Chapter 8. Urinalysis
Background physiology
The formation of urine
The production of urine is the result of the body’s need to maintain an appropriate environment for optimum function of the systems of the body. The body must excrete the waste products of metabolism whilst maintaining essential constituents and managing the balance of water. For example, the concentration of urine varies depending on the needs of the body. On a hot day, when a woman loses fluid through perspiration, and has had little to drink, the body still attempts to conserve fluid by producing dark, concentrated urine. The constituents of urine also reflect potential pathology. Urine does not normally contain glucose or protein, for example, however there are situations that make such an observation more likely.
Urine is formed as blood flows through the kidneys. The kidneys are composed of approximately one million nephrons, the functional units responsible for the filtration of blood. This production of urine is a complex process that can be divided into three steps:
1. Filtration
This occurs in the glomerulus (a coil of capillaries) from which water and other substances in the blood are forced out under pressure into the Bowman’s capsule (a tube, the closed end of which surrounds the glomerulus, the open end leads to the urinary collecting ducts and the renal pelvis).
Access an anatomy text to find a picture of a Bowman’s capsule or look at the diagram at http://en.wikipedia.org/wiki/Bowman’s_capsule
Substances with a molecular mass of greater than 68 kilodaltons (kDa) remain in the capillary (Coad 2006). The rate at which blood is filtered by both kidneys is the glomerular filtration rate (GFR) and this is about 120ml per minute (Blows 2001).
2. Re-absorption
This selective reabsorption returns all the glucose and amino acids to the capillaries surrounding the nephron. The amount of water that returns to the blood is controlled by the antidiuretic hormone (ADH) and the reabsorption of sodium is controlled by aldosterone. Calcium and phosphate reabsorption is controlled by the hormone calcitonin.
3. Secretion
Potassium, ammonia and hydrogen ions are actively secreted into the renal tubule from the capillaries. Drugs and toxins may also enter the filtrate by this process (Coad 2006). The end product is now urine, which collects in the renal pelvis and is conducted via the ureters to the bladder. Urine is voided from the bladder via the urethra.
Physiology in relation to pregnancy
There are a number of changes that take place in the urinary system during pregnancy that may have an impact on the production and constitution of urine.
■ The kidneys enlarge in weight and size with dilatation of the renal pelves and calyces.
■ The ureters dilate and stretch, leading to curving.
■ Relaxation of the muscles of the internal urethral sphincter takes place.
■ The glomerular filtration rate increases leading to greater amounts of urea, creatinine, glucose and folate being excreted.
■ Reduction of bladder capacity.
■ Urine may be more alkaline.
The composition of urine
Urine is 96% water, 4% dissolved substances (Johnson & Taylor 2006). Urinalysis is performed to detect the presence, or measure the value, of the potential constituents of the sample.
Specific gravity
The ability of the kidneys to concentrate urine is reflected by this test. It measures how much more dense the urine is compared to water (Blows 2001). The normal range for specific gravity is 1.002–1.030 (McKinney et al 2000); the higher the value, the more concentrated the urine.
Acidity – pH
Urine is less acidic (higher pH) when a urinary tract infection is present. Acidic urine predisposes to the formation of kidney stones (Johnson & Taylor 2006). Normal pH is between 4.6 and 8.0 (McKinney et al 2000).
Protein
There should be no protein in urine. A positive result may indicate infection, contamination or developing pre-eclampsia. Further tests would be indicated to assess the amount of protein and the presence of infection. Early morning specimens are more concentrated and are most appropriate for the detection of protein. In pregnancy, due to the action of progesterone, the ureters become distended and tortuous. Compounded by compression from the enlarged ovarian arteries and veins and the gravid uterus, there is urinary stasis in the ureters, increasing the risk of infection (Coad 2006). Pre-eclampsia is a potentially life-threatening condition requiring close monitoring (Lewis 2007).
Blood
There should be no blood in urine. A positive result may indicate infection, trauma (following catheterization for example) or contamination and requires further investigation.
