Kidney disease is a chronic condition that affects the kidneys’ ability to filter waste products and excess fluid from the blood. An ultrasound is a non-invasive imaging technique that uses high-frequency sound waves to create real-time images of internal body structures, often used as a first step in evaluating kidney health. While it identifies physical changes, it cannot provide a complete picture of how well the kidneys are functioning. A comprehensive diagnosis relies on combining structural information from the ultrasound with specific laboratory tests.
Visualizing Kidney Structure and Pathology
Ultrasound is useful for detecting physical changes and structural abnormalities within the kidney and the surrounding urinary tract. The technique measures the size and shape of the kidneys; a normal adult kidney typically measures between 9 and 13 centimeters in length. Changes in size, such as atrophy (shrinking) in chronic disease or enlargement in acute conditions, provide initial clues about the kidney’s health status.
The imaging is effective at identifying obstructions within the urinary system. For example, kidney stones or tumors blocking the ureters can cause hydronephrosis, which is the swelling of the kidney’s collecting system due to urine backup. Ultrasound images clearly show this fluid dilation and can pinpoint the location of the blockage.
Beyond size and obstruction, the ultrasound assesses the internal texture of the kidney tissue, known as its echogenicity. Increased echogenicity, where the tissue appears brighter, often suggests fibrosis, inflammation, or scarring within the renal cortex, common findings in progressive chronic kidney disease.
The scan can also reveal the presence of cysts (fluid-filled sacs) or solid masses that might be tumors, helping determine if the disease is structural. Furthermore, the thickness of the renal cortex, normally 7 to 10 millimeters, can be measured. Reduced thickness often indicates advanced tissue damage and correlates with decreased function.
Why Ultrasound Alone Cannot Determine Kidney Function
While ultrasound excels at visualizing the physical structure of the kidneys, it cannot measure the organ’s physiological performance. The primary role of the kidneys is to filter blood, and imaging technology cannot directly assess the speed or efficiency of this filtration process. The Glomerular Filtration Rate (GFR) estimates how much blood passes through the filters in the kidneys each minute.
The ultrasound image cannot generate a GFR value, meaning it cannot detect subtle or early-stage functional decline when the kidney’s physical structure still appears normal. Many types of kidney disease begin with microscopic damage to the filtering units, which does not immediately cause visible changes on the scan. Structural findings, such as increased echogenicity or reduced cortical thickness, offer only an indirect correlation with functional impairment.
Furthermore, the ultrasound often cannot determine the specific underlying cause or type of kidney disease. Differentiating between chronic kidney disease (developing slowly) and acute kidney injury (happening rapidly) requires functional testing. The visual appearance may look similar in various conditions, and the scan cannot distinguish between specific diseases like glomerulonephritis or hypertensive nephropathy, which require different treatment strategies.
Essential Companion Tests for a Complete Diagnosis
Because ultrasound only provides structural information, a complete diagnosis and staging of kidney disease require specific laboratory tests that measure actual kidney function. The most common companion test is a blood test measuring serum creatinine, a waste product generated by muscle metabolism. Creatinine levels rise when the kidneys are not filtering efficiently, marking impaired function.
The serum creatinine result is used in a mathematical formula to calculate the estimated Glomerular Filtration Rate (eGFR). The eGFR is the most reliable indicator of overall kidney function. A persistently low eGFR (typically below 60 milliliters per minute per 1.73 square meters) defines chronic kidney disease. This functional assessment determines the stage of the disease and guides treatment decisions.
A urine test, or urinalysis, is also crucial for detecting early damage that the ultrasound misses. This test checks for the presence of protein, specifically albumin, and blood in the urine. The leakage of albumin into the urine (albuminuria) is often one of the earliest signs of damage to the kidney’s filtering membrane.
If the ultrasound and initial blood work are inconclusive or suggest a complex issue, advanced imaging may be necessary. Options include a CT scan or MRI. A kidney biopsy may also be performed to obtain a tissue sample, which provides the most definitive diagnosis of the underlying pathology.