The physical and chemical properties of urine have long been recognized as important indicators of health. It is the purpose of this article to present an explanation of the tests included in a routine urinalysis, and also to include some of the screening tests which are requested to be done on random urine samples.
The term “screening” implies that a positive result should be followed up by further studies such as quantitative tests. The scope of this article will not include these quantitative procedures.
A routine urinalysis, which is frequently referred to as an R+M (routine and microscopic), includes examination for urinary color, appearance, specific gravity, pH, protein, glucose, ketones, and occult blood, as well as a microscopic examination of the sediment. Because of the recent production of dipsticks that are capable of measuring seven or eight parameters, some laboratories now include bilirubin, nitrite, and urobilinogen in the routine urinalysis. An R+M on a young child should also include a screening test for reducing substances in order to allow for the early detection of congenital defects in carbohydrate metabolism.
In spite of all the technical advances in the clinical laboratory, the value of a urinalysis is dependent upon the ability of the technologist who performs it. Care must be taken to properly interpret and evaluate the various tests. It is the aim of this article to provide a simple explanation of these tests, and by means of photomicrographs to familiarize the reader with the structures found in the urinary sediment.
The Formation of Urine
The kidneys are paired organs which are located in the small of the back on each side of the spine. They are responsible for maintaining homeostasis including the regulation of body fluids, acid-base balance, electrolyte balance and the excretion of waste products. They are also concerned with the maintenance of blood pressure and erythropoiesis. Renal function is influenced by the blood volume, pressure, and composition, as well as by the adrenal and pituitary glands.
The formation of urine involves the complex processes of blood filtration, the reabsorption of essential substances including water, and the tubular secretion of certain substances. After formation in the kidney, the urine passes down the ureter into the bladder, where it is temporarily stored before being excreted through the urethra.
The nephron is the functional unit of the kidney and there are approximately one million nephrons in each kidney. The nephron consists of a capillary network, called the glomerulus, and a long tubule which is divided into three parts: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. Each nephron empties into a collecting tubule to which other nephrons are connected. The urine then collects in the renal pelvis and empties into the ureter. The glomerulus and the convoluted tubules are located in the cortex of the kidney, while the loop of Henle extends down into the medulla.
Approximately 20-25% of the blood that leaves the left ventricle of the heart enters the kidneys by way of the renal arteries. This means that in a normal adult the blood passes through the kidneys at a rate of about 1200 ml/min, or 600 ml/min/kidney. After the renal artery enters the kidney it breaks up into smaller branches until thousands of tiny arterioles are formed. These arterioles are called afferent arterioles because they carry the blood to the nephrons. Each afferent arteriole then forms the capillary network of a glomerulus.
The glomerulus is surrounded by a structure called the Bowman’s capsule, and the space that is formed between the capsule and the glomerulus is the Bowman’s space. As a result of its special structure, the glomerular wall acts as an ultrafilter which is very permeable to water. The pressure of the blood within the glomerulus forces water and dissolved solutes with a molecular weight of less than 50,000 through the semipermeable capillary membrane and into the Bowman’s space (Shaw and Benson 1974). The remainder of the blood including blood cells, plasma proteins, and large molecules, leaves the glomerulus via the efferent arteriole and enters a second capillary network, called the peritubular capillaries, which surrounds the tubules.
Approximately 120 ml/min, or one-fifth, of the renal plasma is filtered through the glomeruli forming what is known as the ultra-filtrate. The ultrafiltrate has the same composition as blood plasma but it is normally free of protein except for about 10 mg/dl of lowmolecular-weight protein (Sisson 1976). Some of the filtered products include water, glucose, electrolytes, amino acids, urea, uric acid, creatinine, and ammonia.
As the glomerular filtrate passes through the proximal tubules, a large portion of the water, sodium chloride, bicarbonate, potassium, calcium, amino acids, phosphate, protein, glucose, and other threshold substances needed by the body are reabsorbed and pass back into the bloodstream. These substances are reabsorbed in varying proportions so that while proteins and glucose, for example, appear to be almost completely reabsorbed, sodium chloride is only partly reabsorbed, and there is no reabsorption of creatinine. Over 80% of the filtrate is reabsorbed in the proximal tubule. The unique structure of the proximal tubule makes this reabsorption possible. The epithelial cells that line this portion of the tubule have a brush border of microvilli which provides a large surface area for reabsorption and secretion. These microvilli contain various enzymes such as carbonic anhydrase which help in these processes (Bennett and Glassnock n.d.).
Threshold substances are those substances which are almost completely reabsorbed by the renal tubules when their concentration in the plasma is within normal limits. When the normal plasma level is exceeded, the substance is no longer totally reabsorbed and therefore appears in the urine. Glucose is a high threshold substance because it usually does not appear in the urine until the plasma concentration exceeds about 160 to 180 mg/dl. Some of the other threshold substances include sodium chloride, amino acids, potassium, creatine, and ascorbic acid.
As the filtrate moves through the tubules, various substances are added to it by the process of tubular secretion. In the proximal tubule, sulfates, glucuronides, hippurates, hydrogen ions, and drugs such as penicillin are some of the substances which are secreted. In the proximal as well as the distal tubule, the hydrogen ions are exchanged for the sodium ions of sodium bicarbonate. The hydrogen ions then combine with the bicarbonate in the filtrate to form carbonic acid which in the presence of carbonic anhydrase breaks down to water and carbon dioxide. The carbon dioxide then diffuses back out of the tubule, and thus, both the sodium and bicarbonate are reabsorbed.
