Urinalysis can reveal diseases that have gone unnoticed because they do not
produce striking signs or symptoms. Examples include diabetes mellitus, various
forms of glomerulonephritis, and chronic urinary tract infections.
The most cost-effective device used to screen urine is a paper or plastic
dipstick. This microchemistry system has been available for many years and
allows qualitative and semi-quantitative analysis within one minute by simple
but careful observation. The color change occurring on each segment of the strip
is compared to a color chart to obtain results. However, a careless doctor,
nurse, or assistant is entirely capable of misreading or misinterpreting the
results. Microscopic urinalysis requires only a relatively inexpensive light
The first part of a urinalysis is direct visual observation. Normal, fresh
urine is pale to dark yellow or amber in color and clear. Normal urine volume
is 750 to 2000 ml/24hr.
Turbidity or cloudiness may be caused by excessive cellular material or
protein in the urine or may develop from crystallization or precipitation of
salts upon standing at room temperature or in the refrigerator. Clearing of the
specimen after addition of a small amount of acid indicates that precipitation
of salts is the probable cause of tubidity.
A red or red-brown (abnormal) color could be from a food dye, eating fresh
beets, a drug, or the presence of either hemoglobin or myoglobin. If the sample
contained many red blood cells, it would be cloudy as well as red.
Examples of appearances of urine
URINE DIPSTICK CHEMICAL ANALYSIS
A dipstick is a paper strip with patches impregnated with chemicals that undergo a color change when certain constituents of the urine are present or in a certain concentration. The strip is dipped into the urine sample, and after the appropriate number of seconds, the color change is compared to a standard chart to determine the findings.
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Findings: Leukocyte esterase 3+, Nitrite Pos; pH 7.0; Protein Neg; Blood Neg; Sp Gr 1.015; Ketones 1+, Glucose 1+; Bilirubin Neg
The glomerular filtrate of blood plasma is usually acidified by renal tubules
and collecting ducts from a pH of 7.4 to about 6 in the final urine. However,
depending on the acid-base status, urinary pH may range from as low as 4.5 to as
high as 8.0. The change to the acid side of 7.4 is accomplished in the distal
convoluted tubule and the collecting duct.
Specific Gravity (sp gr)
Specific gravity of urine is determined by the presence of solutes represented by particles of varying sizes, from small ions to larger proteins. Urine osmolality measures the total number of dissolved particles, regardless of their size. The most common method of measurement is freezing point depression. A refractometer measures the change in direction of a light path (refraction) based upon particle concentration and size in a fluid. Larger particles such as glucose and albumin will alter refraction to a greater degree. The urine dipstick measurement of specific gravity is an approximation that is most sensitive to cationic concentration in urine. Therefore, dipstick specific gravity is altered by very high or low urine pH, but not large particles like proteins.
Urine specific gravity (U-SG) is directly proportional to urine osmolality (U-Osm). A U-Osm of 400 mOsm/Kg equates to sp gr of 1.010, and 800 mOsm/kg to sp gr of 1.020 (Note: the amount of solute in a kilogram of solvent is termed osmolality, and the amount per liter of solvent is osmolarity). The ability of the kidneys to concentrate or dilute the urine over that of plasma is being measured.
Specific gravity between 1.002 and 1.035 on a random sample should be
considered normal if kidney function is normal. Since the sp gr of the
glomerular filtrate in Bowman's space ranges from 1.007 to 1.010, any
measurement below this range indicates hydration and any measurement above it
indicates relative dehydration.
If sp gr is not > 1.022 after a 12 hour period without food or water,
renal concentrating ability is impaired and the patient either has generalized
renal impairment or nephrogenic diabetes insipidus. In end-stage renal disease,
sp gr tends to become 1.007 to 1.010.
Any urine having a specific gravity over 1.035 is either contaminated,
contains very high levels of glucose, or the patient may have recently received
high density radiopaque dyes intravenously for radiographic studies or low molecular weight dextran solutions. Subtract 0.004 for every 1% glucose to determine non-glucose solute concentration.
Dipstick screening for protein is done on whole urine, but semi-quantitative
tests for urine protein should be performed on the supernatant of centrifuged
urine since the cells suspended in normal urine can produce a falsely high
estimation of protein. Normally, only small plasma proteins filtered at the
glomerulus are reabsorbed by the renal tubule. However, a small amount of
filtered plasma proteins and protein secreted by the nephron (Tamm-Horsfall
protein) can be found in normal urine. Normal total protein excretion does not
usually exceed 150 mg/24 hours or 10 mg/100 ml in any single specimen. More
than 150 mg/day is defined as proteinuria. Proteinuria > 3.5 gm/24 hours is
severe and known as nephrotic syndrome.
