Doctors use blood tests for a variety of reasons.
They can use them to:
- monitor your kidney function
- monitor for complications of kidney disease, such as anaemia, acidosis and bone disease
- check that your dialysis is working properly
- ensure that you’re receiving the right dosage of drugs after a kidney transplant.
Monitoring kidney function
These tests are generally used to gauge how well your kidneys are working.
Estimated glomerular filtration rate (eGFR) – is a blood test which is used to indicate roughly how well the kidneys are working to filter out waste products such as creatinine from your blood.
- eGFR is reported in millilitres per minute per 1.73m2, using a formula that takes body size into account. A normal eGFR is greater than 90, but values as low as 60 are considered normal if there is no other evidence of kidney disease.
- eGFR is often shown as a percentage of normal because people find it useful to think of kidney function as a percentage, going from 100% (fully functioning) to 0% (no function).
Cystatin C – is a new test, not yet widely available, that might increase the accuracy of measuring kidney function compared to use of creatinine alone. Cystatin C is also produced at a constant rate, and is only cleared by the kidneys, but its production does not vary with muscle mass.
Creatinine (Creat) – is a waste product of muscle turnover. It is produced at a fairly constant rate from one day to the next, and is only removed from the body by kidney filtration. This means that the creatinine level in the blood stream can be used to track how well the kidneys are clearing waste products: if a person’s kidney function falls, their creatinine level will go up.
- Creatinine clearance (measuring how much creatinine a person makes in 24 hours) can give a reasonable estimate of actual GFR. However, it’s a real nuisance to do a 24-hour urine collection, so this test is not routinely done. Instead, a formula is used that predicts creatinine production from age, gender, and ethnic origin, which then allows calculation of estimated GFR. The formula, approved in the UK now, is called the CKD-EPI formula.
- However, the estimates obtained from this formula are not perfectly accurate and will give false results in a person whose muscle mass is very different to the average for that person’s age, gender and ethnic origin. For instance, body builders (who make more creatinine than average) often have a slightly low eGFR, but usually turn out to have completely normal kidney function when it is measured in another way.
Some minerals in the blood need to be closely monitored because ‘normal’ levels can often change in kidney disease and can cause life-threatening complications, if left untreated. Blood mineral levels are measured in units called milimoles per litre (mmol/L).
- sodium (Na) – normal levels are 135-145 mmol/litre
- potassium (K) – normal levels are 3.5-5.0 mmol/litre
- calcium (Ca) – normal levels are 2.2-2.6 mmol/litre; but some calcium is bound to the protein albumin in the blood, so variations in albumin level can cause variations in the calcium level. Therefore, the calcium level is often expressed as an ‘adjusted calcium’ using a formula that corrects for abnormal blood albumin levels
- phosphate (PO4) – normal levels are 0.9-1.3 mmol/litre but the aim is to keep levels at or below 1.8 for people with kidney failure
- magnesium (Mg) – normal levels are 0.7-1.0 mmol/L, but are lower if the blood albumin level is low.
Counting blood cells and components
Problems such as anaemia (a low blood count) and infection can be detected by counting amounts of these blood components in a blood sample:
- haemoglobin (Hb) (found in red blood cells, haemoglobin is a combination of an iron-containing molecule ‘haem’ and the protein ‘globin’. It gives blood its red colour and carries oxygen in the blood and releases it in our organs) – normal levels are approximately 110-180 grams/litre in men and 115-165 grams/litre in women
- white blood cells (wbc) (which fight infection) – a normal count is approximately 4-11 thousand million per litre
- platelets (plats) (which help to clot blood) – a normal count is approximately 150-350 thousand million per litre
- albumin (Alb) (a blood protein) – normal levels are 35-45g/litre.
Checking the effectiveness of dialysis
A Urea Reduction Ratio (URR) test may be used to work out the percentage reduction of waste products (such as urea) in the blood, after one haemodialysis session. Tests are usually carried out every three months. This involves taking a sample of your blood before and after your dialysis session.
- URR should ideally be over 65% if you have dialysis three times each week and have little or no remaining kidney function.
Regular tests will also be carried out to ensure that your blood is flowing quickly and freely through your fistula or graft during your haemodialysis sessions and that any fluid build-up remains as low as possible.
Peritoneal dialysis (PD)
An Adequacy Kt/V and creatinine clearance test may be used to check that your peritoneal dialysis is adequate for your needs and working efficiently. This test measures the amount of creatinine and urea both in your blood and in a 24-hour PD fluid sample. This is usually measured every six months.
- Adequacy Kt/V should ideally be greater than 1.7
- Creatinine clearance should be greater than 50L/wk/1.73m2
A Peritoneal Equilibrium Test (PET test) will normally be carried out once a year to measure the movement of fluid and waste products across your peritoneal membrane and your urine volume will be measured every six months. You may have to reduce the amount of liquids you can drink over a 24-hour period if your urine levels drop.
Protecting your new kidney
Blood tests are often taken to ensure that immuno-suppressant drugs – such as Ciclosporin, (Cyclosporine, CyA), Tacrolimus (Tacro) and Sirolimus – are at the right level to protect transplanted kidneys and prevent harmful side effects.
- Ask your transplant team what your target levels are.
Reacting to results
Don’t panic if you get an ‘abnormal’ test result; but don’t ignore it either. Speak to your kidney doctor or nurse if you are concerned and always go to A&E if you’re told to do so – even if you feel perfectly fine.
Become your own expert
Many kidney patients choose to take an active role in their care and treatment. You can too by logging on to the Patients Know Best website, where you may be able to keep track of all your medications, test results, scans and medical letters. You may also be able to see your medical history (in the form of a linear chart) and get information about a wide range of tests (eg how they are done, what they are used for, how they can be interpreted, and what ‘normal’ ranges mean for you in practical terms).
You need to be referred to Patient View by your kidney unit so make sure you ask them to do this.
- For more ways to take control of your kidney disease visit our How can I help myself? section.
And for further information about kidney tests visit:
Take control of your kidney disease
What are the stages of kidney disease?
How to get involved in kidney research
Have you been diagnosed with kidney disease?
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