Creatinine and Kidney Function: What Your Blood Test Results Actually Mean

Creatinine is a waste product your muscles produce, which healthy kidneys filter out of your blood at a fairly constant rate. A blood test measures it to calculate your eGFR, an estimate of how well your kidneys are filtering. A single raised result doesn't confirm kidney disease. It's a starting point for further checks, not a diagnosis, and it should always be read alongside your age, muscle mass and, ideally, a previous result for comparison.

The question I get asked most after a kidney function test is simple: is 90 good or bad? People assume one number should tell them everything. It doesn't. Creatinine only really makes sense next to your eGFR, your age, your muscle mass, and ideally a previous result to compare against. I've seen genuinely fit, muscular patients flagged with a slightly reduced eGFR purely because they have more muscle than average, and slighter, older patients with a completely normal-looking eGFR that's actually hiding a real decline.

What is creatinine and what does it actually measure?

Creatinine is produced constantly as your muscles break down and rebuild a molecule called creatine. It's released into your bloodstream at a fairly predictable rate for any given person, which is exactly what makes it useful as a marker.

Your kidneys filter creatinine out of your blood and pass it into urine. Healthy kidneys do this efficiently and consistently, so your blood creatinine level stays within a fairly narrow range from day to day.

When kidney function drops, creatinine isn't cleared as efficiently, so it builds up in the blood. Creatinine itself doesn't cause harm at these levels. It's simply a convenient, measurable stand-in for something much harder to measure directly: how much blood your kidneys are filtering per minute.

What does a raised creatinine level mean?

A raised creatinine can indicate reduced kidney filtration, but it isn't the only explanation. Dehydration, a high-protein meal shortly before testing, creatine supplements, and recent intense exercise can all push creatinine up temporarily without any underlying kidney problem.

Having more muscle mass than average also raises your baseline creatinine, simply because you're producing more of it. This is one of the most common reasons a genuinely healthy, muscular person gets an unexpectedly high result.

Evidence summary

NICE guidance (NG203) notes that eGFR calculated from creatinine can be less reliable during acute illness, in pregnancy, in people with reduced muscle mass, and in those who are malnourished or taking creatine or protein supplements. In these situations, an alternative marker called cystatin C may be used to confirm the picture.

What does a low creatinine level mean?

A low creatinine is far less commonly discussed, but it usually reflects lower muscle mass rather than a problem with the kidneys themselves. This is common in older adults, people who are frail, and those with reduced mobility.

This matters clinically because low muscle mass can mask early kidney decline. Someone producing very little creatinine to begin with can lose a meaningful amount of kidney function before their creatinine rises high enough to look abnormal.

Creatinine or eGFR: which number should you actually look at?

Your creatinine result is a raw number in micromoles per litre. Your eGFR, or estimated glomerular filtration rate, is a calculation that takes that creatinine and adjusts it for your age and sex, estimating how many millilitres of blood your kidneys filter each minute.

In 2021, NICE updated its recommended formula, the CKD-EPI creatinine equation, to remove race as a variable. This is a genuinely significant change. It means eGFR is now calculated the same way for everyone tested, rather than being adjusted differently depending on ethnicity, which earlier equations did.

Two people with an identical creatinine result can end up with quite different eGFRs depending on their age and sex. In almost all cases, the eGFR is the more clinically useful of the two numbers, which is why it's usually reported alongside creatinine rather than instead of it.

What do the kidney function stages actually mean?

Kidney function is usually staged from G1, normal or high, through to G5, kidney failure, based entirely on eGFR. This staging is often looked at alongside a separate urine test for protein, called ACR (albumin to creatinine ratio), which checks a different aspect of kidney health entirely.

A mildly reduced eGFR sitting in the G2 or early G3a range on a single test is common, particularly in older adults, and often isn't something to worry about on its own. A significantly low result, or one that has dropped sharply from a previous test, is a different matter and worth discussing with a healthcare professional promptly.

Myth versus fact: normal creatinine means healthy kidneys

Myth: If my creatinine is normal, my kidneys must be completely fine.

