What is a kidney function test, how to test it & what does it show?

Kidneys play an essential part in sustaining our overall health and wellbeing. They filter the blood, maintain fluid balance, help control blood pressure, and aid the production of urine (which eliminates waste from the body). Kidneys also play an important role in the production of red blood cells and converting vitamin D into the active form that the body can use.

Kidney function tests are generally undertaken with a simple blood test, but a urine sample can also be used to assess additional elements such as the presence of abnormal substances in the urine.

What is a kidney function test?

A kidney function test is undertaken by taking a urine or blood sample to determine your kidneys’ filtering efficiency and to identify any signs of kidney damage.

Blood test

The process of a kidney function test simply involves drawing a small sample of blood from a person’s inner arm with a needle. This is commonly undertaken by a nurse or phlebotomist, and the process is quick and usually only produces mild discomfort for a short time. The sample is then sent to a lab to be examined. A blood sample can also be taken with a simple finger prick, which is generally an option taken by those who would prefer to take their own sample from the comfort of their own home.

Blood tests are a way to assess kidney function by checking for signs of damage and reduced function.

Urine sample

Kidney function can also be assessed with urinalysis, which is a panel of tests used to measure and detect a number of substances in the urine, including microscopic particles of blood. Certain particles are to be expected in the urine, as they are merely by-products of normal bodily functions. However, blood in the urine can sometimes be a sign of infection, kidney stones, other kidney problems and sometimes cancers of the kidney, bladder or prostate.

This test only requires a small sample of urine, which can be self-collected at home. The sample is then sent to a lab for microscopic examination as well as a dipstick test, which is undertaken by dipping a chemically treated strip into the sample. The strip changes colour changes in the presence of abnormal amounts of protein, pus, blood, sugar and bacteria.

What does a kidney function test show?

Kidney function tests help to show whether your kidneys are functioning properly. Different elements are tested depending on the test chosen (blood or urine).

Blood test

Kidney function can be evaluated with a group of three blood tests, which measure the amount of sodium (salt) and two chemical waste products (urea and creatinine) in the blood. It also comprises of a calculation of eGFR (estimated glomerular filtration rate), which is a useful way to detect reduced kidney function.


Sodium is essential for the regulation of blood pressure and fluid balance in the body. Anomalous levels of sodium can put greater strain on the kidneys, which can lead to kidney disease.


Urea is a waste product made by the breakdown of proteins in the body. Healthy kidneys are able to eliminate urea from the blood. Abnormally high levels of urea may indicate reduced kidney function.


Creatinine is a waste product made by the normal breakdown of muscle tissue. Healthy kidneys are able to eliminate creatinine from the blood, which means that raised creatinine levels may be a result of kidney damage.

eGFR (estimated glomerular filtration rate)

eGFR (estimated glomerular filtration rate) is a calculation of the amount of blood filtered through the kidneys over a period of time, therefore it is a useful way to evaluate how efficiently the kidneys are working. A persistently low eGFR is also associated with an increased risk of cardiovascular disease.


The main objective of urinalysis is to detect the presence of tiny particles of blood which are invisible to the naked eye. In addition to this, urine is also generally checked for:

  • Protein
  • pH
  • Glucose
  • Organisms
  • Ketones
  • Urobilinogen
  • White blood cells
  • Red blood cells
  • Casts
  • Epithelial cells
  • Crystals

What should your kidney function levels be?

The recommended ranges of sodium, urea, creatinine and eGFR in the blood are as follows:

Units Recommended range (Male) Recommended range (Female)
Sodium mmol/L 135-145.99 135-145.99
Urea mmol/L 0-8.30 0-8.30
Creatinine umol/L 0-112 0-92
eGFR 90+ 90+

What are the signs that something is wrong with your kidneys?

Kidney problems are generally symptomless until it reaches an advanced stage, therefore kidney function tests are extremely useful to detect any hidden issues.

Results below the recommended range Results above the recommended range
eGFR Results lower than the recommended range may indicate that the kidneys are not working as efficiently as they should. A persistently low eGFR is also associated with increased risk of cardiovascular disease.

If you have low eGFR levels, it is recommended that you follow a low salt diet, maintain a healthy body weight for your height, and take regular physical activity.

The eGFR is a measure of how much blood is being ‘cleaned’ by your kidneys every minute. Therefore a higher figure demonstrates that the kidneys are working efficiently.
Creatinine Decreased creatinine levels in the blood may be due to older age, female sex, reduced muscle mass, vegetarian diets or malnutrition. Lower creatinine levels in older adults, or in someone who is slightly underweight or with low muscle mass, are not generally a cause for concern.

Creatinine levels do not correlate smoothly with kidney function as creatinine levels are also affected by age, race, gender, and body size.

Raised creatinine levels may be caused by high muscle mass, high protein intake, impaired kidney function and some medications.  Creatinine level do not correlate smoothly with kidney function as creatinine levels are also affected by age, race, gender, and body size.
Sodium Reduced sodium levels (hyponatraemia) may be caused by pneumonia, brain haemorrhages, being elderly and certain medications (for instance thiazide diuretics). Low sodium levels are common and are typically not associated with any symptoms.

However, it is recommended that it is brought to the attention of a GP if symptoms such as confusion, nausea, restlessness or irritability, headaches, muscle cramps or weakness are also experienced.

Raised sodium levels (hypernatremia) can be caused by excessive fluid loss (for instance from sweating, vomiting, diarrhoea or burns), uncontrolled diabetes, and some endocrine syndromes.
Urea Low urea levels are not usually a cause for concern, and are frequently caused by very-low protein diets or pregnancy. Although they can sometimes be seen in cases of severe liver disease or malnutrition, they are not used to diagnose or monitor these conditions. Raised urea levels may be caused by dehydration, a high protein diet, gastro-intestinal (gut) haemorrhage, renal failure or urinary outflow obstruction. Very high urea levels can sometimes suggest poor kidney function, but there are many things besides kidney disease that can affect urea levels.

How can I get a kidney function test?

At Bluecrest, we offer a range of packages that include kidney function tests, so you can assess your kidney health.

At a clinic

You can book a health check with one of our Certified Healthcare professionals at one of over 2,000 mobile clinics nationwide. All of our Health MOT Packages include a kidney function test (blood test), in addition to a wide array of health markers for a detailed view of your health. You can choose between 3 levels of testing to suit your own personal needs.

At Home

Alternatively, you can get a kidney function test from the comfort of your own home with one of our Home Test Kits (blood test), which all include a kidney function test and also assess a variety of other key readings pertaining to your general health. All you need to do is order your kit, take your own sample, send it back in our pre-addressed return envelope, and wait to receive your results online within 8 days.

The above tests are all done via blood test, however you may wish to consider our Urinalysis test, which examines a urine sample for the presence of abnormal substances. The urinalysis test pack is sent to you via mail so you can collect your sample in the comfort of your own home.