PHYSICAL AND CHEMICAL ASPECTS OF URINALYSIS

28 PHYSICAL AND CHEMICAL ASPECTS OF URINALYSIS



1 Why is urinalysis performed?


Urinalysis (UA) is performed in healthy patients to screen for occult disease. UA is an essential component in the diagnostic evaluation of a diseased patient. UA can also be used to monitor disease progression, response to therapy, and safety of potentially nephrotoxic drugs.



2 What are the components of a complete routine urinalysis, and why should all the components be done?


A complete UA includes assessment of several physical and chemical characteristics of urine by gross inspection of the urine, specific chemical testing, and microscopic examination of sediment.


Performance of a complete UA allows effective interpretation of data obtained from measurement of serum biochemical markers of renal function and of the individual components of the UA. For example, it is impossible to assess the importance of proteinuria without knowing the result of microscopic examination of the urine sediment. Also, in the presence of azotemia, knowledge of the urine specific gravity helps to localize the azotemia to prerenal, renal, or postrenal causes.



3 Describe ways that the urine collection method can influence results.


When interpreting the results of urine sediment examination, it is useful to consider the method of urine collection. Urine can be collected by cystocentesis, transurethral catheterization, and free catch of a voided sample.


Each method may be associated with contaminants. Urine obtained by cystocentesis should be free of bacteria and may contain low numbers (<5 per high-power field) of epithelial cells. A variable degree of iatrogenic microscopic hematuria is frequently induced by cystocentesis and cannot be readily distinguished from disease-induced hematuria. This contamination can be particularly pronounced when the bladder wall is inflamed. Samples collected by catheterization and free catch may be contaminated by variable amounts of epithelial cells, bacteria, and debris from the lower urinary tract. However, properly performed catheterization is ideally sterile. Voided urine collected from a tabletop or from the floor may contain additional contaminants (e.g., cleanser residue) that can affect results of chemical testing.


The collection method also influences the results of urine culture and sensitivity. Ideally, urine culture and sensitivity are performed on samples collected by cystocentesis. However, this is not always possible. When necessary, quantitative urine culture and sensitivity can be performed on samples collected by catheterization or free catch.



4 Describe the optimal method of urine sample handling.


For optimal results, urine should be analyzed within 30 minutes of collection. When this is not possible, urine may be refrigerated in a sterile, opaque, airtight container for up to 12 hours. Before analysis the urine should be allowed to warm to room temperature. The container in which the urine is placed can also affect UA results. Ideally, urine should be collected in a sterile container. The container should be airtight to prevent evaporation of volatile substances, such as the ketone acetone, and to prevent loss of carbon dioxide, which raises the pH. Containers that are contaminated with a detergent or food may affect the results of enzymatic and chemical tests, such as pH measurement. If UA will be delayed more than 30 minutes, use of an opaque container that prevents exposure of the urine sample to light may be useful; this will limit photodegradation of light-sensitive analytes such as bilirubin.



5 What artifacts can refrigeration cause in urinalysis results?


Before analysis of refrigerated urine samples, it is useful to allow them to warm to room temperature to avoid artifacts that refrigeration may induce. Refrigeration can decrease the solubility of crystallogenic substances that are in solution, which can cause in vitro crystal formation. The cold temperature of the sample may inhibit enzymatic reactions in the dipstick, such as the glucose test, leading to falsely decreased results. The specific gravity of cold urine is higher than that of room temperature urine, because cold urine is more dense.



6 What artifacts may be observed when urine is stored for a prolonged period at room temperature before analysis?


Prolonged storage at room temperature may cause an increase in pH due to the escape of carbon dioxide from the sample and in vitro bacterial overgrowth. The bacterial overgrowth not only will affect the sediment, but may affect other measurements as well. Bacterial overgrowth may falsely increase the turbidity of the sample. Bacteria may consume glucose that is present in the sample, leading to an in vitro reduction of this analyte. Depending on the type of bacteria, a variable in vitro change in pH may occur. If the overgrowth is caused by non-urease-producing bacteria, an acidic pH may result from their catabolism of glucose. If urease-producing bacteria are present, the urine may become alkaline. Alkalinization of urine may cause a false-positive dipstick protein reaction, lysis of cells, and degeneration of casts, and alter the type and amount of crystals present.



