Urinalysis
Urinalysis, a portmanteau of the words urine and analysis, is a panel of medical tests that includes physical examination of the urine, chemical evaluation using urine test strips, and [|microscopic examination]. Macroscopic examination targets parameters such as color, clarity, odor, and specific gravity; urine test strips measure chemical properties such as pH, glucose concentration, and protein levels; and microscopy is performed to identify elements such as cells, urinary casts, crystals, and organisms.
Background
is produced by the filtration of blood in the kidneys. The formation of urine takes place in microscopic structures called nephrons, about one million of which are found in a normal human kidney. Blood enters the kidney though the renal artery and flows through the kidney's vasculature into the glomerulus, a tangled knot of capillaries surrounded by Bowman's capsule. The glomerulus and Bowman's capsule together form the renal corpuscle. A healthy glomerulus allows many solutes in the blood to pass through, but does not permit the passage of cells or high-molecular weight substances such as most proteins. The filtrate from the glomerulus enters the capsule and proceeds to the renal tubules, which reabsorb water and solutes from the filtrate into the circulation and secrete substances from the blood into the urine in order to maintain homeostasis.The first destination is the proximal convoluted tubule. The filtrate proceeds into the loop of Henle, then flows through the distal convoluted tubule to the collecting duct. The collecting ducts ultimately drain into the renal calyces, which lead to the renal pelvis and the ureter. Urine flows through the ureters into the bladder and exits the body through the urethra.
Besides excreting waste products, the process of urine formation helps to maintain fluid, electrolyte and acid-base balance in the body. The composition of urine reflects not only the functioning of the kidneys, but numerous other aspects of the body's regulatory processes. The ease with which a urine sample can be obtained makes it a practical choice for diagnostic testing.
Medical uses
Urinalysis involves assessment of the physical properties of urine, such as color and clarity; chemical analysis using urine test strips; and microscopic examination. Test strips contain pads impregnated with chemical compounds that change color when they interact with specific elements in the sample, such as glucose, protein and blood, and microscopic examination permits the counting and classification of solid elements of the urine, such as cells, crystals, and bacteria.Urinalysis is one of the most commonly performed medical laboratory tests. It is frequently used to help diagnose urinary tract infections and to investigate other issues with the urinary system, such as incontinence. It may be used to screen for diseases as part of a medical assessment. The results can suggest the presence of conditions such as kidney disease, liver disease and diabetes. In emergency medicine urinalysis is used to investigate numerous symptoms, including abdominal and pelvic pain, fever, and confusion. During pregnancy, it may be performed to screen for protein in the urine, which can be a sign of pre-eclampsia, and bacteria in the urine, which is associated with pregnancy complications. The analysis of urine is invaluable in the diagnosis and management of kidney diseases.
Specimen collection
Samples for urinalysis are collected into a clean container. The sample can be collected at any time of the day, but the first urine of the morning is preferred because it is more concentrated. To prevent contamination, a "midstream clean-catch" technique is recommended, in which the genital area is cleaned before urinating and the sample is collected partway through the urination. Samples can also be collected from a urinary catheter or by inserting a needle through the abdomen and into the bladder. In infants and young children, urine may be collected into a bag attached to the genital region, but this is associated with a high risk of contamination. If the sample is not tested promptly, inaccurate results can occur because bacteria in the urine will multiply and elements such as cells and casts will degrade. It is recommended that urinalysis is performed within two hours of sample collection if the urine is not refrigerated.Macroscopic examination
Color and clarity
Normal urine has a yellow hue, which is primarily caused by the pigment urochrome. The color can range from pale yellow to amber based on the individual's hydration status. Urine can develop a variety of abnormal colors, which may suggest disease in some cases. A total lack of color indicates that the urine is extremely dilute, which may be caused by excessive fluid intake, diabetes insipidus, or diabetes mellitus. Dark yellow-brown to green urine may suggest a high concentration of bilirubin, a state known as bilirubinuria. Red urine often indicates the presence of red blood cells or hemoglobin, but can also be caused by some medications and the consumption of foods containing red pigments, such as beets. Myoglobin, a product of muscle breakdown, can give urine a red to reddish-brown color. Dark brown or black urine can occur in a genetic disorder called alkaptonuria and in people with melanoma. Purple urine occurs in purple urine bag syndrome.A spectrum of abnormal colors can result from the intake of drugs. An unusually bright yellow color can occur after consumption of B vitamin supplements, while phenazopyridine, used to treat urinary tract-related pain, can turn the urine orange. Methylene blue may turn it blue to bluish-green. Phenolphthalein, a stimulant laxative previously found in Ex-Lax, can produce colors ranging from red to purple, and levodopa, used to treat Parkinson's disease, may result in "cola-colored" urine.
The clarity of urine is also recorded during urinalysis. Urine is typically clear; materials such as crystals, cells, bacteria, and mucus can impart a cloudy appearance. A milky appearance can be caused by a very high concentration of white blood cells or fats, or by chyluria. Unpreserved urine will become cloudier over time.
Smell
The odor of urine can normally vary from odorless to a much stronger odor when the subject is dehydrated and the urine is concentrated. Transient changes in urine odor can occur after consuming certain foods, most notably asparagus. The urine of diabetics experiencing ketoacidosis may have a fruity or sweet smell, while urine from individuals with urinary tract infections often has a foul smell. Some inborn errors of metabolism cause characteristic odors, such as maple syrup urine disease and phenylketonuria. Odor is rarely reported during urinalysis.Specific gravity
is a measure of the concentration of the urine, which provides information about hydration status and kidney function. It normally ranges from 1.003 to 1.035; lower values indicate that the urine is dilute, while higher values mean that it is concentrated. A urine specific gravity that consistently remains around 1.010 can indicate kidney damage, as it suggests that the kidneys have lost the ability to control urine concentration. It is not possible for the kidneys to produce urine with a specific gravity greater than 1.040 but such readings can occur in urine that contains high-molecular weight substances, such as contrast dyes used in radiographic imaging. Specific gravity is commonly measured with urine test strips, but refractometers may also be used. Reagent strip readings are based on the concentration of ions in the sample, while refractometer readings are affected by other substances such as glucose and protein.Urine test strip
s or "dipsticks" allow for the rapid measurement of numerous urine parameters and substances. The strip is dipped into the urine sample and the color changes on the reagent pads are read after a defined period of time, either by eye or using an automated instrument. The tests included vary depending on the type of dipstick, but common ones are glucose, ketones, bilirubin, urobilinogen, blood, white blood cells, protein, nitrite, pH, and specific gravity. Nitrite is reported as negative or positive; other elements may be scored on a scale or reported as an approximate concentration based on the intensity of the color change.False positive and false negative results may occur. General sources of error include abnormally colored urine, which interferes with the interpretation of color changes; high levels of ascorbic acid, which can cause false negative results for blood, bilirubin, glucose, and nitrite; and variations in the concentration of the sample.