Dipsticks detect protein by production of color with an indicator dye,
Bromphenol blue, which is most sensitive to albumin but detects globulins and
Bence-Jones protein poorly. Precipitation by heat is a better semiquantitative
method, but overall, it is not a highly sensitive test. The sulfosalicylic acid
test is a more sensitive precipitation test. It can detect albumin, globulins,
and Bence-Jones protein at low concentrations.
In rough terms, trace positive results (which represent a slightly hazy
appearance in urine) are equivalent to 10 mg/100 ml or about 150 mg/24 hours
(the upper limit of normal). 1+ corresponds to about 200-500 mg/24 hours, a 2+
to 0.5-1.5 gm/24 hours, a 3+ to 2-5 gm/24 hours, and a 4+ represents 7 gm/24
hours or greater.
Less than 0.1% of glucose normally filtered by the glomerulus appears in
urine (< 130 mg/24 hr). Glycosuria (excess sugar in urine) generally means
diabetes mellitus. Dipsticks employing the glucose oxidase reaction for
screening are specific for glucos glucose but can miss other reducing sugars
such as galactose and fructose. For this reason, most newborn and infant urines
are routinely screened for reducing sugars by methods other than glucose oxidase
(such as the Clinitest, a modified Benedict's copper reduction test).
Ketones (acetone, aceotacetic acid, beta-hydroxybutyric acid) resulting from
either diabetic ketosis or some other form of calorie deprivation (starvation),
are easily detected using either dipsticks or test tablets containing sodium
A positive nitrite test indicates that bacteria may be present in
significant numbers in urine. Gram negative rods such as E. coli are more
likely to give a positive test.
A positive leukocyte esterase test results from the presence of white blood
cells either as whole cells or as lysed cells. Pyuria can be detected even if
the urine sample contains damaged or lysed WBC's. A negative leukocyte esterase
test means that an infection is unlikely and that, without additional evidence
of urinary tract infection, microscopic exam and/or urine culture need not be
done to rule out significant bacteriuria.
A sample of well-mixed urine (usually 10-15 ml) is centrifuged in a test
tube at relatively low speed (about 2-3,000 rpm) for 5-10 minutes until a
moderately cohesive button is produced at the bottom of the tube. The supernate
is decanted and a volume of 0.2 to 0.5 ml is left inside the tube. The sediment
is resuspended in the remaining supernate by flicking the bottom of the tube
several times. A drop of resuspended sediment is poured onto a glass slide and
The sediment is first examined under low power to identify most crystals,
casts, squamous cells, and other large objects. The numbers of casts seen are
usually reported as number of each type found per low power field (LPF).
Example: 5-10 hyaline casts/L casts/LPF. Since the number of elements found in
each field may vary considerably from one field to another, several fields are
averaged. Next, examination is carried out at high power to identify crystals,
cells, and bacteria. The various types of cells are usually described as the
number of each type found per average high power field (HPF). Example: 1-5
Red Blood Cells
Hematuria is the presence of abnormal numbers of red cells in urine due to:
glomerular damage, tumors which erode the urinary tract anywhere along its
length, kidney trauma, urinary tract stones, renal infarcts, acute tubular
necrosis, upper and lower uri urinary tract infections, nephrotoxins, and
physical stress. Red cells may also contaminate the urine from the vagina in
menstruating women or from trauma produced by bladder catherization.
Theoretically, no red cells should be found, but some find their way into the
urine even in very healthy individuals. However, if one or more red cells can
be found in every high power field, and if contamination can be ruled out, the
specimen is probably abnormal.
RBC's may appear normally shaped, swollen by dilute urine (in fact, only
cell ghosts and free hemoglobin may remain), or crenated by concentrated urine.
Both swollen, partly hemolyzed RBC's and crenated RBC's are sometimes difficult
to distinguish from WBC's in the urine. In addition, red cell ghosts may
simulate yeast. The presence of dysmorphic RBC's in urine suggests a glomerular disease such as a glomerulonephritis. Dysmorphic RBC's have odd shapes as a consequence of being distorted via passage through the abnormal glomerular structure.
