Análisis de sangre rutinario de la función renal

What is a kidney function test?

Kidney function tests are one of the most commonly-performed blood tests.

When doctors talk about a blood test for the kidneys, they usually mean a single blood test that measures the following four things:

• Urea.

• Creatinine.

• Sodium.

• Potassium.

Some laboratories also measure chloride and bicarbonate routinely, whereas in other labs these are only done if the clinician specifically asks for them.

These blood tests are called a 'renal profile', 'renal function test', or 'urea and electrolytes' ('U&E' for short). In the US, it's part of a 'basic metabolic panel'.

Kidney function is measured in lots of situations. For example, it can be used:

• As part of a general health assessment.

• If you have suspected low body water content (dehydration), when the urea level increases.

• If you have suspected kidney failure. The higher the blood levels of urea and creatinine, the less well the kidneys are working. The level of creatinine is usually used as a marker as to the severity of kidney failure. Creatinine in itself is not harmful but a high level indicates that the kidneys are not working properly.

• Before and after starting treatment with certain medicines. Some medicines occasionally cause kidney damage as a side-effect. Some medicines also have to be given at different doses, or avoided completely, at certain levels of kidney function. Therefore, kidney function is often checked before and after starting treatment with certain medicines.

See also the leaflets on Lesión renal aguda y Enfermedad renal crónica.

Urea is a waste product formed from the breakdown of proteins. Urea is usually passed out in the urine. High urea can be caused several things, such as:

• The kidneys not working properly (uraemia).

• Deshidratación.

• Eating a high-protein diet.

• Heavy bleeding into the stomach (blood in the gut has lots of protein, which is absorbed) - people with this will usually have other symptoms of bleeding, such as vomiting blood, or passing black, sticky, tar-like poo (melaena).

Creatinine is a waste product released by the muscles. Creatinine passes into the bloodstream, is filtered by the kidneys, and then passed out in urine. Creatinine is usually a more accurate marker of kidney function than urea.

So, a high level of creatinine usually means that the kidneys are not working properly, because they are unable to remove creatinine from the blood, causing the levels to increase.

Creatinine levels are also affected by muscle mass. Someone with lots of muscle will naturally release more creatinine into their blood.

So, creatinine is often not used on its own as a marker of kidney function, but used as part of a calculation that takes into account someone's body mass and muscle mass. The estimated GFR (eGFR) is one example.

Creatinine levels can also be raised by taking creatine supplements.

The 'glomerular filtration rate' (GFR) is an important measure of kidney function. It describes the amount of fluid that the glomeruli - part of the kidneys - can filter per minute. Problems with kidney function cause the GFR to decrease.

GFR can be measured directly, usually by injecting a specific substance into the blood and doing repeated blood or urine tests to determine how quickly it is removed from the blood by the kidneys. However, this is quite a long procedure, and so it's only done in a few situations, usually when it is necessary to get a very precise measurement of kidney function.

Instead, we usually estimate the GFR using an equation, based on a blood creatinine measurement. This is called the estimated GFR, or eGFR. The eGFR is much easier to measure, as it only requires a single blood test. It provides a good estimate of kidney function in most situations.

eGFR is calculated based on:

• The blood creatinine level.

• Your age.

• Your sex.

eGFR values can also be adjusted for ethnicity (a 'correction factor' has been used to adjust eGFR results in Black people). This is no longer felt to be accurate or helpful, and it may falsely overestimate kidney function in Black people. Most labs have stopped doing this.

A 'normal' eGFR is usually greater than 90. However, an eGFR above 60 is also considered normal if there are no other signs of kidney damage - such as if the urine albumin-creatinine ratio measurement is normal (see below).

See the separate leaflet called Estimated glomerular filtration rate.

Kidney blood tests also routinely measure sodium and potassium, and sometimes chloride and bicarbonate as well. These are dissolved 'salts' that are normally found in the blood. They are sometimes referred to as 'electrolytes'. There are a lot of different reasons for these salts to be high or low - sometimes, this can be due to a kidney problem.

The routine kidney blood test is a general marker of kidney function. It may miss early signs of kidney damage.

A urine test, called the albumin:creatinine ratio, or ACR, is often used to look for these early signs, particularly in people with diabetes.

Other ways of measuring kidney function include:

• The estimated creatinine clearance (CrCL). This is similar to the eGFR, but is a slightly more accurate measure of kidney function in elderly people, people who are very underweight, and people who are very overweight. This is calculated from the blood creatinine level, sex, age, body weight, and sometimes also height. It is often used when prescribing drugs that are very sensitive to kidney function.

• Cystatin-C. Blood levels of cystatin-C can be used to estimate kidney function, in a similar way to creatinine. Cystatin-C is less affected by muscle mass than creatinine is. It may be useful if the creatinine result is thought to be an unreliable measure of someone's kidney function. It is mostly used in research studies, and not yet widely available to clinicians. It might become more commonly-used in future.

• Other specialised tests, such as a directly-measured GFR, or a directly-measured creatinine clearance, using a 24-hour urine collection. Both of these tests give a very accurate measurement of kidney function, but are difficult and time-consuming to do. They are very rarely done.

If the kidney function test is abnormal, further tests might be needed to find out what is causing the kidney problem. There are lots and lots of possible tests, and which ones are needed depends on the exact situation. Examples include blood tests, ultrasound scans, and more specialised tests, like kidney biopsies.