Blood tests · Pillar guide
Understanding Kidney Function Tests
Kidney function tests, often reported as U&Es, look at how well your kidneys are filtering and how balanced your body's salts are. Here is what each value means in everyday language.
Medipher explains medical wording in plain English — it does not diagnose or replace a clinician. If this is a medical emergency, call your local emergency number immediately.
Part of the Blood Tests guides
This guide covers one panel in detail. If you are new to blood test reports, the general guide is a good place to start.
What are these tests?
Kidney function tests are a group of blood results, commonly reported together as "U&E" (urea and electrolytes) or "renal profile". They look at waste products your kidneys clear from the blood, and at the balance of key salts your body keeps within a narrow range.
The kidneys filter blood continuously, so measuring what is left circulating gives an indirect but useful picture of how that filtering is going.
These tests are among the most frequently requested blood tests. They are often part of routine monitoring or a medication check rather than an investigation of a suspected kidney problem.
Why is it measured?
Being asked for these tests is common and does not, by itself, suggest that anything has been found. Healthcare professionals request them for many everyday reasons.
Common reasons these tests are requested
- As part of a routine health check or long-term condition review
- Before or during treatment with medicines that are cleared by the kidneys
- To check hydration and salt balance during illness
- To monitor conditions such as high blood pressure or diabetes
- Before certain scans or procedures
- To look into symptoms such as swelling, tiredness or changes in passing urine
What do the individual results mean?
Reference ranges vary between laboratories and between individuals, so always compare a value with the range printed on your own report.
Creatinine
CreatinineA waste product made by normal muscle activity and removed by the kidneys.
- Why it is included
- Because creatinine is produced fairly steadily, the amount left in the blood gives an indication of how much is being filtered out.
- If the result is higher
- A higher creatinine can be associated with reduced filtering, but it is also influenced by dehydration, larger muscle mass, intense exercise and some medicines.
- If the result is lower
- A lower creatinine can simply reflect smaller muscle mass and is often not a concern on its own.
Estimated glomerular filtration rate
eGFRA calculated estimate of how much blood your kidneys filter each minute.
- Why it is included
- eGFR is worked out from your creatinine result along with factors such as age and sex. It is an estimate, not a direct measurement.
- If the result is higher
- Higher values generally suggest more filtering capacity. Very high estimates are usually interpreted with caution rather than treated as a finding.
- If the result is lower
- A lower eGFR may be seen when filtering is reduced, but a single low estimate can also follow dehydration or a temporary illness. Clinicians look at the trend over time.
Urea
UreaA waste product formed when the body breaks down protein.
- Why it is included
- Urea is cleared by the kidneys, so it moves with kidney filtering — but it is also sensitive to hydration and diet.
- If the result is higher
- A raised urea can be associated with dehydration, a high-protein diet, bleeding in the digestive system or reduced kidney filtering. It is read alongside creatinine.
- If the result is lower
- A lower urea may be seen with a low-protein diet, pregnancy or liver-related changes.
Sodium
SodiumA salt your body keeps within a narrow range to manage fluid balance.
- Why it is included
- Sodium reflects the balance between water and salt in the body rather than how much salt you eat.
- If the result is higher
- A higher sodium is often associated with not enough fluid in the body.
- If the result is lower
- A lower sodium can be associated with several things, including some medicines, illness and hormone changes. Even mild changes are usually reviewed by a clinician.
Potassium
PotassiumA salt involved in nerve signalling and heart rhythm.
- Why it is included
- The body keeps potassium tightly controlled, so results outside the range are usually reviewed promptly.
- If the result is higher
- A raised potassium can be associated with some medicines and with reduced kidney filtering — but it can also be a false reading if red cells break down in the sample tube, so a repeat test is common.
- If the result is lower
- A lower potassium may be seen with some medicines such as diuretics, or after vomiting or diarrhoea.
What this doesn't necessarily mean
- A single raised creatinine does not automatically mean kidney disease — hydration, muscle mass and exercise all affect it.
- A slightly reduced eGFR does not automatically mean your kidneys are failing; it is an estimate and is read alongside your history.
