Analysis of the renal function

"The different determinations that are analyzed in the laboratory make it possible to detect whether the kidney is functioning correctly or not".

The functions of the kidneys are:

  • The filtration of the blood that reaches the glomerular capillaries.
  • The tubular reabsorption of substances that should not be eliminated.
  • The tubular secretion of substances that can also suffer the two previous processes.
  • The balance of these processes depends on the correct formation of urine that presents a composition, density, pH and volume.
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When is kidney function studied?

In order to know the state of the kidney function, a set of biochemical tests are performed from blood and urine samples collected during 24 hours, together with the microscopic observation of the urinary sediment. 

These tests help the doctor to focus well on the diagnosis and treatment of a disease.

For the correct collection of the 24-hour urine sample, the first urination of 08:00 hours should be discarded, collecting all urinations for the next 24 hours in a suitable sterile container, which can be provided by the hospital or purchased from a pharmacy.

Most frequent indications of this test:

  • Glomerulonephritis.
  • Urinary tract infections.
  • Acute renal failure.
  • Chronic renal insufficiency.
  • Pielonephritis.

Do you have any of these diseases?

You may need to have your kidney function tested

Types of kidney function tests

Creatinine is a substance of muscular origin made up of three amino acids. The amount of creatinine that appears in an individual's blood depends on his or her muscle mass.

This concentration will be constant for each individual if their muscle mass does not vary (reference values: women: 0.4-1.3 mg/dL; men: 0.5-1.2 mg/dL). Creatinine undergoes glomerular filtration but is not reabsorbed and its tubular secretion is minimal.

According to this, the increase of creatinine in the blood would indicate a great muscular exchange either pathological, because the muscle is "breaking", or physiological, if the individual presents a great muscular mass, as in the case of sportsmen.

On the other hand, the increase of creatinine in the blood may be due to a bad glomerular filtration. It is evaluated by determining the 24-hour urine creatinine, establishing the relationship between it and the concentration of creatinine in the blood.

This parameter is called Creatinine Clearance. Its units are ml/min. and values the glomerular filtration. The normal value of creatinine clearance ranges from 100-130 ml / min. Its decrease indicates that the glomerulus is filtering less than it should while its elevation would indicate an abnormally high filtration.

It is the non-toxic form of ammonia that is generated in the body from the degradation of proteins from both the diet and physiological turnover.

Due to its small size, it presents a variable reabsorption and secretion in the renal tubule accompanying the water. The values normally observed in blood for an individual in fasting are: 0.1-0.5 g./l.

The retention of urea in the blood reflects renal malfunction globally, although it is affected by a diet rich in protein, by liver function and by catabolic states.

In addition, in the tubule, urea accompanies water, so that if diuresis is elevated, water excretion is greater and therefore urea will be eliminated. On the contrary, if the subject presents a low diuresis (dehydration, hemorrhage, heart failure, renal failure, etc.) the reabsorption will increase, and therefore the concentration of urea in the blood.

They are free ions (such as Sodium (Na), Potassium (K), Chlorine (Cl) and Bicarbonate (HCO3)) that exist in body fluids. All metabolic processes in the body affect the concentration of electrolytes in blood and urine. Their concentration (mmol/l) is decisive for the osmolarity, hydration status and pH of body fluids.

Along the nephron, the electrolytes are reabsorbed or secreted as needed to regulate their blood concentration and to regulate both the osmotic charge and the pH of the urine.

The existence of a renal pathology will be reflected in the imbalance of the concentration of these substances in both blood and 24-hour urine. The interpretation of these determinations is complex since many pathologies, other than renal, alter their concentration.

Together with other tests such as clarification of creatinine, determination of blood and urine urea, determination of Calcium (Ca) and Phosphorus (PO4H3-) in blood and urine, etc., represent a good approximation of renal function.

Protein in the urine or proteinuria does not normally appear in urine except in certain circumstances such as pregnancy, after sport, after a long time on your feet, etc. However, there are pathological causes that manifest themselves with proteinuria.

The laboratory study of proteinuria begins with the determination of the concentration of total protein in urine. If its presence is detected, it must be ruled out that it is due to a non-renal pathology involving increased production, as in myeloma, fever, inflammatory processes, burns, etc. Once these possibilities have been ruled out, the cause will be renal. In this case, proteinuria may be due to an alteration of the glomerulus that allows proteins to filter and/or an alteration of the tubule, which does not reabsorb them.

  • Glomerular Proteinuria: To study it, a protein of a limit size for filtration is determined, for example albumin, and another large protein that under normal conditions does not filter, such as immunoglobulins. If these proteins appear in urine, they indicate a glomerular lesion.
  • Tubular Proteinuria: to study it, a protein is determined that filters into the glomerulus but is totally reabsorbed in the tubule, so it should not appear in the urine. Small proteins are determined, such as retinol transport protein, a1-microglobulin, b2-microglobulin, etc., if they appear in urine it is because the tubule is not reabsorbing correctly.
  • Mixed Proteinuria: all types of proteins appear in urine because they are damaged both in the glomerulus and the tubule of the nephrons. All the parameters described above will indicate that there is kidney damage.

Where do we do it?


The Clinical Biochemistry Service
of the Clínica Universidad de Navarra

The Clinical Biochemistry Service of the Clinica Universidad de Navarra is responsible for performing the biochemical analyses requested by the medical specialists of our center.

We carry out technical quality controls to guarantee the proper functioning of the equipment and the highest precision in the results obtained from the samples.

In order to guarantee excellence in patient service, we offer the response with the results of the analyses in the shortest possible time, responding in only 46 minutes in some cases of general analysis.

Organized in care units

  • General biochemistry.
  • Electrolytes.
  • Hormones, urine and proteins.
  • Markers.
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Why at the Clinica?

  • Maximum speed in offering the analytical results.
  • We carry out quality controls to guarantee the correct and most precise results obtained.
  • We work in a multidisciplinary way with all the departments of the Clinic.