day. Your kidneys filter it out, so creatinine rises when filtering declines. It works like a mirror of kidney function. Still, creatinine alone has a drawback: it also depends on muscle mass, age and sex.

That is why labs convert creatinine into an eGFR, the estimated glomerular filtration rate, which corrects for age and sex (Levey 2009). See also creatinine normal values.

eGFR explained: what the numbers roughly mean

eGFR is expressed in millilitres per minute. Roughly, a higher number means better filtering. Doctors often group the outcome to help place a result.

One number is a snapshot. eGFR can fluctuate through something as simple as drinking too little, fever or heavy exercise before the test. A mildly low result is often repeated after a few weeks (Levey 2012). What a result means for you is a conversation for your GP.

Urea and uric acid: the supporting values

Alongside creatinine and eGFR you often see urea and uric acid. Urea forms during protein breakdown and reacts strongly to how much you drink and eat (Weiner 2015). Uric acid is a breakdown product of purines, and a high level is mostly linked to gout. Read more at uric acid and gout and urea in your blood.

Which symptoms may point to kidney problems?

Kidneys are quiet organs. A mild to moderate drop in filtering often causes no symptoms at all. When symptoms do appear, they are usually non-specific: tiredness, swollen ankles, changes in how often you pass urine, or itchy skin. One symptom on its own says little. It is about the pattern, and whether it persists. Your GP can help decide whether your kidneys play a role.

Who is more likely to have reduced kidney function?

Reduced kidney function can affect anyone, but the two best-known risk factors are high blood pressure and diabetes, because both strain the small blood vessels in the kidneys (Bidani 2004). Kidney disease in the family and cardiovascular disease also play a role. If you recognise this and symptoms persist, it is worth discussing with your GP.

Kidneys and blood pressure are connected

Your kidneys and blood pressure sit in a two-way relationship. Your kidneys help regulate blood pressure via your fluid and salt balance, while high blood pressure can damage the kidneys over time. I expand on this in the article on kidneys and blood pressure.

When to have your kidney function tested?

There is no single rule for everyone. Some people choose to measure their kidney function when kidney disease runs in the family, or when they have high blood pressure or diabetes. A practical tip: drink normally in the days before the draw and avoid heavy exercise beforehand. More in the article on kidney function blood test.

To measure your kidney values without a referral, you can have a kidney function test at Vitalcheck. Every result is reviewed by a BIG-registered doctor.

What can you do about abnormal values?

An abnormal value is not a diagnosis, but a signal. The next step is almost always a conversation with your GP. Kidney values can fluctuate, so a mildly abnormal eGFR is often repeated after a few weeks. To place your result next to other values, see understanding blood test results. Discuss an abnormal result with your GP before changing anything yourself.

References

• Webster AC, et al. Chronic Kidney Disease. Lancet. 2017. PMID: 27887750.
• Levey AS, et al. Chronic kidney disease. Lancet. 2012. PMID: 21840587.
• Levey AS, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009. PMID: 19414839.
• Weiner ID, et al. Urea and Ammonia Metabolism and the Control of Renal Nitrogen Excretion. Clin J Am Soc Nephrol. 2015. PMID: 25078422.
• Nierstichting. How your kidneys work.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.