尿分析

During the visual examination of the urine, the laboratorian observes the urine's color and clarity. These can be signs of what substances may be present in the urine. They are interpreted in conjunction with results obtained during the chemical and microscopic examinations to confirm what substances are present. (See The Chemical Examination and The Microscopic Examination for details).

Color—urine can be a variety of colors, most often shades of yellow, from very pale or colorless to very dark or amber. Unusual or abnormal urine colors can be the result of a disease process, several medications (e.g., multivitamins can turn urine bright yellow), or the result of eating certain foods. For example, some people can have red-colored urine after eating beets; the color is from the natural pigment of beets and is not a cause for worry. However, red-colored urine can also occur when blood is present in the urine and can be an indicator of disease or damage to some part of the urinary system. Another example is yellow-brown or greenish-brown urine that may be a sign of bilirubin in the urine (see The Chemical Examination section).

Clarity—urine clarity refers to how clear the urine is. Usually, laboratories report the clarity of the urine using one of the following terms: clear, slightly cloudy, cloudy, or turbid. "Normal" urine can be clear or cloudy. Substances that cause cloudiness but that are not considered unhealthy include mucus, sperm and prostatic fluid, cells from the skin, normal urine crystals, and contaminants such as body lotions and powders. Other substances that can make urine cloudy, like red blood cells, white blood cells, or bacteria, indicate a condition that requires attention.

To perform the chemical examination, most clinical laboratories use commercially prepared test strips with test pads that have chemicals impregnated into them. The laboratorian dips the strip into urine, chemical reactions change the colors of the pads within seconds to minutes, and the laboratorian determines the result for each test. To reduce timing errors and eliminate variations in color interpretation, automated instruments are frequently used to "read" the results of the test strip.

The degree of color change on a test pad can give an estimate of the amount of substance present. For example, a slight color change in the test pad for protein may indicate a small amount of protein present in the urine whereas a deep color change may indicate a large amount.

The chemical examination is often done in conjunction with or may be followed by a microscopic examination of the urine if there are any abnormal results. Results from both sets of tests are then considered together for interpretation. Abnormal findings may be followed by additional urine and/or blood tests.

The most frequently performed chemical tests using reagent test strips are:

• Specific Gravity (SG)
• pH
• Protein
• Glucose
• Ketones
• Blood (hemoglobin) and Myoglobin
• Leukocyte Esterase
• Nitrite
• Bilirubin
• Urobilinogen

Some reagent test strips also have a test pad for ascorbic acid (vitamin C).

 Specific Gravity (SG) 
Specific gravity is a measure of urine concentration. This test simply indicates how concentrated the urine is. Specific gravity measurements are a comparison of the amount of substances dissolved in urine as compared to pure water. If there were no substances present, the specific gravity of the urine would be 1.000 (the same as pure water). Since all urine has some substances in it, a urine SG of 1.000 is not possible. If a person drinks excessive quantities of water in a short period of time or gets an intravenous (IV) infusion of large volumes of fluid, then the urine specific gravity may be very close to that of water. The upper limit of the test pad, a specific gravity of 1.035, indicates concentrated urine, one with many substances in a limited amount of water.

Knowing the urine concentration helps healthcare practitioners understand whether a urine specimen they are evaluating is the best one to detect a particular substance. For example, if they are looking for very small amounts of protein, a concentrated morning urine specimen would be the best sample.

 pH 
As with specific gravity, there are typical but not "abnormal" pH values. The urine is usually slightly acidic, about pH 6, but can range from 4.5-8. The kidneys play an important role in maintaining the acid-base balance of the body. Therefore, any condition that produces acids or bases in the body, such as acidosis or alkalosis, or the ingestion of acidic or basic foods can directly affect urine pH.

Some of the substances dissolved in urine will precipitate out to form crystals when the urine is acidic; others will form crystals when the urine is basic. If crystals form while the urine is being produced in the kidneys, a kidney stone or "calculus" can develop. By modifying urine pH through diet or medications, the formation of these crystals can be reduced or eliminated. (For additional information, see the articles on Kidney Stone Analysis and Kidney Stone Risk Panel.)