- An unexpected potassium result is sometimes caused by the sample itself, which is why a repeat test is common.
Understanding the results together
Kidney results are read as a group. Creatinine and eGFR are two views of the same underlying measurement, and urea adds information about hydration and protein turnover, so a clinician looks at how they move relative to each other.
Direction of travel matters more than a single snapshot. A stable result over several years is interpreted very differently from the same number appearing after a period of normal results.
Urea raised proportionally more than creatinine
This pattern is often associated with dehydration, and clinicians will usually consider fluid intake and recent illness before anything else.
A slightly reduced eGFR that has been stable for years
Stability over time is reassuring context. A clinician will usually compare against your previous results rather than the reference range alone.
A newly changed creatinine after starting a medicine
Some medicines change kidney results predictably. A clinician may review the dose, recheck the blood test, or simply monitor.
These are illustrations of how results are read together — they are not a way of working out a diagnosis from your own numbers.
What can affect the results?
An unexpected result usually has more than one possible explanation. Some of the more common influences include:
- How much fluid you have had before the test
- Muscle mass, body build and recent intense exercise
- A high-protein diet or protein supplements
- Certain medicines, including some painkillers, blood pressure tablets and diuretics
- Recent illness, vomiting or diarrhoea
- Age — eGFR estimates naturally change across the lifespan
- How the blood sample was taken and handled (particularly potassium)
What happens next?
There is no single route after an unusual result. Depending on the whole picture, a healthcare professional may consider any of the following — none of them is guaranteed.
- Repeating the test once you are well hydrated or after an illness has settled
- Comparing against your previous kidney results to look at the trend
- Reviewing medicines that affect the kidneys
- Checking blood pressure and, where relevant, blood sugar
- Testing a urine sample for protein
- Monitoring at planned intervals rather than acting immediately
Questions you could ask
These are prompts to help you have a more informed conversation — not a way to interpret results yourself.
- How does this result compare with my previous kidney tests?
- Could dehydration or any of my medicines be affecting this?
- Will this test need to be repeated, and when?
- Should my blood pressure or urine also be checked?
- Is there anything I should monitor day to day?
- Do any of my regular medicines need reviewing because of this result?
Frequently asked questions
What is the difference between creatinine and eGFR?
Creatinine is measured directly in your blood. eGFR is calculated from that creatinine value along with factors such as age and sex, to estimate how much blood the kidneys filter each minute.
Can drinking water before the test change my results?
Hydration can influence urea, creatinine and sodium. If you were dehydrated at the time, a clinician may simply repeat the test.
Why do my kidney results change between tests?
Small variation between tests is normal. Illness, hydration, diet, exercise and laboratory measurement all contribute, which is why clinicians look at patterns.
Does a low eGFR mean I need to see a specialist?
Not necessarily. Many people have stable results that are monitored in primary care. Whether a referral is considered depends on the whole picture, not one number.
Related medical terms
Each term has its own plain-English page in the Medipher medical glossary.
- CreatinineA waste product from normal muscle activity that the kidneys filter out of your blood.
- eGFRAn estimate of how well your kidneys are filtering blood, calculated from your creatinine.
- UreaA waste product formed when your body breaks down protein. The kidneys filter it out of your blood.
- AlbuminThe main protein in your blood, made by the liver. It helps keep fluid inside your blood vessels.
Sources
Medipher guides are written from publicly available UK health information. They are educational and do not replace advice from your own healthcare professional.
- 01NHSBlood tests — Health A to Z
- 02Association for Laboratory MedicineLab Tests Online UK — Tests index
- 03National Institute for Health and Care Excellence (NICE)NICE guidance and advice list
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Medipher can help explain the medical language in plain English, highlight the terms used and suggest questions to take to your healthcare professional. It does not provide a diagnosis or medical advice.
This guide is general health information, not medical advice, and it cannot diagnose any condition. Reference ranges and clinical interpretation vary between laboratories and individuals — always discuss your own results with a GP, doctor, nurse or pharmacist.