Glucose
There should be no glucose in urine. During pregnancy, however, there is a tendency for glomerular filtration to exceed the renal threshold for glucose and glycosuria results (Blows 2001). Assessment of glucose in urine is currently not recommended (NICE 2008). However, practitioners should be aware of those women who may be at risk of diabetes. Ideally all women should have had pre-conception assessment to establish if they are diabetic or have a family history of the condition. However, most women will not have had this opportunity and establishing appropriate screening for diabetes in pregnancy is currently being developed (NICE 2007a).
Find out what macrosomia means and what risks there are to the baby if a woman develops diabetes in pregnancy.
Ketones
There should be no ketones in urine. Ketones are formed following the breakdown of fat. They may be present if the woman has not been eating (during prolonged labour) or has been vomiting excessively (hyperemesis gravidarum). Women with uncontrolled diabetes may also have ketonuria. Links have been made between maternal ketonuria among women with post-term pregnancy and an increased rate of oligohydramnios and fetal heart rate anomalies (Onyeije & Dixon 2001).
Nitrites
There should he no nitrites in urine. They are produced by bacteria and are therefore suggestive of infection (McKinney et al 2000).
Bilirubin
There should be no bilirubin in urine. Its presence may indicate hepatic disease and requires further investigation. A false negative result may occur if the sample is exposed to sunlight (Johnson & Taylor 2006).
UrobiIinogen
This is normally present in urine in small amounts, 0.09–4.23μmol in 24 hours (Blows 2001) but raised levels are suggestive of excessive haemolysis or liver disease.
National guidance
The current NICE Guidelines for antenatal care indicate that urinalysis for proteinuria should be carried out at each antenatal appointment (NICE 2008). Further screening for asymptomatic bacteriuria, a factor involved in preterm birth, should be offered at an early stage in pregnancy (Smaill 2007, NICE 2008).
Access the Action on Pre-eclampsia PECOG guidelines at http://www.apec.org.uk/guidelines.htm and consider those on urinalysis. Are these guidelines followed in your area of work?
It is not recommended to screen for diabetes routinely, by testing for glucose in the urine, though this is sometimes the practice (Anderson 2007). Within the guidelines for intrapartum care, urinalysis is included as part of the initial assessment of labour, with regularly ensuring the woman empties her bladder throughout the first stage (NICE 2007b). There is no specific guidance relating to urinalysis in the NICE guidelines for postnatal care (NICE 2006), although a midwife should use professional judgement if either a woman or a baby are suspected to be unwell and a urinary tract infection (UTI) is suspected.
Professional guidelines
Midwives must work within the professional framework of the Nursing and Midwifery Council in order to practise in the United Kingdom.
The Midwives rules and standards (NMC 2004b) outline the responsibility of midwives to recognize any deviation from normal in either the woman or the baby and to refer such cases to an appropriate health professional. The midwife is also responsible for monitoring antenatal, intrapartum and postnatal progress and this includes undertaking urinalysis in line with local and national guidelines.
Urinalysis
Urinalysis generally refers to the activity involving the immersion of a reagent strip into a specimen of urine followed by visual interpretation of the results. The accuracy of this method has come into question and automated analysis may be more appropriate in some situations (Waugh et al 2005). However, urine is also examined through observation and the use of smell, and under the microscope in a laboratory. The reagent strips may be for the detection of a single substance, such as glucose, or have multiple test pads on a single strip. There is a range of products on the market and each surgery or Hospital Trust will purchase reagents that meet the needs of their clients and budgets.
Urinalysis is performed on a range of samples:
Early morning specimen of urine (EMU)
Urine is collected during the first visit of the day to the toilet. This specimen is the most concentrated of the day, thus enhancing the detection of solutes such as protein and hormones.
Midstream specimen of urine (MSU)
The woman is asked to start urinating in the toilet and then to catch some urine in a clean container (sterile if going to the laboratory). The bladder is then completely emptied in the toilet. This specimen should therefore be free from contamination from the urethra as the urine that first flushes through is not collected for examination.
Catheter specimen of urine (CSU)
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