 Protein 
The protein test pad provides a rough estimate of the amount of albumin in the urine. Albumin makes up about 60% of the total protein in the blood. Normally, there will be no protein or a small amount of protein in the urine. When urine protein is elevated, a person has a condition called proteinuria.

Proteinuria may occasionally be seen in healthy individuals. Healthy people can have temporary or persistent proteinuria due to stress, exercise, fever, aspirin therapy, or exposure to cold, for example. Repeat testing may be done once these conditions have resolved to determine whether the proteinuria is persistent.

If trace amounts of protein are detected, and depending on the person's signs, symptoms and medical history, a repeat urinalysis and dipstick protein may be performed at a later time to see if there is still protein in the urine or if it has dropped back to undetectable levels.

If a large amount of protein is detected on a urinalysis and/or if the protein persists in repeated tests, a 24-hour urine protein test may be used as a follow-up test. Since the dipstick primarily measures albumin, the 24-hour urine protein test also may be ordered if a healthcare practitioner suspects that proteins other than albumin are being released into the urine.

Protein in the urine may be a sign of kidney disease. Small amounts of albumin may be found in the urine when kidney dysfunction begins to develop. A different test called a urine albumin test detects and measures small amounts of albumin in the urine. The urine albumin test is more sensitive than a dipstick urinalysis and is routinely used to screen people with chronic conditions that put them at risk for kidney disease, such as diabetes and high blood pressure. (See the article on Urine Albumin.)

Proteinuria may also be associated with many other diseases and conditions. A healthcare practitioner may order other types of follow-up tests to help determine the cause of protein in the urine. For more information, see the articles on Proteinuria and Kidney Disease.

 Glucose 
Glucose is normally not present in urine. When glucose is present, the condition is called glucosuria. It results from either:

1. An excessively high glucose level in the blood, such as may be seen with people who have uncontrolled diabetes
2. A reduction in the "renal threshold;" when blood glucose levels reach a certain concentration, the kidneys begin to eliminate glucose into the urine to decrease blood concentrations. Sometimes the threshold concentration is reduced and glucose enters the urine sooner, at a lower blood glucose concentration.

Some other conditions that can cause glucosuria include hormonal disorders, liver disease, medications, and pregnancy. When glucosuria occurs, other tests such as a fasting blood glucose are usually performed to further identify the specific cause.

 Ketones 
Ketones are not normally found in the urine. They are intermediate products of fat metabolism. They are produced when glucose is not available to the body's cells as an energy source. They can form when a person does not eat enough carbohydrates (for example, in cases of fasting, starvation, or high-protein diets) or when a person's body cannot use carbohydrates properly. When carbohydrates are not available, the body metabolizes fat instead to get the energy it needs to keep functioning. Strenuous exercise, exposure to cold, frequent, prolonged vomiting, and several digestive system diseases can also increase fat metabolism, resulting in ketonuria.

In a person who has diabetes, ketones in urine may also be an early indication of insufficient insulin. With insufficient insulin, a diabetic cannot process glucose and instead metabolizes fat. This can cause ketones to build up in the blood, resulting first in ketosis and then progressing to ketoacidosis, a form of metabolic acidosis. Excess ketones and glucose are dumped into the urine by the kidneys in an effort to flush them from the body. This condition, called diabetic ketoacidosis (DKA), is most frequently seen with uncontrolled type 1 diabetes and can be a medical emergency. (See the articles on Ketones and Diabetes.)

 Blood (Hemoglobin) and Myoglobin
This test is used to detect hemoglobin in the urine (hemoglobinuria). Hemoglobin is an oxygen-transporting protein found inside red blood cells (RBCs). Its presence in the urine indicates blood in the urine (known as hematuria).

A small number of RBCs are normally present in urine and usually result in a "negative" chemical test. An increased amount of hemoglobin and/or increased number of RBCs are detected as a "positive" chemical test result. Results of this test are typically interpreted along with those from the microscopic examination of the urine to determine whether RBCs are present in the urine. A positive result on this test with no RBCs present may indicate the presence of hemoglobin in the urine (which can occur when RBCs have broken apart) or myoglobin from muscle injury. (See the article on Myoglobin.)

