This page describes some blood tests that are commonly used in patients with kidney diseases, or to measure kidney function in people who may have kidney disease.
|How well are my kidneys working?
|Creatinine clearance (CrCl)
|Glomerular filtration rate (GFR and eGFR)
|Things the kidneys normally put out
|Other blood tests
|White cell count (wbc)
|Parathyroid hormone (PTH)
|Is my dialysis good enough?
|Urea reduction ratio (URR)
|Ciclosporin (cyclosporine, CyA)
Creatinine – Creatinine is the best routine blood test for measuring how well kidneys are working. It is produced by muscles and put out through the kidneys. This means that ‘normal’ levels depend on how much muscle you have, and this can cause some problems in spotting kidney disease.
Creatinine comes from muscle. The little girl has normal kidney function and a creatinine level less than 60. The body-builder has normal kidney function and a creatinine of 120. For the girl, a creatinine of 120 would be very poor kidney function.
Big changes in creatinine are likely to be important, whatever the creatinine level. Although different labs may give significantly different results for the same creatinine sample, because there are different ways of doing the test.
|Normal blood creatinine is 60-120 micromol/litre (0.7-1.4mg/dl) – In many people, ‘normal’ creatinine can sometimes conceal 50% loss of kidney function.
Urea – Urea is a small molecule that is produced in the liver from protein that you have eaten. It is normally put out by the kidneys, so blood levels rise as kidneys fail. However other things change the level of urea in your blood too, so that it is not a simple guide to kidney function. Here are some of the things:
- Fluid – if you are short of fluid (e.g. drinking very little), your kidneys keep more urea in the blood
- How much protein you have eaten
- Liver disease can stop urea being produced normally
Urea is still a very useful test when used together with creatinine. It can also be used to measure how efficient dialysis is.
|Normal blood urea is 3.5-6.5 mmol/litre (20-30mg/dl)
You can get round many of the problems of blood creatinine measurements by collecting urine for 24 hours and measuring how much creatinine is in the urine at the same time as finding out how much is in the blood. This is called creatinine clearance . If any urine is not collected during the 24 hours, the result will not be accurate.
|Normal creatinine clearance is about 100mls/minute
This is a test of how much the kidneys are filtering. It is normally about 100 mls/minute. This means that the kidneys are removing all the creatinine found in 100mls of blood every minute – almost 150 litres per day! Most of this is treated, and then absorbed back into the body, so that only 1-2% of the filtrate appears as urine. Creatinine clearance (see above) gives quite a good measure of GFR, but requires a 24 hour urine collection for measurement. Two other ways of measuring GFR are:
- Injecting a tiny amount of a radioactive substance and measuring how quickly it disappears from the blood, or appears in the urine.
- Using blood tests, age, sex, and sometimes other information to estimate the GFR (eGFR). This isn’t as good as measuring it, but is much simpler as it requires just one blood test. It is being used increasingly to spot kidney disease earlier than previously, and earlier than would be possible using just creatinine measurements.
|Normal GFR is about 100mls/minute/1.73m2
Sodium (Na) – Sodium comes from salt, which is sodium chloride. Although there is often too much of it in your body when your kidneys don’t work properly, its level in the blood does not change much. This is probably because sodium makes you thirsty, so you drink more until the level is normal again. Having too much sodium in the body causes high blood pressure, oedema (swelling), and eventually severe problems such as heart failure and fluid on the lungs (pulmonary oedema). Removing sodium is an important part of dialysis treatment. Diuretic drugs (e.g. furosemide) increase the sodium put out by the kidney. Most people with kidney disease need to keep down the amount of salt in their food.
|Normal blood sodium levels are 135 – 145 mmol/litre
Potassium (K) – Potassium comes from food, especially fruit, vegetables and nuts. Extra potassium you eat is normally put out in urine, so its levels in blood may rise in kidney failure. Blood potassium is normally precisely controlled and important. High levels can be very dangerous as they can cause serious heart rhythm abnormalities, including cardiac arrest, even before you have any other symptoms from it.
|Normal blood potassium levels are 3.5 – 5.0 mmol/litre
How high is dangerous?
