Urinalysis

Definition

Aurinalysis is a group of manual and/orautomated qualitative and semi-quantitative tests performed on a urine sample. A routine urinalysis usually includes the following tests: color, transparency, specific gravity, pH, protein, glucose, ketones, blood, bilirubin, nitrite, urobilinogen, and leukocyte esterase. Some laboratories include a microscopic examination of urinary sediment with all routine urinalysis tests. If not, it is customary to perform the microscopic exam, if transparency, glucose, protein, blood, nitrite, or leukocyte esterase is abnormal.

Purpose

Routine urinalysis is performed for several reasons:

• general health screening to detect renal and metabolic diseases
• diagnosis of diseases or disorders of the kidneys or urinary tract
• monitoring of patients with diabetes

In addition, quantitative urinalysis tests may be performed for diagnosis of many specific disorders, such as endocrine diseases, bladder cancer, osteoporosis, and phorphyrias. This often requires the use of a timed urine sample. Examples include the d-xylose absorption test for malabsorption, creatinine clearance test for glomerular function, the 24-hour urinary metanephrine test for pheochromocytoma, and the microalbumin test. The urinary microalbumin test measures the rate of albumin excretion in the urine using immunoassay. This test is used to monitor the renal vascular function of persons with diabetes mellitus. In diabetics, the excretion of greater than 200 μg/mL albumin is predictive of impending glomerular disease.

Precautions

Voided specimens

All patients should avoid intense athletic training or heavy physical work before the test, as these activities may cause small amounts of blood to appear in the urine. Many urinary constituents are labile, and samples should be tested within one hour of collection or refrigerated. Samples may be stored at 36-46°F (2-8°C) for up to 24 hours for chemical urinalysis tests; however, the microscopic exam should be performed within four hours, if possible. To minimize sample contamination, women who require a urinalysis during menstruation should insert a fresh tampon before providing a urine sample.

Over two-dozen drugs are known to interfere with various chemical urinalysis tests. These include:

• ascorbic acid
• chlorpromazine
• L-dopa
• nitrofurantoin (Macrodantin, Furadantin)
• penicillin
• phenazopyridine (Pyridium)
• rifampin (Rifadin)
• tolbutamide

Preservatives used to prevent loss of glucose and cells may affect biochemical test results. The use of preservatives should be avoided whenever possible.

Description

Routine urinalysis consists of three testing groups, physical characteristics, biochemical tests, and microscopic evaluation.

Physical tests

Physical tests are color, transparency (clarity), and specific gravity. In some cases, volume (daily output) may be measured. Color and transparency are determined from visual observation.

COLOR. Normal urine is straw to amber in color. Abnormal colors include bright yellow, brown, black (gray), red, and green. These pigments may result from medications, dietary sources, or diseases. For example, red urine may be caused by blood or hemoglobin, beets, medications, and some porphyrias. Black-gray urine may result from melanin (melanoma) or homogentisic acid (alkaptonuria). Bright yellow urine may be caused by bilirubin. Green urine may be caused by biliverdin or medications. Orange urine may be caused by some medications or excessive urobilinogen. Brown urine may be caused by excessive amounts of prophobilin, or urobilin.

TRANSPARENCY. Normal urine is transparent. Cloudy or turbid urine may be caused by both normal or abnormal processes. Normal conditions giving rise to turbid urine include precipitation of crystals (usually urates or phosphates), mucus, or vaginal discharge. Abnormal causes of turbidity include the presence of blood cells, yeast, and bacteria. Turbidity is typically graded by visual comparison to standard solutions of barium sulfate.