Blood in the urine is not a normal finding, but it is not uncommon and not necessarily a cause for alarm. Your healthcare practitioner will investigate further to try to determine the source and underlying cause of the blood and may request repeat testing to determine whether the blood is persistent.

See The Microscopic Exam: Red Blood Cells and the article on Hematuria for more information.

 Leukocyte Esterase 
Leukocyte esterase is an enzyme present in most white blood cells (WBCs). A few white blood cells are normally present in urine and usually give a negative chemical test result. When the number of WBCs in urine increases significantly, this screening test will become positive. Results of this test will be considered along with a microscopic examination for WBCs in the urine.

When this test is positive and/or the WBC count in urine is high, it may indicate that there is inflammation in the urinary tract or kidneys. The most common cause for WBCs in urine (leukocyturia) is a bacterial urinary tract infection (UTI), such as a bladder or kidney infection. In addition to WBCs, bacteria and RBCs may also be seen in the microscopic examination. If bacteria are present, the chemical test for nitrite may also be positive (see below).

 Nitrite 
This test detects nitrite and is based upon the fact that many bacteria can convert nitrate (a normal substance in urine) to nitrite. Normally, the urinary tract and urine are free of bacteria and nitrite. When bacteria enter the urinary tract, they can cause a urinary tract infection. A positive nitrite test result can indicate a UTI. However, since not all bacteria are capable of converting nitrate to nitrite, someone can still have a UTI despite a negative nitrite test. The results of this test will be considered along with the leukocyte esterase (above) and a microscopic examination.

 Bilirubin 
This test screens for bilirubin in the urine. Bilirubin is not present in the urine of normal, healthy individuals. It is a waste product that is produced by the liver from the hemoglobin of RBCs that are broken down and removed from circulation. It becomes a component of bile, a fluid that is released into the intestines to aid in food digestion.

In certain liver diseases, such as biliary obstruction or hepatitis, excess bilirubin can build up in the blood and is eliminated in urine. The presence of bilirubin in urine is an early indicator of liver disease and can occur before clinical symptoms such as jaundice develop.

The results of this test will be considered along with the result of urobilinogen (below). If positive, the healthcare practitioner will likely follow up with other laboratory tests, such as a liver panel, to help establish a diagnosis.

For more information, see the articles on Bilirubin and Liver Disease.

 Urobilinogen 
This test screens for urobilinogen in the urine. The results are considered along with those for urine bilirubin (above).

Urobilinogen is normally present in urine in low concentrations. It is formed in the intestine from bilirubin, and a portion of it is absorbed back into the blood. Positive test results may indicate liver diseases such as viral hepatitis, cirrhosis, liver damage due to drugs or toxic substances, or conditions associated with increased RBC destruction (hemolytic anemia). When urine urobilinogen is low or absent in a person with urine bilirubin and/or signs of liver dysfunction, it can indicate the presence of hepatic or biliary obstruction.

 Ascorbic Acid (Vitamin C)
Occasionally, people taking vitamin C or multivitamins may have large amounts of ascorbic acid in their urine. When this is suspected to be the case, a laboratorian may test the sample for ascorbic acid (vitamin C) because it has been known to interfere with the accuracy of some of the results of the chemical test strip, causing them to be falsely low or falsely negative. Examples of tests that may be affected include the urine dipstick tests for glucose, blood, bilirubin, nitrite, and leukocyte esterase.

A microscopic examination may or may not be performed as part of a routine urinalysis. It will typically be done when there are abnormal findings on the physical or chemical examination and the results from all will be taken into account for interpretation.

The microscopic exam is performed on urine sediment – urine that has been centrifuged to concentrate the substances in it at the bottom of a tube. The fluid at the top of the tube is then discarded and the drops of fluid remaining are examined under a microscope. Cells, crystals, and other substances are counted and reported either as the number observed "per low power field" (LPF) or "per high power field" (HPF). In addition, some entities, if present, are estimated as "few," "moderate," or "many," such as epithelial cells, bacteria, and crystals. Cells and other substances that may be seen include the following:

• Red Blood Cells (RBCs)
• White Blood Cells (WBCs)
• Epithelial Cells
• Bacteria, Yeast and Parasites
• Trichomonas
• Casts
• Crystals

 Red Blood Cells (RBCs) 
Normally, a few RBCs are present in urine sediment (0-5 RBCs per high power field, HPF). A positive chemical test for hemoglobin and an increase in the number of RBCs seen under the microscope indicates that there is blood in the urine. However, this test cannot be used to identify where the blood is coming from. For instance, contamination of urine with blood from hemorrhoids or vaginal bleeding cannot be distinguished from a bleed in the urinary tract. This is why it is important to collect a urine specimen correctly and for women to tell their healthcare provider that they are menstruating when asked to collect a urine specimen.