There isn’t a simple danger level, but levels above 5.5 usually cause concern, and levels much above 6.0 may be dangerous. Levels above 7.0 are certainly dangerous.
- Diet is the most important way of controlling potassium levels in renal failure.
- Potassium is lowered by dialysis, but in patients treated by haemodialysis it rises between treatments, so it is important to be careful what you eat.
- Some medicines make blood potassium higher. This is more of a problem when kidneys don’t work properly.
- Sometimes high potassium levels may not be accurate, for instance because of delays in getting the sample to the lab.
Treatments for dangerously high potassium may include giving sugar (glucose) and insulin, and some other measures. If it is very high and kidney function is poor, dialysis may also be necessary.
Calcium (Ca) – The body’s calcium is mostly in bones, but in kidney failure there are problems with calcium and bones. Low levels of calcium may lead to thinning of bones, very low levels lead to weakness, tingling, and other problems. High levels cause sickness. It is measured regularly in patients with kidney diseases because:
- Blood calcium may be low in renal failure, without treatment.
- Special types of vitamin D (calcitriol, alfacalcidol) are given to prevent renal bone disease and to raise blood calcium.
- Medicines (phosphate binders) given to keep blood phosphate low may also raise blood calcium.
- After years of kidney failure, parathyroid hormone levels often rise, causing high blood calcium levels
- bone disease to be worse
- calcium deposits to form in blood vessels and in other parts of the body – these may cause serious disease.
Parathyroid hormone (PTH) – Parathyroid hormone is measured from time to time in patients with kidney diseases to prevent renal bone disease (renal osteodystrophy) After years of kidney failure, PTH levels often rise, causing calcium to be removed from bone and blood calcium levels to be high. The parathyroid glands may need to be removed if this cannot be controlled, an operation called parathyroidectomy . The glands are in the neck, behind the thyroid gland. Normal levels of PTH are different when measured in different labs. In kidney disease it may not be best to keep PTH levels completely normal – some guidelines suggest just keeping it less than 2 or 3 times the normal level.
White blood cell count (wbc) – White blood cells fight infection. Levels are often high during infections, but may be made low by some drugs, and sometimes also by infections. If the numbers are very low there is an increased risk of serious infection. Different kinds of white blood cell are added together to get the wbc. Neutrophils are the type that is most important for fighting common infections, and they are particularly likely to be affected by drugs.
Platelets (plats) – Platelets are needed to form blood clots. If the numbers are very low (below 50) the risk of serious bleeding increases. Patients with kidney disease quite often have slightly low platelet counts, but other diseases, and drugs, may cause low platelet counts.
- Leakage into urine
- Poor diet and starvation
- Infections or other causes of inflammation
- Liver disease
In dialysis patients it has been found that low albumin levels can be a marker for not doing well on dialysis. Unfortunately different methods used to measure albumin mean that different labs can get quite different results for the same samples from patients with kidney diseases.
Urea Reduction Ratio (URR) – This is a measure of how much the urea has been changed by one haemodialysis treatment. It is used to check that enough haemodialysis is being given. If it is too low more than once, you should find out why.
Ciclosporin, Tacrolimus and Sirolimus are drugs that are used to prevent your immune system from rejecting your transplant. They all need blood tests to make sure that you are taking enough, but not too much. Too much may cause serious side effects, while too little may not prevent rejection. The exact levels you are aiming for depend on a number of things, for instance:
- How long ago your transplant was
- The other medicines you are taking
- The risk of rejection in your own case
– so we have not given a target range here. If you want to know, you should ask your transplant team what your target levels are.
Please be aware that while we have made all effort to ensure that this brief information is accurate, we cannot guarantee that there are no mistakes. Also that the best management for individual patients may differ from that outlined here. Only the doctors caring for the patient will be able to advise on this.