SPECIFIC GRAVITY. The specific gravity of urine is a measure of the concentration of dissolved solutes, and it reflects the ability of the renal tubules to concentrate the urine (conserve water). It is usually measured by determining the refractive index of a urine sample (refractometry) or by chemical analysis. Specific gravity varies with fluid and solute intake. It will be increased (above 1.035) in persons with diabetes mellitus and persons taking large amounts of medication. It will also be increased after radiologic studies of the kidney owing to the excretion of x-ray contrast dye. Consistently low specific gravity (1.003 or less) is seen in persons with diabetes insipidus. In renal failure, the specific gravity remains equal to that of the plasma (1.008-1.010) regardless of changes in salt and water intake. Urine volume below 400 mL per day is termed oliguria, and may occur in persons who are dehydrated and those with glomerular disease owing to reduced glomerular filtration. Volume in excess of 2 liters per day is termed polyuria and is common in persons with diabetes mellitus and diabetes insipidus.

Biochemical tests

Biochemical testing of urine is performed using dry reagent strips, often called dipsticks. A urine dipstick consists of a white plastic strip with absorbent microfiber cellulose pads attached to it. Each pad contains dried reagents needed for a specific test.

When performing dry reagent strip testing, health professionals should adhere strictly to the manufacturer's instructions. General instructions for performing the test manually are as follows:

• Mix the sample by inverting the container several times.
• Insert the reactive portion of the dipstick, completely, but briefly.
• Remove the dipstick from the container by sliding the back of the dipstick along the rim to remove excess urine.
• Adhere to the reaction time stated on the package insert; and note that not all the tests are to be read at the same time.
• Compare the color of the test areas on the dipstick with the color chart on the bottle label by holding the strip close to the color blocks.
• Record the results for each test using the concentration given by the closest color match.

A dry reagent strip reader may be used as an alternative to visual comparison of color reactions. This device consists of a special colorimeter that measures the optical density of each reagent pad by reflectance. All reactions are read at the precise timed interval, resulting in greater precision than visual interpretation of color intensity.

Additional tests are available to measure bilirubin, protein, glucose, ketones, and urobilinogen in urine. In general, these individual tests provide greater sensitivity, and therefore, permit detection of a lower concentration of the respective substance. A brief description of the most commonly used dry reagent strip tests follows.