Blood in the urine is not a normal finding, but it is not uncommon and is not necessarily a cause for alarm. Hematuria is a sign or an indicator that prompts a healthcare practitioner to investigate further to try to determine the underlying cause of the blood. As part of the investigation, a healthcare practitioner will evaluate an individual's medical history, physical examination, and accompanying signs and symptoms. Additional urine and blood tests may be done to help determine the source.

Some of the underlying causes of hematuria are benign, temporary states that do no lasting harm and resolve with little or no specific treatment. If there is blood in the urine along with white blood cells and bacteria, it may be caused by a urinary tract infection that can be easily treated with antibiotics. Some causes of hematuria, however, may be critical conditions or represent a chronic condition that requires treatment and monitoring.

For more detailed information on causes and tests, read the article on Hematuria.

 White Blood Cells (WBCs) 
The number of WBCs in urine sediment is normally low (0-5 WBCs per high power field, HPF). WBCs can be a contaminant, such as those from vaginal secretions.

An increased number of WBCs seen in the urine under a microscope and/or positive test for leukocyte esterase may indicate an infection or inflammation somewhere in the urinary tract. If also seen with bacteria (see below), they indicate a likely urinary tract infection.

 Epithelial Cells 
Epithelial cells are usually reported as "few," "moderate," or "many" present per low power field (LPF). Normally, in men and women, a few epithelial cells can be found in the urine sediment. In urinary tract conditions such as infections, inflammation, and malignancies, an increased number of epithelial cells are present. Determining the kinds of cells present may sometimes help to identify certain conditions. For example, epithelial cells containing large amounts of broken-down hemoglobin (called hemosiderin) may indicate that there were red blood cells or hemoglobin in the urine recently, even if there are none now.

 Bacteria, Yeast and Parasites 
In healthy people, the urinary tract is sterile and, if the urine sample is collected as a "clean-catch" sample, there will be no microbes seen in the urine sediment under the microscope. Special care must be taken during specimen collection, particularly in women, to prevent bacteria that normally live on the skin or in vaginal secretions from contaminating the urine sample.

If microbes are seen, they are usually reported as "few," "moderate," or "many" present per high power field (HPF).

• Bacteria from the surrounding skin can enter the urinary tract at the urethra and move up to the bladder, causing a urinary tract infection (UTI). If the infection is not treated, it can eventually move to the kidneys and cause kidney infection (pyelonephritis). If a person has an uncomplicated lower urinary tract infection, then the person may be treated without need for a urine culture. However, if the person has had recurrent UTIs, suspected complicated infection, or is hospitalized, a urine culture and susceptibility testing may be performed to help guide treatment. (See the article on Urine Culture for details.)
• In women (and rarely in men), yeast can also be present in urine. They are most often present in women who have a vaginal yeast infection because the urine has been contaminated with vaginal secretions during collection. If yeast are observed in urine, then the person may be treated for a yeast infection.
• Trichomonas vaginalis is a parasite that may be found in the urine of women, or rarely, men. As with yeast, T. vaginalis infects the vaginal canal and their presence in urine is due to contamination during sample collection. If these are found during a urinalysis, then Trichomonas testing may be performed to look for a vaginal infection.

 Casts 
Casts are cylindrical particles sometimes found in urine that are formed from coagulated protein released by kidney cells. They are formed in the long, thin, hollow tubes of the kidneys known as tubules and usually take the shape of the tubule (hence the name). Under the microscope, they often look like the shape of a "hot dog" and in healthy people they appear nearly clear. This type of cast is called a "hyaline" cast. Normally, healthy people may have a few (0–5) hyaline casts per low power field (LPF). After strenuous exercise, more hyaline casts may be detected.

