Creatinine and Urea [Test for kidney function]


Creatinine and urea are two substances present in the bloodstream that can be measured by blood tests when you want to evaluate the function of your kidneys.

When a patient’s kidneys begin to function inadequately and their ability to filter blood is affected, the concentrations of urea and creatinine in the blood tend to rise. The higher the blood creatinine, the more severe the kidney failure.

In this article we will explain what urea and creatinine are, which diseases can cause them to be elevated, and how to interpret their results.

Attention: do not confuse creatinine with creatine. They have similar names, but the substances are different.

What is creatinine?

Our muscles need energy to perform their functions. The “fuel” that generates this energy is a protein called creatine phosphate, which is synthesized from the proteins in our food. Creatine phosphate is produced in the liver and then stored in the muscles.

Our muscles are permanently active, even when we are at rest. This means that we are consuming creatine phosphate all the time. Creatinine is a kind of metabolic waste resulting from this constant consumption. After it is generated, creatinine is released into the bloodstream and is eliminated from the body in the urine through the kidneys.

Summarizing this cycle:

proteins ingested in the diet → production of creatine phosphate by the liver → consumption of creatine phosphate by muscles for energy generation → production of creatinine → elimination of creatinine by the kidneys.

Every day, about 2% of all creatine phosphate stored in our body is converted into creatinine by the metabolism of muscles. It is this resulting creatinine that we measure in blood tests.

Why is creatinine used to evaluate the function of the kidneys?
Creatinine is a harmless substance in the blood; it is constantly produced and eliminated by the body. If the patient’s muscle mass is more or less stable, but has increased blood creatinine levels, this is a strong signal that his or her elimination process from the body is compromised, i.e., the kidneys are having some problem excreting it.

If the kidneys are not managing to eliminate the creatinine produced daily by the muscles, they are probably also having problems eliminating various other substances from our metabolism, including toxins. Therefore, an increase in the concentration of creatinine in the blood is a sign of kidney failure.

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Urea is another substance produced in the liver, also as a result of the metabolization of proteins from food. Like creatinine, urea is also eliminated by the kidneys. Elevations in blood levels of urea are a sign of kidney malfunction. We usually test both substances to evaluate kidney function, but creatinine is more specific and reliable.

How important are creatinine and urea tests?

It is estimated that worldwide there are millions of people with some degree of kidney dysfunction; 70% of these are not even aware that they may be ill. The most effective method for the early diagnosis of kidney disease is through the measurement of creatinine.

Numerous diseases can lead to chronic kidney disease, but six of these account for the majority of cases:

  • Hypertension.
  • Diabetes.
  • Polycystic kidney disease
  • Glomerulonephritis
  • Recurrent urinary tract infection
  • Recurrent kidney stones.

It is very common for us doctors to hear the following phrase:

  • Oh, doctor, my kidneys are great, I urinate very well and they don’t hurt.

This is a big misunderstanding! Chronic renal failure doesn’t usually cause symptoms until the very advanced stages of the disease. The fact that you don’t feel pain in your kidneys doesn’t mean anything. In general, the kidney only causes pain when there is a kidney stone or infection. All other kidney diseases are not usually associated with pain in the kidneys.

There is also a myth that good urination is a sign of kidney health. In fact, the control of body water is only one of the kidneys’ tasks. Initially the kidney becomes unable to filter out toxins, but it can eliminate water without any major problems. The reduction in urine volume is a very late sign, which often occurs only after the kidney failure is very severe and the patient already needs to enter a hemodialysis program.

Therefore, the fact of urinating good volumes and the absence of kidney pain is no guarantee of kidney health .

The creatinine test is important to detect kidney failure at an early stage and thus avoid the complications of the disease. The kidneys, besides controlling the body water, also act in the

  • Excretion of blood substances, such as drugs or toxins.
  • Blood levels of electrolytes, such as potassium, sodium, magnesium, calcium, and phosphorus.
  • Production of hormones that control red blood cells (RBCs).
  • Control of bone mass.
  • Control of blood clotting function.
  • Controls blood pH.
  • Controls blood pressure.
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Chronic kidney disease is a disease that usually progresses slowly and silently over a period of years, causing all of the above functions to be compromised. Not diagnosing kidney disease early means not acting on these problems in a timely manner.

Who should be tested?

Anyone at risk of developing kidney disease should have their blood creatinine measured. This includes people who have

  • Hypertension.
  • diabetes
  • Are older than 50 years of age
  • A family history of polycystic kidneys
  • Family history of glomerulonephritis
  • Family history of chronic renal failure
  • Chronic use of anti-inflammatory drugs
  • Frequent urinary tract infection
  • Recurrent kidney stones
  • Edemas (swellings) without defined cause.
  • Anemia without a definite cause
  • Severe heart disease, especially heart failure
  • Changes in urine such as bleeding (urine the color of Mate or Coca-Cola) or excessive foaming, which is a sign of proteinuria
  • People with weight loss, loss of appetite, morning nausea, and intense weakness without apparent cause
  • Obese people.
  • Smokers
  • Children with growth problems

So how do I know if my kidneys are working properly?

In general, the urea and creatinine levels are usually ordered together to evaluate the kidneys. However, creatinine is a better marker, since urea can be altered in cases of dehydration, use of diuretics, digestive bleeding, high protein diet, liver disease, etc.

The reasoning is simple: the two substances (urea and creatinine) are constantly produced by the body and are eliminated by the kidneys. In this way their concentration always remains stable. If the kidneys start to malfunction, they start to accumulate in the blood. Therefore, the worse the kidney function is, the higher the urea and creatinine values will be.

In addition to the creatinine and urea tests, your doctor may also order a simple urine test, called a BSE or urine type 1 test.

What are the normal values?

Normal creatinine levels range from 0.6 to 1.3 mg/dl. These values, however, are not absolute and should be interpreted by your doctor.

Since creatinine is produced by muscles, muscular people have higher basal levels. A young, muscular athlete may have up to 1.4 mg/dl creatinine without having kidney disease, while a thin, elderly woman with 1.2 mg/dl may have kidney disease.

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Therefore, creatinine is not to be interpreted as an absolute value. One has to take into account the gender, age, and weight of the patient. In general, however, creatinine values above 1.5 or 1.6 mg/dl are a sign of kidney disease in the vast majority of cases.

From the creatinine results your doctor can calculate the glomerular filtration rate (also called creatinine clearance), which is basically the volume of blood filtered by the kidney every minute. Normal kidneys filter up to 180 liters of blood per day (approximately 120 ml/min). Values below 60 ml/min are indicative of chronic renal failure.

Calculate your Glomerular Filtration Rate

From the creatinine results your doctor can calculate your GFR (also called creatinine clearance), which is basically the volume of blood filtered by your kidneys every minute. Normal kidneys filter up to 180 liters of blood per day (approximately 120 ml/min). Values below 60 ml/min are indicative of chronic renal failure.

How to lower creatinine?

There is no specific drug to lower creatinine. It is important to remember that creatinine is only a marker of kidney dysfunction. It is not creatinine itself that is bad for the body. It rises when the kidneys are malfunctioning.

The recovery of kidney function is possible in cases of acute renal failure, when the aggression to the kidney is punctual. In these cases, treatment must be directed at the acute disease that is causing the kidney damage.

In cases of chronic renal failure, the kidneys already have irreversible damage from years of aggression caused by diabetes, hypertension, drugs toxic to the kidneys, etc. Therefore, it is unlikely that there is still a possibility to start some treatment that will restore the kidney function and, consequently, lower the creatinine values.