If you are advised to have a kidney blood test it is normal to feel worried, but understanding more about why kidney blood tests are carried out will help put your mind at rest. The first thing you and your doctor can find out is whether or not your kidneys are functioning properly. Kidney blood tests can also help diagnose a specific illness that may affect your kidneys such as kidney disease, kidney stones, or severe dehydration and the tests can also be used for monitoring. If you are known to have a kidney problem, it is important to have a kidney blood test to find out whether it is getting better or worse.

Your doctor may also perform a kidney blood test before prescribing drugs that may put strain on your kidneys. The tests will reveal if they’re healthy enough to withstand the effects of the medication.

This article on kidney blood tests is by Kathryn Senior, a freelance journalist who writes health, medical, biological, and pharmaceutical articles for national and international journals, newsletters and web sites.

What does the kidney do?

Your kidney’s main function is to filter waste products out of your bloodstream. As blood is circulated through the kidneys, waste substances are extracted and then excreted into the urine, which then passes out of the body. Besides being a filter, your kidneys also:

  • Regulate your body’s water balance by excreting excess water via urine
  • Maintaining the acid/alkaline balance of the body

What do kidney blood tests do?

The kidney’s main function is to excrete waste from your body, so if your kidneys aren’t working properly, there are likely to be higher than normal levels of waste products in your blood. Kidney blood tests therefore look at the blood levels of these waste products to see how well your kidneys are performing their excretion function.

How is the test done?

Usually, your doctor will ask you not to eat excessive amounts of meat or other protein for 24 hours before a kidney blood test. During the test, your doctor will extract blood from your veins into a tube, which will then be sent off for testing at a laboratory. Test results are usually available within a few days, or even on the same day if you’re having the test in a hospital.

What do these tests look for?

A kidney blood test measures the levels of different substances in the blood:

Blood Urea Nitrogen (BUN) Test

This type of kidney blood test looks at the nitrogen levels in your blood, which is an indicator of urea levels. When your body digests proteins, urea is the waste product that is produced by your liver. In a normal healthy body, your kidneys filter this urea out of your blood and excrete it through your urine. If your kidneys aren’t working correctly they won’t be able to filter this urea properly and so the levels of nitrogen in your blood will be elevated.

Creatinine Test

When your body metabolises energy in muscles, the waste product creatinine is produced. In a normal, healthy body, creatinine is absorbed by the kidneys and then excreted through your urine. However, when your kidneys aren’t functioning properly, this filter system isn’t as efficient, and so your blood will contain higher than normal amounts of creatinine. Normally, creatinine levels in the blood tend to be very stable. This is because the amount produced depends on your muscle mass, which typically doesn’t fluctuate very much. This type of kidney blood test can therefore be a more sensitive indicator of kidney problems than a BUN test.

Other kidney blood tests

Kidney blood tests can also measure the blood levels of other chemicals that are regulated by your kidneys, such as sodium, calcium, magnesium, phosphorus, uric acid and glucose. Any abnormal levels of these substances need further investigation.

What can results of a kidney blood test show?

High levels of urea and creatinine in a kidney blood test result can indicate that you may have:

  • Kidney disease
  • Dehydration
  • Blockage of the urinary system or kidney stones
  • A condition that causes you to bleed into your stomach

You can develop high levels of urea in your blood if you eat a lot of protein, take in excess fluid, or if you have a problem with liver function. In addition, some antibiotics and drugs such as diuretics can cause abnormal BUN test results.

Conversely, if a kidney blood test reveals a low level of urea, this may indicate that you have:

  • A diet deficient in protein
  • Malnutrition
  • Severe liver damage
  • The habit of drinking excessive amounts of liquid

Women and children naturally have lower urea levels than men and urea levels also tend to be lower than usual if you have a kidney blood test during the second and third trimester of pregnancy. Because there can be other causes for abnormal levels of BUN or creatinine, kidney blood tests are usually done in conjunction with other tests such as a liver function test.

The value of kidney blood tests

If you have a problem with your kidneys or liver, it is better to have a prompt test and find out exactly what is at the root of that problem so that you can get effective treatment faster. If your kidney blood tests show that nothing is wrong, this can set your mind at rest.

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