1. pH: A combination of pH indicators (methyl red and bromthymol blue) react with hydrogen ions (H⁺) to produce a color change over a pH range of 5.0 to 8.5. pH is useful in determining metabolic or respiratory disturbances in acid-base balance. For example, kidney disease often results in retention of H⁺ (reduced acid excretion). pH varies with a person's diet, tending to be acidic in those who eat meat but more alkaline in vegetarians. It is also useful for the classification of urine crystals. Crystals commonly found in acid urine are uric acid, urate, and oxalate, while those commonly found in alkaline urine include phosphates and carbonates.
2. Protein: Based upon a phenomenon called the "protein error of indicators," this test uses a pH indicator, such as tetrabromphenol blue, that changes color (at constant pH) when albumin is present in the urine. The protein affects the ionization constant (pKa) of the dye, making it behave as if it were exposed to a more alkaline solution. The test for protein is far more sensitive to albumin than to globulins. Albumin is important in determining the presence of glomerular damage. The glomerulus is the network of capillaries that filters low molecular weight solutes such as urea, glucose, and salts, but normally prevents passage of protein or cells from blood into filtrate. Albuminuria occurs when the glomerular membrane is damaged, a condition called glomerulonephritis.
3. Glucose: Glucose is measured by the glucose oxidase reaction. Glucose oxidase catalyzes the oxidation of glucose by oxygen. This produces hydrogen peroxide and gluconic acid. The peroxide reacts with potassium iodide or another chromogen, producing iodine or other colored product. The glucose test is used to monitor persons with diabetes. When blood glucose levels rise above 160 mg/dL, glucose will be detected in urine. Consequently, glycosuria may be the first indicator that diabetes or another hyperglycemic condition is present. Copper sulfate tests should not be used to test urine for glucose because the reagent reacts with many nonglucose-reducing substances. The copper sulfate test may be used to screen newborns for galactosuria and other disorders of carbohydrate metabolism that cause urinary excretion of a sugar other than glucose.
4. Ketones: At alkaline pH, sodium nitroprusside or ferricyanide forms a violet-colored complex with acetoacetic acid and acetone. These ketones are produced in excess in disorders of carbohydrate metabolism, especially Type 1 diabetes mellitus. In diabetes, excess ketoacids in the blood may cause life-threatening acidosis and coma. These ketoacids and their salts spill into the urine causing ketonuria. Ketones are also found in the urine in several other conditions including fever, pregnancy, glycogen storage diseases, and in persons on a carbohydrate restricted diet.
5. Blood: Hemoglobin (also myoglobin) is capable of catalyzing the reduction of hydrogen peroxide. In the presence of hemoglobin, hydrogen peroxide will oxidize a dye such as benzidine to form a colored product. Red cells and hemoglobin may enter the urine from the kidney or lower urinary tract. This test detects abnormal levels of either, which may be caused by excessive red cell destruction, glomerular disease, kidney or urinary tract infection, malignancy, or urinary tract injury.
6. Bilirubin: Bilirubin is a breakdown product of hemoglobin. Most of the bilirubin produced is conjugated by the liver and excreted into the bile, but a very small amount of conjugated bilirubin is reabsorbed by the portal circulation and reaches the general circulation to be excreted in the urine. Normally, the level of urinary bilirubin is below the detection limit of the test. Bilirubin reacts with a diazonium salt to form azobilirubin, which is violet. Bilirubin in the urine is derived from the liver, and a positive test indicates hepatic disease or hepatobiliary obstruction.
7. Specific gravity: Solutes in the urine promote ionization of malic acid bound to a polyelectrolyte. As the malic acid residues ionize, H⁺ is released; this changes the color of a pH indicator, bromthymol blue. High ionic strength causes the indicator to behave as if the solution were more acidic, and the indicator becomes green. Specific gravity is a measure of the concentrating ability of the kidneys.
8. Nitrite: Some bacteria including the lactose positive Enterobactericeae, Staphylococcus, Proteus, Salmonella, and Psuedomonas are able to reduce nitrate in urine to nitrite. A positive test for nitrite indicates bacteruria. Nitrite reacts with p-arsenilic acid or sulfanilamide to form a diazonium compound. The diazo group reacts with a quinoline dye to form a red product.
9. Urobilinogen: Urobilinogen reacts with p-dimethylaminobenzaldehyde (Ehrlich's reagent) or methoxybenzene-diazonium tetrafluoroborate at an acid pH to form a red or orange color. Urobilinogen is formed in the gastrointestinal tract by the bacterial reduction of conjugated bilirubin. Increased urinary urobilinogen occurs in prehepatic jaundice (hemolytic anemia), hepatitis, and other forms of hepatic necrosis that impair the enterohepatic circulation. The test is helpful in differentiating these conditions from obstructive jaundice which results in decreased production of urobilinogen. The Watson-Schwartz test is used to confirm the presence of urobilinogen or differentiate between urobilinogen and porphobilinogen. This is a quantitative test using Ehrlich's reagent and a timed urine sample. Urobilinogen is differentiated from porphobilinogen based upon its solubility in chloroform.
10. Leukocytes: Nonspecific esterases in polymorphonuclear white blood cells (neutrophils) will hydrolyze a pyrole ester of alanine or indoxycarbonic acid to form a pyrole alcohol. The product reacts with a diazonium compound forming a purple azo complex. The presence of white blood cells in the urine usually signifies a urinary tract infection, such as cystitis, or renal disease, such as pyelonehritis or glomerulonephritis.