Other types of casts are associated with different kidney diseases, and the type of casts found in the urine may give clues as to which disorder is affecting the kidney. Cellular casts, such as red blood cell and white blood cell casts, indicate a kidney disorder. Some other examples of types of casts include granular casts, fatty casts, and waxy casts. When a disease process is present in the kidney, cells or other substances can become trapped in the protein as the cast is formed. When this happens, the cast is identified by the substances inside it, for example, as a red blood cell cast or white blood cell cast.

For more on the types of conditions and disorders that can affect kidneys, read the article on Kidney Disease.

 Crystals 
Urine contains many dissolved substances (solutes) – waste chemicals that the body needs to eliminate. These solutes can form crystals, solid forms of a particular substance, in the urine if:

1. The urine pH is increasingly acidic or basic;
2. The concentration of dissolved substances is increased; and
3. The urine temperature promotes their formation.

Crystals are identified by their shape, color, and by the urine pH. They may be small, sand-like particles with no specific shape (amorphous) or have specific shapes, such as needle-like. Crystals are considered "normal" if they are from solutes that are typically found in the urine; these usually form as urine cools after collection and were not present in the body. Some examples of crystals that can be found in the urine of healthy individuals include:

• Amorphous urates
• Crystalline uric acid
• Calcium oxalates
• Amorphous phosphates

If the crystals are from substances that are not normally in the urine, they are considered "abnormal." Abnormal crystals may indicate an abnormal metabolic process. Some of these include:

• Calcium carbonate
• Cystine
• Tyrosine
• Leucine

Normal or abnormal crystals can form within the kidneys as urine is being made and may group together to form kidney "stones" or calculi. These stones can become lodged in the kidney itself or in the ureters, tubes that pass the urine from kidney to the bladder, causing extreme pain. For more details, read Kidney Stone Analysis and Kidney Stone Risk Panel.

Medications, drugs, and x-ray dye can also crystallize in urine. Therefore, the laboratorian must be familiar with and trained in the identification of urine crystals.

尿分析可作为筛查和/或诊断工具，因为它可以辅助检测尿中与不同的代谢紊乱和肾疾病相关的物质和细胞。尿分析检测到任何异常都必须进行后续试验。通常，在病人意识到自身不适之前，一些物质如蛋白质或糖就开始出现在尿中。尿分析也用于尿路感染（UTI）和尿路的其他疾病的检测。在急或慢性疾病中，如肾脏疾病，要求间断性做尿分析来辅助诊断器官的功能、状态和治疗反馈。

当您入院时，要进行尿分析。也作为健康检查、怀孕期的检查或手术计划前的身体检查的一部分。

假如您的病人有腹痛、背痛、尿痛或尿频、血尿和泌尿道感染（UTI），那么有可能需要做尿分析试验。这个试验也可以用于监测病人病情的好坏。

尿分析结果有很多种解释。它启动警示作用，警告存在不正常的结果，需要做进一步地分析。一般来说，异常物质浓度越高（如糖、蛋白质或红细胞浓度大量升高），就越可能存在问题，这些问题需要解决。但这结果并不能明确地告诉医生发现的（异常）是什么引起的或是否为临时性的还是慢性疾病。正常的尿分析结果同样不能保证病人没有疾病。有些人在病程早期不会释放大量升高的物质而有些人在一天中偶发地释放该物质（这意味着仅仅通过一次尿标本检测可能会错过诊断疾病）。在非常稀释的尿液中不能检测到少量的化学物质。

尿分析是一组筛查试验，能够提供一个人健康的综合概况。您的医生必须将尿分析结果与您的健康主诉、临床发现相联系，并用其他试验（如CMP，CBC）或尿培养（查看尿路感染）寻找引起异常物质的原因。

　　因为这是一个综合性的筛查试验，尽管早晨第一次排尿是首选的，但时间通常不重要。然而，如果您的医生在检测一个特别的异常物质，他有可能让您在特定的时间内采集尿液样本。例如，如果他要检测糖排泄情况，那最好在饭后采集标本。如果他在寻找低水平的蛋白质，那最好收集浓缩的样本，如晨尿标本。