Like the proximal tubule, the descending limb of the loop of Henle is very permeable to water, but the resorption of solutes does not occur in this part of the loop (Murphy and Henry 1979). The ascending limb, however, is nearly impermeable to water, but there is active resorption of sodium, chloride, calcium, and magnesium. As a result of the loss of sodium chloride, the fluid that leaves the loop of Henle has a lower osmolality than plasma. In this section of the tubule and in the remaining tubule, hydrogen ion and ammonia are secreted.
The mechanism that provides for the absorption of water from the descending loop, and the resorption of solute without water in the ascending limb, is called countercurrent multiplication. There is a set of blood vessels called the “vasa recta” that is parallel to and shaped the same as Henle’s loop. In the vasa recta, solute diffuses out of the interstitium of the medulla and into the ascending limb, and then out of the ascending limb back into the interstitium. Water, however, moves in the opposite direction or out of the descending limb and back into the ascending one. The net effect is to retain only solute, and not water, in the interstitium of the medulla. This process coupled with the resorption of solute from the ascending loop of Henle results in an interstitium which is hypertonic, thus, causing water to be absorbed from the descending loop and the collecting tubule.
About 90% of the glomerular filtrate is reabsorbed by the time it reaches the distal tubule (Wilson 1975). The main function of the distal and collecting tubules is the adjustment of the pH, osmolality, and electrolyte content of the urine, as well as the regulation of those substances still present in the filtrate. Potassium, ammonia, and hydrogen ions are secreted by this portion of the nephron, and sodium and bicarbonate are reabsorbed by the same mechanism as in the proximal tubule. Potassium ions are also exchanged for sodium ions, and this exchange is enhanced by aldosterone which is secreted by the adrenal cortex. The ammonia that is secreted combines with hydrogen ions to form ammonium ions and this helps to regulate the hydrogen ion concentration of the urine. In the collecting duct, urea is also reabsorbed.
The absorption of water in the distal portion of the nephron is regulated by antidiuretic hormone (ADH) which is secreted by the pituitary gland. When the body needs to conserve water, ADH is secreted, and the walls of the distal and collecting tubules are made very permeable, thereby allowing water to be reabsorbed. If the body has excess water, less ADH is produced, the walls of the tubules become less permeable, and the volume of excreted urine increases.
Of the approximate 120 ml/min that was filtered at the glomerulus, only an average of 1 ml/min is finally excreted as urine. This quantity can range from 0.3 ml in dehydration to 15 ml in excessive hydration. For an adult the normal average daily volume of urine is about 1200-1500 ml, with more urine produced during the day than at night. However, the normal range may be from 600-2000 ml/24 h (Bradley et al. 1979). Polyuria is an abnormal increase in the volume of urine (more than 2500 ml), as in diabetes insipidus and diabetes mellitus. Oliguria is a decrease in urinary volume, such as occurs in shock and acute nephritis. In an adult it is frequently defined as being less than 500 ml/24 h (Wagoner and Holley 1978, Muth 1978) or less than 300 ml/m2/24 h. The term anuria designates the complete suppression of urine formation, although in the wider sense of the term it is sometimes defined as being less than 100 ml/24 h during 2 to 3 consecutive days, in spite of a high fluid intake (Renyi-Vamos and Babics 1972).
The main constituents of urine are water, urea, uric acid, creatinine, sodium, potassium, chloride, calcium, magnesium, phosphates, sulfates, and ammonia. In 24 hours the body excretes approximately 60 g of dissolved material, half of which is urea (Race and White 1979). In some pathologic conditions, certain substances, such as ketone bodies, protein, glucose, porphyrins, and bilirubin, appear in large quantities. Urine can also contain structures such as casts, crystals, blood cells, and epithelial cells.
Some of the renal disorders that a urinalysis can help in diagnosing include: cystitis, which is the inflammation of the bladder; nephritis, which is the inflammation of the kidney and can either be present with bacterial infection (pyelonephritis), or without infection (glomerulonephritis); and nephrosis, which is the degeneration of the kidney without inflammation.
Collection of Specimen
The performance of an accurate urinalysis begins with the proper collection technique. There are several methods available, depending on the type of specimen needed.
The first important step is the use of a clean, dry container. Disposable containers are preferred by most laboratories, since they avoid the possibility of contamination from improperly washed glass urine bottles. Samples that are to be cultured must be collected in a sterile container. If the specimen is being collected into a bedpan first, then the bedpan must also be sterile.
One method frequently used is that of collecting the entire voided sample. The problem with this method is that the specimen cannot be used for bacterial examination. Also, in female patients the sample is often contaminated with vaginal discharge.
Catheterization of the bladder is sometimes necessary to obtain a suitable specimen. This method may be used if the patient is having difficulty voiding. It can also be used in a female patient to avoid vaginal contamination, especially during menstruation. However, since this procedure carries with it the possibility of introducing organisms into the bladder which may, in turn, cause infection, it should not be routinely used for the collection of culture specimens.
Suprapubic aspiration of the bladder is sometimes used in place of catheterization for obtaining a single urine sample. It involves the insertion of a needle directly into the distended bladder. This technique avoids vaginal and urethral contamination and can also be useful in getting urine from infants and small children. The specimen obtained by this method can also be used for cytology studies.
The “clean-catch” or clean-voided midstream specimen is usually the method of choice. It is easy to perform and it provides a sample that can be used for bacteriologic examination as well as for routine urinalysis. Prior to collection, the external genitalia are thoroughly cleansed with a mild antiseptic solution. During the collection the initial portion of the urine stream is allowed to escape while the midstream portion is collected into a sterile container.