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".
DR. ESTIBALIZ ALEGRE SPECIALIST. BIOCHEMISTRY DEPARTMENT
What are they and what are they for?
The evaluation of kidney function combines laboratory tests and imaging techniques, selected according to each individual case. Blood and urine tests are the first step to detect abnormalities in filtration and the balance of fluids and electrolytes. Ultrasound is the preferred imaging technique because it is safe and uses no radiation. CT scans and magnetic resonance imaging (MRI) are reserved for specific indications, with special precautions in patients with chronic kidney disease due to the use of contrast agents. Nuclear medicine studies can measure the function of each kidney separately and reveal scarring. In selected cases, a kidney biopsy provides highly detailed diagnostic information by examining tissue under different types of microscopy. Additionally, bioelectrical impedance analysis helps assess nutritional status and body composition. Each of these tests provides different insights, and the physician will determine which ones are needed based on the patient’s clinical situation.
When is kidney function studied?
Kidney function is evaluated when there are signs or risk factors of kidney damage.
These studies are requested in the presence of swelling in the legs or face, high blood pressure, changes in urination, or abnormal results in previous tests. They are also recommended for people with diabetes, autoimmune diseases, or a family history of kidney disease.
In hospitals, these tests are commonly performed before administering certain medications or radiologic contrast agents. Likewise, periodic monitoring is carried out in patients with chronic kidney disease or those who have undergone a transplant, to assess how their kidney function is progressing.
Diseases for which renal function tests are requested
- Chronic kidney disease (CKD): to assess disease progression and adjust treatment.
- Diabetic kidney disease: due to its direct link with the development of diabetic nephropathy.
- Kidney disease and hypertension: since high blood pressure can damage the renal blood vessels.
- Glomerulonephritis: to evaluate filtering function and inflammation of the glomeruli.
- Acute kidney injury: to determine the cause, severity, and recovery of kidney function.
Do you have any of these diseases?
You may need to have your kidney function tested
Types of kidney function tests
Blood and urine tests
These are the most common, non-invasive, and first-line tests to assess kidney function. They provide complementary information and, in most cases, guide both diagnosis and follow-up.
- Serum creatinine and estimated glomerular filtration rate (eGFR): the laboratory calculates the eGFR based on creatinine levels. Together they indicate the degree of kidney function and help classify chronic kidney disease by stages.
- Cystatin C: an alternative marker that can be more reliable when creatinine does not accurately reflect kidney function (e.g., very low or high muscle mass, elderly individuals, athletes). It helps confirm eGFR results when there is uncertainty.
- Albuminuria/Proteinuria: the urine albumin-to-creatinine ratio (ACR) in a single sample is the preferred test to detect protein loss. The persistent presence of albumin is an early marker of kidney and cardiovascular damage.
- Urea (BUN): less specific than creatinine; it varies with hydration, diet, and certain medications but complements the overall assessment.
- Urinalysis (dipstick and sediment): detects blood, white cells, nitrites, glucose, or crystals—useful for identifying infections, kidney stones, or glomerular diseases.
- 24-hour urine collection: reserved for specific situations (quantifying high proteinuria, evaluating kidney stones, measuring creatinine clearance) when a spot sample is insufficient.
How to prepare: maintain normal hydration the day before, avoid intense exercise and high-protein meals right before testing, and inform your doctor of all medications (some, such as anti-inflammatories or certain antibiotics, may affect results). For urine testing, provide a sample of the first morning urine or a mid-stream sample, avoiding contamination (especially during menstruation).
Interpretation and next steps: a reduced eGFR and/or persistent albuminuria in two separate tests usually indicate kidney damage and require medical evaluation. Sometimes it is necessary to repeat the tests (due to dehydration, urinary infection, or recent physical exertion) or to complement them with other techniques depending on the clinical situation.
Bioelectrical Impedance Analysis
A non-invasive and quick method used to estimate body composition: total body water (intra- and extracellular), lean/muscle mass, and fat mass. In nephrology, it helps detect fluid overload or dehydration, assess nutritional status, and guide decisions regarding dialysis, diet, and exercise.
- How it works: a very low electrical current is applied, and the body’s resistance and reactance are measured. The test is painless and involves no radiation.
- Preparation: fasting is not usually required; come well hydrated unless otherwise instructed, and remove any metallic objects.
- Limitations: results may vary with hydration status, edema, or implanted devices. They should always be interpreted alongside clinical assessment and laboratory findings.
