Kidney Function Tests Health Article

Definition

Kidney function tests are a cadre of tests that are used to screen for and manage renal disease. Tests commonly used for this purpose are plasma creatinine, blood urea nitrogen (BUN), electrolytes, and routine urinalysis. Additional laboratory tests are performed to evaluate abnormal renal function and help differentiate between causes. The most commonly used follow-up tests are creatinine clearance, plasma and urine osmolality, and urine sodium.

Purpose

Renal function tests are used to screen for renal disease, to differentiate the cause of renal disease, and to determine the extent of renal dysfunction. These tests attempt to define the clinical state of renal dysfunction and not the process of injury. The latter is determined primarily by a combination of clinical data and biopsy to determine the histological pattern of injury.

Precautions

A complete history should be taken prior to kidney function tests to assess the patient's symptoms and food and drug intake. A wide variety of prescription and over-the-counter medications can affect blood and urine kidney function test results, as can some food and beverages. Renal function tests are performed on both blood and urine. Blood samples are collected by venipuncture from a vein in the crease of the arm. The nurse or phlebotomist performing the procedure should observe universal precautions for the prevention of transmission of blood-borne pathogens. The creatinine clearance test requires a timed urine sample. Explicit written instruction must accompany the explanation of how to collect this sample. It is imperative that the patient empty his or her bladder at the start of the test and not include this urine in the collection. It is equally important that all urine produced during the time of the test be saved and refrigerated, and that the bladder be emptied completely and this urine added to the collection at the end of the test.

Description

The kidneys are a pair of organs located in the back of the abdominal cavity on either side of the vertebral column. Their purpose is to filter the blood and remove wastes and excess water. They also selectively reabsorb compounds that have been filtered, thus conserving essential nutrients, electrolytes, amino acids and other biomolecules. Approximately one-quarter of the cardiac output, 1200 mL of blood per minute are received by the kidneys. Each kidney is made up of functional micro-scopic units called nephrons. Each nephron contains a capillary tuft, the glomerulus and a tubule. Blood flows into the kidneys, and engorges the capillary tufts. Water and small solutes pass through the vessel walls forming a filtrate of the plasma which enters the underlying space, Bowman's capsule. The walls of the capsule form a tubule that traverses the kidney. Blood leaves the glomerular capillaries through an efferent arteriole which forms a capillary network, the vasa recta, that follows the path of the tubules. The cells of the renal tubule modify the filtrate along its length ultimately forming urine that passes out of the body. The tubule is responsible for two processes, reabsorption and secretion. Reabsorption is the process of moving solutes from the tubular lumen into the interstitium that bathes the tubules, so that they can be absorbed by the vasa recta. Some substances such as glucose and sodium are one-hundred percent reabsorbed until the plasma level exceeds a certain concentration called the renal threshold. Secretion is the process of transporting solutes from the interstitium into the tubular lumen, so that they can be excreted in the urine. Secretion allows substances such as hydrogen ions to be eliminated at a rate that exceeds glomerular filtration. These processes are controlled by the selective permeability of different segments of the tubule to water, salt and urea, and the response of the distal collecting tubules to hormones such as aldosterone, antidiuretic hormone, and parathyroid hormone.

When kidney function becomes compromised by disease, the processes of glomerular filtration and tubular reabsorption and secretion become affected to different extents. This can result in retention of waste products that are incompletely filtered, loss of essential solutes that are not reabsorbed, and failure of the tubules to respond to hormonal control of electrolyte and water balance. Blood and urine biochemistry tests reflect the extent of this dys-function and are used to characterize the clinical state of the patient. Fortunately, the kidneys have a large reserve capacity, and a significant amount of damage must be incurred before kidney function tests become significantly abnormal.

There are several renal states that can be categorized by renal function test results, but the two major ones are acute and chronic renal failure. Renal failure is a term used to describe a loss of renal function characterized by uremia, the retention of nitrogenous wastes in the blood. The acute form is rapid in onset and often reversible. It can occur as three different states, termed prerenal, renal (intrarenal), and postrenal failure. Prerenal failure results from decreased blood flow to the kidneys, and its most common cause is congestive heart failure. Renal failure results from injury to the glomerulus and the tubules. The most common causes are glomerulonephritis which is mediated by autoantibodies that damage the glomerulus and obstruct the tubules; pyelonephritis which is caused by a bacterial infection of the interstitium; and tubular damage caused by drugs, heavy metals, and viral infections. Post renal failure is caused by obstruction below the kidneys. This can result from urinary tract stones, tumors, or anatomic obstruction as in benign prostatic hypertrophy. The chronic form is characterized by slow onset without accompanying symptoms in its early stage. Chronic renal failure often follows episodes of acute renal failure, and it is not reversible. Chronic renal failure is most commonly a sequalae to acute glomerulonephritis or pyelonephritis which together account for more than half the cases. Other causes of chronic renal failure are chronic diseases such as diabetes mellitus, renal vascular disease (e.g., atherosclerosis of the renal vessels), hypertension, polycystic kidney disease, drug damage, and kidney stones. Kidneys from patients with chronic renal failure will appear smaller than average and a biopsy of the kidney will demonstrate scarring of the tubules.