Microscopic examination

The urine may contain cells that originated in the blood, the kidney, and lower urinary tract, and the microscopic examination of urinary sediment can provide valuable clues regarding many diseases and disorders involving these systems. The microscopic exam is performed after concentrating a 12 mL volume of urine by centrifugation. The supernatant is poured off and the sediment resuspended in a small volume of residual supernatant. A drop of the sediment is placed on a glass slide and a cover glass is applied. Alternatively, a special centrifuge tube and plastic slide may be used to achieve uniform concentration and chamber depth. The sediment is examined under low power for casts, crystals, and mucus threads. Casts are deposits of gelled protein that form in the renal tubules and are washed into the filtrate over time. The number and type of casts per low power field is recorded, and the amount and type of crystals and mucus are graded semi-quantitatively. The magnification is increased to high power (400 ×) in order to count the number of red blood cells, white blood cells, and epithelial cells per field. Bacteria, yeast, and trichomonads are identified at high power, and are reported in semi-quantitative terms (e.g., small, moderate, large).

The presence of bacteria or yeast and white blood cells differentiates a urinary tract infection from possible contamination in which case the WBCs are not seen. The presence of cellular casts (casts containing RBCs, WBCs, or epithelial cells) identifies the kidneys (versus the lower urinary tract) as the source of such cells. Cellular casts and renal epithelial cells signify the presence of renal disease. The microscopic exam also identifies both normal and abnormal crystals in the sediment. Abnormal crystals are those formed as a result of an abnormal metabolic process and are always clinically significant. These include bilirubin, cystine, tyrosine, leucine, and cholesterol crystals. Normal crystals are formed from normal metabolic processes, but may be implicated in formation of urinary tract stones (calculi).

Routine urinalysis including microscopic exam may be fully automated using the Yellow Iris workstation. This instrument uses a dry reagent strip reader, harmonic oscillation (for specific gravity), and flow-focused image analysis to perform all of the steps of the urinalysis.

Preparation

A urine sample is collected in an unused disposable plastic cup with a tight-fitting lid. A randomly voided sample is suitable for routine urinalysis although the first-voided morning urine is most concentrated and therefore, preferred. The best sample is one collected in a sterile container after the external genitalia have been cleansed using the midstream void (clean-catch) method. This sample may be cultured, if findings indicate bacteruria.

• Females should use a clean cotton ball moistened with lukewarm water (or antiseptic wipes provided with collection kits) to cleanse the external genital area, before collecting a urine sample. To prevent contamination with menstrual blood, vaginal discharge, or germs from the external genitalia, they should release some urine before beginning to collect the sample. A urine specimen obtained this way is called a midstream or clean-catch sample.
• Males should use a piece of clean cotton, moistened with lukewarm water or antiseptic wipes to cleanse the head of the penis and the urethral meatus. They should draw back the foreskin, if not circumcised. After the area has been thoroughly cleansed, they should use the midstream void method to collect the sample.
• For infants, a parent or health care worker should cleanse the child's outer genitalia and surrounding skin. A sterile collection bag should be attached to the child's genital area and left in place until the child has urinated. It is important to not touch the inside of the bag, and to remove it as soon as a specimen has been obtained.

Urine samples can also be obtained via bladder catheterization, a procedure used to collect uncontaminated urine when the patient cannot void. A catheter is a thin flexible tube that a health care professional inserts through the urethra into the bladder to allow urine to flow out. To minimize the risk of infecting the patient's bladder with bacteria, many clinicians use a Robinson catheter, which is a plain rubber or latex tube that is removed as soon as the specimen is collected. If urine for culture is to be collected from an indwelling catheter, it should be aspirated from the line using a syringe and not removed from the bag in order to avoid contamination by urethral flora.

Suprapubic bladder aspiration is a collection technique sometimes used to obtain urine from infants younger than six months or urine directly from the bladder for culture. The doctor withdraws urine from the bladder into a syringe through a needle inserted through the skin.

Aftercare

The patient may return to normal activities after collecting the sample and may start taking medications that were discontinued before the test.

Complications

There are no risks associated with voided specimens. The risk of bladder infection from catheterization with a Robinson catheter is about 3%.

KEY TERMS

Acidosis— A condition of the blood in which bicarbonate levels are below normal.