Radiologic imaging tests
Imaging techniques help assess the shape, size, and structure of the kidneys and detect problems such as stones, cysts, tumors, or obstructions. The choice of test depends on the clinical situation and the patient’s kidney function.
- 2D and Doppler renal ultrasound: safe, radiation-free, quick, and the most commonly used. It allows visualization of kidney size, cysts, or dilatations (hydronephrosis), measures post-void residual urine in the bladder, and evaluates the blood flow in renal arteries and veins using Doppler.
- PoCUS (Point-of-Care Ultrasound): an ultrasound performed directly at the patient’s bedside for immediate evaluation, without the need to move the patient to another department. In nephrology, it helps assess the kidneys and bladder, estimate the fluid status (vena cava, lungs), and detect fluid accumulation in the abdomen or around the heart and lungs. It reduces waiting times, avoids radiation exposure, and supports timely clinical decisions such as adjusting dialysis or identifying complications early.
- CT scan (computed tomography): very useful for detecting kidney stones (without contrast) and for characterizing masses or assessing complications (usually with iodinated contrast). Contrast is used with caution in patients with chronic kidney disease or at risk of acute kidney injury; prior hydration and medication adjustments (e.g., metformin, as directed by the physician) may be required.
- Magnetic resonance imaging (MRI): an alternative when it is necessary to avoid radiation or when CT is not suitable. In advanced kidney failure, gadolinium (contrast agent) may be contraindicated; many MRI scans are performed without contrast or coordinated with dialysis, depending on the case.
Safety and preparation: inform your healthcare team if you are pregnant, have a pacemaker or other metal implants, allergies to contrast agents, or a history of kidney disease. Arrive well hydrated unless your doctor advises otherwise. If contrast is used, it is normal to feel a brief warm sensation or a metallic taste; afterward, drink fluids (if not restricted) and follow your medical team’s instructions.
Nuclear Medicine Tests
Nuclear Medicine Tests
These are specialized studies that show how each kidney functions separately. They involve a small intravenous injection of a low-dose radiopharmaceutical and capturing images with a special camera. The test is painless, the radiation exposure is minimal, and it is generally very safe.
- Dynamic renal scintigraphy (MAG3, DTPA): evaluates in real time how the radiotracer enters the kidney, is processed, and excreted into the bladder. It allows measurement of the individual function of each kidney (right/left percentage) and helps detect urinary tract obstructions.
- Static renal scintigraphy (DMSA): shows which parts of the kidney function best and helps identify scarring or areas with reduced activity. It is useful after recurrent urinary tract infections or to assess congenital abnormalities.
- Isotopic measurement of glomerular filtration rate (GFR): the most accurate method to determine kidney filtration capacity. It is not performed routinely but is indicated when an exact measurement is required, such as before certain treatments or surgeries.
How to prepare: fasting is usually not required; come well hydrated unless instructed otherwise. Inform your healthcare team if you are pregnant or breastfeeding. After the test, it is recommended to drink plenty of fluids (unless restricted) to help eliminate the radiotracer more quickly.
Kidney biopsy
A kidney biopsy is a test with diagnostic, prognostic, and therapeutic value. At our center, it is performed under ultrasound guidance, as this is the safest and most accurate technique. Using local anesthesia, a thin needle is inserted to obtain small tissue samples from the kidney.
It allows specialists to identify the type and cause of kidney disease, estimate its progression, and adjust treatment (immunosuppressants, therapeutic changes, etc.). Once collected, the tissue sample is examined using different techniques:
- Light microscopy: examines glomeruli, tubules, and blood vessels to detect inflammation, scarring, or abnormal deposits.
- Immunofluorescence: uses antibodies that emit fluorescence to reveal immune complexes or protein deposits that may be damaging the kidney.
- Electron microscopy: provides an ultrastructural level of detail (e.g., glomerular basement membrane), essential when other methods are inconclusive.
Together, these complementary techniques provide a more accurate diagnosis of the type and cause of kidney disease.
Where do we do it?
IN NAVARRE AND MADRID
The Nephrology Service
of the Clínica Universidad de Navarra
The Nephrology Service of the Clínica Universidad de Navarra has more than five decades of experience, both in the diagnosis and treatment of all kidney pathologies and in the transplant of this organ.
Our specialists have completed their training in centers of national and international reference.
We have the best facilities in the Dialysis Unit in order to offer the highest quality care to our patients.
Why at the Clinica?
- National reference in kidney transplantation, pioneer in living donor kidney transplantation.
- Specialized nursing for the care and follow-up of our patients.
- Cardiovascular and renal damage prevention program.
Our team of professionals