7 What are the most common causes of discolored urine?


Normal urine may be various shades of yellow due to the presence of urochromes. This color can be altered by the presence of substances not normally found in urine, such as the following:


a. Hemoglobin or myoglobin, which makes urine dark-reddish brown

b. Bilirubin, which imparts a dark-orange color to urine

c. Drugs and drug metabolites, which may cause the urine to turn various shades (e.g., blue or green)

Also, urine may become darker yellow with exposure to light, which causes urochrome degradation.



8 What are the most common causes of cloudy urine?


Urine is made cloudy by the presence of particulate matter that scatters light, such as cells, crystals, lipid, mucus, and bacteria.



9 Why is horse urine normally cloudy?


Horse urine is routinely cloudy due the presence of calcium carbonate crystals and mucus secreted by glands in the renal pelvis. The kidneys are the major route of calcium excretion in the horse. Consequently, a large number of calcium carbonate crystals are typically found in normal horse urine. Horses in renal failure will often develop hypercalcemia, unlike other domestic species, in which this is an uncommon finding associated with chronic renal failure.



10 What is urine specific gravity, and what portion of the nephron does it reflect?


Urine specific gravity is the ratio of the density of urine to the density of distilled water. Urine is denser than pure water because it contains excreted solutes. Therefore the urine specific gravity is always greater than 1.000. Specific gravity is affected by the number of solute particles in solution as well as their molecular weight. It is therefore a way to estimate a solution’s osmolality, which is affected only by the number of solute particles in solution. The urine specific gravity primarily reflects the ability of the renal tubules to respond to the activity of antidiuretic hormone (ADH) to produce concentrated urine. To determine the clinical significance of a given urine specific gravity measurement, the value must be interpreted in light of the patient’s hydration status, blood urea nitrogen (BUN), and serum creatinine.



11 Define isosthenuria, hyposthenuria, and hyperosthenuria.


Urine specific gravity is classified relative to the specific gravity of glomerular filtrate. The specific gravity of glomerular filtrate ranges from 1.008 to 1.012. Urine specific gravity within this range is classified as isosthenuria. Urine specific gravity less than this range is referred to as hyposthenuria. Urine specific gravity greater than that of the glomerular filtrate is classified as hyperosthenuria. However, hyperosthenuria does not necessarily indicate adequate concentrating function by the kidneys. The patient’s signalment and hydration status and the degree of hyperosthenuria need to be considered when making this determination.



12 What urine specific gravity ranges in animals indicate the kidneys likely have adequate capacity to concentrate urine, and what degree of functional deficit must exist before abnormalities are detected by urine specific gravity?


A urine specific gravity of at least 1.025 to 1.030 in horses and bovines, 1.030 to 1.035 in dogs, and 1.035 to 1.045 in cats suggests that the kidneys likely have adequate capacity to concentrate the urine. However, urine specific gravity much lower than this may be seen in randomly collected samples, depending on the patient’s hydration status.


The concentrating function of the kidneys must be compromised by at least two thirds before an abnormal result may be detected.



13 How can urine specific gravity be used to help localize the cause of azotemia (prerenal, renal, or postrenal)?


Azotemia can result from the following:


a. Prerenal causes: decreased delivery of nitrogenous waste to the kidneys due to renal hypoperfusion or increased generation of nitrogenous wastes

b. Intrarenal causes: decreased removal of nitrogenous wastes from circulation due to primary renal dysfunction

c. Postrenal causes: decreased elimination of nitrogenous wastes due to urethral obstruction or bladder rupture

d. Any combination of prerenal, intrarenal, or postrenal causes.

Urine specific gravity can be a useful tool in helping to localize the cause of azotemia. With prerenal azotemia, adequately functioning kidneys respond to decreased perfusion by producing a concentrated urine with urine specific gravity greater than 1.035 in dogs, 1.045 in cats, and 1.030 in cattle and horses.


With renal azotemia, urine specific gravity is usually between 1.008 and 1.029 in dogs and cattle and between 1.008 and 1.035 in cats. Renal azotemia is typically associated with a 75% reduction in renal function, whereas defective urine concentration typically occurs with a 68% renal functional deficit. Therefore, renal azotemia is often found concurrently with inadequately concentrated urine. (However, some cats with renal azotemia maintain their concentrating ability and can have a specific gravity greater than 1.045.)


The urine specific gravity associated with postrenal azotemia is variable. Other clinical findings, such as oliguria or anuria with a firm, possibly distended bladder, can be used to distinguish this cause of azotemia.

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Aug 26, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on PHYSICAL AND CHEMICAL ASPECTS OF URINALYSIS

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