Alkalosis— A condition of the blood and other body fluids in which bicarbonate levels are higher than normal.

Cast— An insoluble gelled protein matrix that takes up the form of the renal tubule in which it deposited. Casts are washed out by normal urine flow.

Catheter— A thin flexible tube inserted through the urethra into the bladder to allow urine to flow out.

Clean-catch specimen— A urine specimen that is collected from the middle of the urine stream after the first part of the flow has been discarded.

Cystine— An amino acid normally reabsorbed by the kidney tubules. Cystinuria is an inherited disease in which the reabsorption of cystine and some other amino acids is defective. Cystine crystals form in the kidney leading to obstructive renal failure.

Ketones— Substances produced during the breakdown of fatty acids. They are produced in excessive amounts in diabetes and certain other abnormal conditions.

pH— A chemical symbol used to describe the acidity or alkalinity of a fluid, ranging from 1 (more acid) to 14 (more alkaline).

Porphobilinogen— An intermediary product in the biosynthesis of heme.

Urethra— The tube that carries urine from the bladder to the outside of the body.

Urinalysis (plural, urinalyses)— The diagnostic testing of a urine sample.

Voiding— Another word for emptying the bladder or urinating.

Results

Normal urine is a clear straw-colored liquid, but may also be slightly hazy. It has a slight odor and some laboratories will note strong or atypical odors on the urinalysis report. It may contain some normal crystals, squamous or transitional epithelial cells from the bladder, lower urinary tract, or vagina. Urine may contain transparent (hyaline) casts especially if collected after vigorous exercise. However, the presence of hyaline casts may signify renal disease when the cause cannot be attributed to exercise, running, or medications. Normal urine contains a small amount of urobilinogen, and may contain a few RBCs and WBCs. Normal urine does not contain detectable glucose or other sugars, protein, ketones, bilirubin, bacteria, yeast cells, or trichomonads. Normal values representative of many laboratories are given below.

• Glucose: negative (quantitative less than 130 mg/day or 30 mg/dL).
• Bilirubin: negative (quantitative less than 0.02 mg/dL).
• Ketones: negative (quantitative 0.5-3.0 mg/dL).
• pH: 5.0-8.0.
• Protein: negative (Quantitative 15-150 mg/day, less than 10 mg/dL).
• Blood: negative.
• Nitrite: negative.
• Specific gravity: 1.015-1.025.
• Urobilinogen: 0-2 Ehrlich units (quantitative 0.3-1.0 Ehrlich units).
• Leukocyte esterase: negative.
• Red blood cells: 0-2 per high power field.
• White blood cells: 0-5 per high power field (0-10 per high power field for some standardized systems).

Health care team roles

Doctors, nurses, or laboratory scientists may provide the patient with instructions for sample collection. Laboratory scientists most often perform the tests, though in a physician's office, the doctor, nurse, or physician assistant may perform the visual examination of the sample and the dipstick test.

Resources

BOOKS

Chernecky, Cynthia C, and Berger, Barbara J. Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia, PA: W. B. Saunders Company, 2001.

Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests, 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

PERIODICALS

Gantzer, Mary Lou. "The Value of Urinalysis: An Old Method Continues to Prove Its Worth." Clinical Laboratory News (1998).

ORGANIZATIONS

American Association of Kidney Patients. 100 S. Ashley Drive, Suite 280, Tampa, FL 33260. (800)749-2257.

American Kidney Fund. 6110 Executive Blvd., Suite 1010, Rockville, MD 20852. (301)881-3052. 〈http://www.arbon.com/kidney〉.

National Kidney and Urologic Diseases Information Clearinghouse. 3 Information Way, Bethesda, MD 20892-3580.

OTHER

ARUP Laboratories. 〈http://www.arup-lab.com/〉.

The University of Iowa. Virtual Hospital. 〈http://secundus.vh.org/Providers/CME/CLIA/UrineAnalysis/UrineAnalysis.html〉 (July 20, 1999).