Fact: Kidneys have significant reserve capacity, and a meaningful amount of function can be lost before creatinine rises measurably. The remaining working kidney tissue compensates by filtering harder, which can keep creatinine looking normal even while underlying function has already declined. This is one of the reasons repeat testing over time is more informative than any single result.

Did you know?

TSH aside, for kidneys specifically: cystatin C, an alternative marker produced by almost all cells in the body at a steady rate, is sometimes used alongside or instead of creatinine when muscle mass makes creatinine-based eGFR unreliable, such as in people who are frail, very muscular, or have had a limb amputated. NICE NG203 specifically recommends considering this in situations where creatinine alone may be misleading.

What to know before you book a kidney function test

No fasting is required for a standard kidney function test on its own, though if it's part of a wider panel that includes glucose or lipids, fasting guidance may differ for those other markers.

Try to be reasonably hydrated beforehand. Significant dehydration at the time of your blood draw can temporarily raise your creatinine and lower your calculated eGFR.

If you've done intense exercise, taken creatine supplements, or eaten a very large protein-heavy meal in the hours before your appointment, it's worth mentioning this, since any of these can affect your result on the day.

If you're already being monitored for a kidney condition or take medication known to affect kidney function, such as long-term NSAIDs or certain blood pressure medications, let your phlebotomist know so this can be noted alongside your result.

What happens after your kidney function results come back?

A result within the normal range for your age and sex generally needs no further action beyond routine monitoring if you have any ongoing risk factors, such as diabetes or high blood pressure.

A mildly abnormal result is usually followed by a repeat test after a reasonable interval, to see whether it was a one-off or part of a genuine trend, rather than immediate further investigation.

A significantly abnormal result, or a sharp change from a previous test, is worth discussing with a healthcare professional promptly. This is worth raising directly rather than waiting for your next scheduled check.

The most important thing to know about kidney function testing is that trends matter more than a single number. One result is a snapshot; two or three results over time, ideally from the same laboratory, tell you whether something is actually changing.

Getting tested with Kuon Healthcare

Kidney function is included in several of our private blood test UK panels, including the Complete Wellness Check and the Advance Vitamin Profile, alongside a full blood count and other markers. You can book a home blood test UK appointment without a GP referral, and results are explained in plain English, not just handed over as raw numbers.

If kidney function is the only thing you want checked, our Bespoke Wellness Profile lets you build a bespoke wellness blood test around exactly that, without paying for markers you don't need.

Questions I often hear from patients

What is a normal creatinine level?

This varies by lab, sex and muscle mass, which is why eGFR, the calculated estimate, is usually more useful than the raw creatinine figure on its own. Your results report will include the reference range used by that specific laboratory.

What does a raised creatinine level mean?

It may indicate that your kidneys are filtering less efficiently than usual, though it can also reflect dehydration, high protein intake or recent intense exercise. This is worth discussing with a healthcare professional, especially if it's a new or significant change from a previous result.

Can dehydration affect creatinine results?

Yes. Being dehydrated when you're tested can temporarily raise your creatinine and lower your calculated eGFR. Drinking normally beforehand, rather than fasting excessively, gives a more representative result.

Do I need to fast before a kidney function blood test?

No fasting is required for a standard kidney function test on its own, though if it's part of a wider panel that includes glucose or lipids, fasting guidance may differ. Your test details will confirm this before you book.

How often should kidney function be checked?

This depends on your individual risk factors and any medication you take. NICE recommends at least annual monitoring for people on medications that can affect kidney function, such as long-term NSAIDs, while others may only need occasional checks as part of a general wellness test.

Explore related tests

- Complete Wellness Check

- Advance Vitamin Profile

- Bespoke Wellness Profile

A single creatinine result rarely tells you what you actually want to know. What matters is whether it's stable over time, and whether it fits the wider picture of your health, not whether it sits a fraction outside a reference range on one occasion.

Ready to take control of your health? Book your private blood test today at kuonhealthcare.co.uk