Urinalysis
Urinalysis | |
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MedlinePlus | 003579 |
Urinalysis, a portmanteau of the words urine and analysis,
Background
The first destination is the
Besides excreting waste products, the process of urine formation helps to maintain
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.[9] 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,[10] and microscopic examination permits the counting and classification of solid elements of the urine, such as cells, crystals, and bacteria.[11]
Urinalysis is one of the most commonly performed medical laboratory tests.[12] It is frequently used to help diagnose urinary tract infections[13] and to investigate other issues with the urinary system, such as incontinence.[14] 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.[12] In emergency medicine urinalysis is used to investigate numerous symptoms, including abdominal and pelvic pain,[15][16] fever,[17] and confusion.[18] During pregnancy, it may be performed to screen for protein in the urine (proteinuria), which can be a sign of pre-eclampsia,[19] and bacteria in the urine, which is associated with pregnancy complications.[16][20] The analysis of urine is invaluable in the diagnosis and management of kidney diseases.[21]
Specimen collection
Samples for urinalysis are collected into a clean (preferably sterile) container.
Macroscopic examination
Color and clarity
Normal urine has a yellow hue, which is primarily caused by the pigment
A spectrum of abnormal colors can result from the intake of drugs. An unusually bright yellow color can occur after consumption of
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.
Smell
The odor (scent) of urine can normally vary from odorless (when very light colored and dilute) to a much stronger odor when the subject is dehydrated and the urine is concentrated.[36] Transient changes in urine odor can occur after consuming certain foods, most notably asparagus. The urine of diabetics experiencing ketoacidosis (urine containing high levels of ketone bodies) 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 (which takes its name from the urine scent) and phenylketonuria (which causes a "mousey" smell).[37] Odor is rarely reported during urinalysis.[38]
Specific gravity
Urine test strip
False positive and false negative results may occur. General sources of error include abnormally colored urine, which interferes with the interpretation of color changes;
Blood
Reagent pads for blood change color in the presence of
White blood cells
Leukocyte esterase, an enzyme found in granulocytes, is measured to estimate the concentration of white blood cells.[53] The action of the enzyme on chemicals in the test pad ends in the creation of a purple azo dye.[54] False positive results can occur if the sample is contaminated with vaginal secretions; false negatives can occur in very concentrated samples or those containing high levels of glucose and protein.[54] Elevated white blood cell counts in urine generally indicate infection or inflammation.[55] People with a low level of neutrophils in the blood (neutropenia) may not have enough white blood cells in their urine to produce a positive reaction.[56]
Nitrite
Some bacteria that cause UTIs can
Protein
Test strips estimate urine protein levels by exploiting the ability of protein to interfere with
pH
Specific gravity
Urine test strips use the concentration of ions in the urine to estimate specific gravity. The test pad contains a polyelectrolyte that releases hydrogen ions in proportion to the concentration of ions in the sample. The consequent pH change is measured using a pH indicator. The reading obtained from reagent strips, as opposed to refractometers, is not affected by substances such as glucose, urea and contrast dyes. Falsely low readings can occur in alkaline urine.[70][71]
Glucose
Test strips for glucose contain the enzyme
Ketones
Ketonuria occurs in uncontrolled
Bilirubin
The presence of bilirubin in the urine (termed
Urobilinogen
Urine urobilinogen is increased in liver disease and
Microscopic examination
Microscopic examination of the urine allows
If microscopy is necessary, the urine may be
There are automated microscopy systems that use flow cytometry technology or pattern recognition to identify microscopic elements in unconcentrated urine.[94] Automated instruments reduce workload in medical laboratories and can accurately identify most common urinary elements, but do not perform as well with unusual findings such as transitional and renal epithelial cells, abnormal casts and rare crystals.[95]
Elements that can be observed on microscopic examination include:
Red blood cells
Under the microscope, normal red blood cells (RBCs) appear as small concave discs. Their numbers are reported per high-power field.[96] In highly concentrated urine they may shrivel and develop a spiky shape, which is termed crenation, while in dilute urine they can swell and lose their hemoglobin, creating a faint outline known as a ghost cell. A small quantity of red blood cells in the urine is considered normal.[93][97]
An increased level of RBCs is termed
White blood cells
Typically, most white blood cells (WBCs) in urine are
Epithelial cells
Squamous epithelial cells line the urethra, as well as the vagina and the outer layer of the skin. They are very large, flat, and thin, with irregular borders and a single, small nucleus. They may fold into various shapes. They are not considered clinically significant, but if they are seen in large numbers they can indicate contamination of the sample by vaginal secretions or the skin of the urogenital area.[105][106]
Transitional epithelial cells, also known as urothelial cells, line the urinary tract from the
Renal tubular epithelial cells (RTEs) line the
