Kidney function and electrolytes, with eGFR interpreted properly against your age and build.
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Kidney function tests measure waste products the kidneys filter out and the salts they balance. Together they show how well the kidneys are working, and whether that has changed.
The kidneys are quiet organs. Function can decline substantially before anyone notices anything, which is why this test is one of the more valuable things to have measured rather than waited for — particularly for anyone with high blood pressure or diabetes, the two commonest causes of kidney disease in the UK.
Creatinine — a waste product from muscle, cleared by the kidneys. It rises as filtration falls.
eGFR — estimated glomerular filtration rate, calculated from creatinine, age and sex. This is the headline number and the one used to stage kidney disease.
An important limitation: because it is derived from a muscle waste product, eGFR is less reliable at the extremes of muscle mass. A very muscular person can appear to have reduced function when they do not; someone frail with little muscle can appear to have better function than they really have. Cystatin C, in the extended panel, is not affected by muscle mass and is the better measure in those situations.
Urea — rises with reduced function, but also with dehydration, a high-protein meal or gut bleeding, so it is read alongside creatinine rather than alone.
Sodium and potassium — the salts the kidneys balance. Potassium matters clinically: it can rise on ACE inhibitors such as ramipril and on potassium-containing salt substitutes, and a high level affects heart rhythm.
Calcium, magnesium and phosphate — all regulated by the kidneys and disturbed as function declines.
Uric acid, in the extended panel, relates to gout.
Standard panel: creatinine · eGFR · urea · sodium · potassium · chloride · calcium · magnesium · phosphate.
Extended panel adds cystatin C and uric acid. Cystatin C is a more reliable measure of kidney function than creatinine in people with high or low muscle mass, where eGFR can be misleading in both directions.
1. Consultation. Medication review is a substantial part of this one — several common prescriptions depend on kidney function for safe dosing, and several others affect it.
2. Referral. To a Randox Health clinic. We commonly request HbA1c and lipids alongside, and urine testing for protein, which detects kidney damage earlier than blood tests do.
3. The sample. A single blood draw. Stay normally hydrated — dehydration distorts the result — and avoid a high-protein meal or heavy exercise for 24 hours beforehand.
4. Results review. A GP explains what the eGFR means in context. A mildly reduced eGFR is common with age and is frequently not disease; what matters is the trend over time and whether protein is present in the urine. Where it is falling, or protein is present, we arrange nephrology referral.
Stay normally hydrated — dehydration distorts results. Avoid a high-protein meal or heavy exercise for 24 hours beforehand.
eGFR is an estimate, not a measurement — calculated from creatinine, age and sex. Above 90 is normal; 60 to 89 is mildly reduced and often entirely normal for age; below 60 sustained for three months defines chronic kidney disease.
A single reduced eGFR means very little. Dehydration alone can drop it substantially, and it recovers with a glass of water. The diagnosis requires it to be sustained over three months, which is why we repeat rather than react.
Creatinine comes from muscle. So:
Cystatin C is not affected by muscle mass, which is why it is worth adding at either extreme — and it is the marker most often missing when an eGFR does not fit the person.
Protein in the urine. Albumin-to-creatinine ratio predicts progression and cardiovascular risk at least as well as eGFR does — and it is the check most frequently omitted.
Someone with a mildly reduced eGFR and no protein is in a very different position from someone with the same eGFR and significant proteinuria.
Potassium matters most. High or low levels affect heart rhythm, and ACE inhibitors, ARBs, spironolactone and diuretics all move it — which is exactly why bloods are repeated within two to four weeks of starting or increasing those.
Low sodium is common in older people, and is frequently caused by a medicine — thiazide diuretics, SSRIs and PPIs among them.
eGFR falls naturally with age, and a reading of 70 in a healthy 75-year-old is usually not a disease. Labelling it as one causes anxiety and occasionally unnecessary treatment.
Equally, a "normal" eGFR in someone with significant proteinuria is not reassurance. The two must be read together.
Dehydration, a heavy protein meal, strenuous exercise and creatine supplements all raise creatinine and lower the calculated eGFR — sometimes substantially. Testing after a hard gym session gives a misleading result, which is worth knowing before anyone acts on it.
Kidney function is checked free on the NHS for anyone with diabetes, high blood pressure, or on medicines that require monitoring — and it is part of the free NHS Health Check for adults aged 40 to 74.
If you are on an ACE inhibitor, ARB or diuretic, this monitoring should already be happening. If it is not, that is worth raising with your practice.
Repeating a single mildly reduced eGFR every few weeks. The diagnosis requires three months, and anxious repeat testing changes nothing.
Kidney "detox" and cleanse products, which do nothing — and several herbal preparations are genuinely nephrotoxic.
Probably not, and a single reading definitely does not establish it. Chronic kidney disease requires an eGFR below 60 sustained over three months.
eGFR also falls naturally with age. 65 in a healthy 75-year-old is usually not a disease — and whether there is protein in the urine matters more than the number itself.
Because it is an estimate calculated from creatinine, and creatinine moves. Dehydration, a protein-heavy meal, strenuous exercise and creatine supplements all raise it and lower the calculated eGFR.
Test well hydrated, and not the morning after a hard session at the gym.
Yes, and it is a common source of unnecessary alarm. Creatinine comes from muscle, so a muscular person can have a raised creatinine and a falsely low eGFR with entirely healthy kidneys.
Cystatin C is not affected by muscle mass, and it is the marker to ask for when the number does not fit the person.
Protein — the albumin-to-creatinine ratio. It predicts progression and cardiovascular risk at least as well as eGFR does, and it is the check most frequently left out.
A mildly reduced eGFR with no protein is a very different situation from the same eGFR with significant proteinuria.
No — but stay normally hydrated, and avoid a high-protein meal or heavy exercise for 24 hours beforehand. Both distort creatinine.
The ones worth knowing about: regular ibuprofen and other NSAIDs, which are the commonest avoidable cause; ACE inhibitors and ARBs, which require bloods within two to four weeks of starting; diuretics; and lithium.
NSAIDs bought over the counter are the ones people do not think of, and taken regularly they matter.
Not on your own. A small fall in eGFR after starting an ACE inhibitor or ARB is expected and does not mean harm — those medicines protect the kidneys over years, particularly with diabetes or proteinuria.
A larger fall does need reviewing, which is exactly why bloods are repeated after starting.
It depends entirely on how high, and it is worth taking seriously rather than ignoring. Significant elevation affects heart rhythm.
A mildly raised result is frequently artefactual — caused by the sample clotting slightly or the fist being clenched during the draw — and repeating it is often the right next step. Medicines are the other common cause.
Controlling blood pressure and blood sugar, above everything else. Then avoiding regular NSAIDs, staying hydrated, stopping smoking, and being careful with any supplement — several herbal products are genuinely nephrotoxic.
Enough to keep your urine pale is sensible. Drinking large volumes beyond that does not improve kidney function and can lower sodium dangerously in some people.
"Kidney cleanse" products do nothing. The kidneys are the cleansing organ.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 23, 2026
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