Casts
Granular casts, so named for their microscopic appearance, incorporate degenerated cellular material or protein aggregates. They are considered an abnormal finding and are associated with diseases of the kidney
Crystals
Various compounds in the urine can
The presence of crystals in the urine has conventionally been associated with the formation of
Organisms
Microorganisms that can be observed in the urine include bacteria,
Other elements
Mucus can occur in the urine, where it appears as translucent wavy strands under the microscope. The presence of mucus is not a clinically significant finding, but it can be confused with hyaline casts. Sperm may occasionally be observed in the urine of both males and females; in female children and vulnerable adults, this can indicate sexual abuse. Reporting the presence of sperm in human urine (spermaturia), especially in women,is a controversial topic.[132][133][134] Fat droplets and oval fat bodies may be present in a condition called lipiduria, which has various causes, most notably nephrotic syndrome. Contaminants from the outside environment, such as starch granules, hair, and clothing fibers, may be seen but are not reported.[135]
Interpretation
Test | Result |
---|---|
Blood | Negative |
Leukocytes | Negative |
Nitrite | Negative |
Protein | Negative to trace |
pH | 5–7 |
Specific gravity | 1.003–1.035 |
Glucose | Negative |
Ketones | Negative |
Bilirubin | Negative |
Urobilinogen | <1 mg/dL |
The interpretation of urinalysis takes into account the results of physical, chemical and microscopic examination and the person's overall condition. Urine test results should always be interpreted using the reference range provided by the laboratory that performed the test, or using information provided by the test strip/device manufacturer.[137] Not all abnormal results signify disease, and false positive results are common. For this reason, the use of urinalysis for screening in the general population has been discouraged, but it remains a common practice.[138]
Urinalysis is commonly used to help diagnose urinary tract infections, but the significance of the results depends on the broader clinical situation.
If a significant quantity of bacteria is present in the urine but there are no symptoms of a UTI, the condition is called
A positive dipstick result for blood could signify the presence of red blood cells, hemoglobin, or myoglobin, and therefore requires microscopic analysis for confirmation.[144] Intact red blood cells will normally be observed under the microscope if present, but they may lyse in dilute or alkaline samples.[97] Hemoglobinuria, if unaccompanied by a high quantity of RBCs, can signify intravascular hemolysis (destruction of red blood cells inside the body).[145] Myoglobinuria occurs in rhabdomyolysis and other conditions that cause breakdown of muscle tissue.[146]
If red blood cells are present, the interpretation takes into account whether the urine is visibly bloody (termed
Elevated levels of protein in the urine are often suggestive of kidney disease, but may have other causes. Proteinuria can occur transiently as a consequence of exercise, fever, stress, or UTI. Proteinuria that occurs only while standing, called
History
The diagnostic value of urine has been recognized since ancient times. Urine examination was practiced in Sumer and Babylonia as early as 4000 BC, and is described in ancient Greek and Sanskrit texts.[155] Hippocrates, Celsus and Galen published important works correlating the characteristics of urine with patients' health.[156] During the Middle Ages the visual inspection of urine―termed uroscopy—gained widespread popularity. The 7th-century manuscript De Urinis by the Byzantine physician Theophilus Protospatharius is thought to be the earliest publication dedicated solely to the examination of urine. Protospatharius notably described a method for precipitating protein out of urine using heat.[155]
Many influential works on urine testing followed. Publications by
Physicians of antiquity interpreted the color of urine using circular charts listing correspondences with disease states. The relation of urine characteristics to disease was based on the theory of the
During the late Middle Ages and the Renaissance, the abuse of uroscopy by disreputable individuals began to draw criticism. "Uromancers" without medical training claimed that they could not only diagnose disease, but detect pregnancy, determine a baby's sex, and even predict the future from a subject's urine. In 1637 the English physician Thomas Brian published The Pisse-Prophet, or, Certaine Pisse-Pot Lectures, excoriating those who claimed to be able to diagnose diseases by uroscopy without examining the patient.[162][163]
The 19th century saw a proliferation in chemical methods for the analysis of urine, but these techniques were labor-intensive and impractical; in one contemporary editorial a physician complained about the dangers of keeping
In 1956 Helen Murray Free and her husband developed Clinistix (also known as Clinistrip), the first dip-and-read test for glucose in urine for patients with diabetes.[167] This breakthrough led to additional dip-and-read tests for other substances.[168] The invention was named a National Historic Chemical Landmark by the American Chemical Society in May 2010.[169] A dipstick test for urine protein, called Albustix, was introduced by Miles Laboratories in 1957,[170] and the first multi-test dipsticks were released in 1959. Automated test strip readers came onto the market in the 1980s.[171]
See also
- Uroscopy, the ancient form of this analysis
- Urinary casts
- Proteinuria
- Urine test strip
- Urine collection device
- Pregnancy test, measures hCG levels in urine
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