Red blood cells in urine
Dysmorphic red blood cells in urine
White Blood Cells
Pyuria refers to the presence of abnormal numbers of leukocytes that may
appear with infection in either the upper or lower urinary tract or with acute
glomerulonephritis. Usually, the WBC's are granulocytes. White cells from the
vagina, especially in the presence of vaginal and cervical infections, or the
external urethral meatus in men and women may contaminate the urine.
If two or more leukocytes per each high power field appear in non-contaminated urine, the specimen is probably abnormal. Leukocytes have lobed nuclei and granular cytoplasm.
White blood cells in urine
Renal tubular epithelial cells, usually larger than granulocytes, contain a
large round or oval nucleus and normally slough into the urine in small numbers. However, with nephrotic syndrome and in conditions leading to tubular
degeneration, the number sloughed is increased.
When lipiduria occurs, these cells contain endogenous fats. When filled with numerous fat droplets, such cells are called oval fat bodies. Oval fat bodies exhibit a "Maltese cross" configuration by polarized light microscopy.
Oval fat bodies in urine
Oval fat bodies in urine, with polarized light
Transitional epithelial cells from the renal pelvis, ureter, or bladder have
more regular cell borders, larger nuclei, and smaller overall size than squamous
epithelium. Renal tubular epithelial cells are smaller and rounder than
transitional epithelium, and their nucleus occupies more of the total cell
Squamous epithelial cells from the skin surface or from the outer urethra can appear in urine.
Their significance is that they represent possible contamination of the specimen with skin flora.
Squamous epithelial cells in urine
Urinary casts are formed only in the distal convoluted tubule (DCT) or the
collecting duct (distal nephron). The proximal convoluted tubule (PCT) and loop of Henle are not locations for cast formation. Hyaline casts are composed primarily of a mucoprotein (Tamm-Horsfall protein) secreted by tubule cells. The Tamm-Horsfall protein secretion (green dots) is illustrated in the diagram below, forming a hyaline cast in the collecting duct:
Even with glomerular injury causing increased glomerular permeability to plasma proteins with resulting proteinuria, most matrix or "glue" that cements urinary casts together is Tamm-Horsfall mucoprotein, although albumin and some globulins are also incorporated. An example of glomerular inflammation with leakage of RBC's to produce a red blood cell cast is shown in the diagram below:
The factors which favor protein cast formation are low flow rate, high salt
concentration, and low pH, all of which favor protein denaturation and
precipitation, particularly that of the Tamm-Horsfall protein. Protein casts
with long, thin tails formed at the junction of Henle's loop and the distal
convoluted tubule are called cylindroids. Hyaline casts can be seen even in
Red blood cells may stick together and form red blood cell casts. Such casts are indicative of glomerulonephritis, with leakage of RBC's from glomeruli, or severe tubular damage.
White blood cell casts are most typical for acute pyelonephritis, but they may also be present with glomerulonephritis. Their presence indicates inflammation of the kidney, because such casts will not form except in the kidney.
When cellular casts remain in the nephron for some time before they are
flushed into the bladder urine, the cells may degenerate to become a coarsely
granular cast, later a finely granular cast, and ultimately, a waxy cast.
Granular and waxy casts are be believed to derive from renal tubular cell casts.
Broad casts are believed to emanate from damaged and dilated tubules and are
therefore seen in end-stage chronic renal disease.
The so-called telescoped urinary sediment is one in which red cells, white
cells, oval fat bodies, and all types of casts are found in more or less equal
profusion. The conditions which may lead to a telescoped sediment are: 1) lupus
nephritis 2) hypertensive emergency 3) diabetic glomerulosclerosis, and
4) rapidly progressive glomerulonephritis.
In end-stage kidney disease of any cause, the urinary sediment often becomes
very scant because few remaining nephrons produce dilute urine.
Hyaline casts in urine
Red blood cell casts forming in tubules
Red blood cell cast in urine
White blood cell cast in urine
Renal tubular cell cast in urine
Granular casts in urine
Granular cast in urine
Waxy cast in urine
Bile stained hyaline casts in renal tubules
Bacteria are common in urine specimens because of the abundant normal
microbial flora of the vagina or external urethral meatus and because of their
ability to rapidly multiply in urine standing at room temperature. Therefore,
microbial organisms found in all but the most scrupulously collected urines
should be interpreted in view of clinical symptoms.
Diagnosis of bacteriuria in a case of suspected urinary tract infection
requires culture. A colony count may also be done to see if significant numbers
of bacteria are present. Generally, more than 100,000/ml of one organism
reflects significant bacteriuria. Multiple organisms reflect contamination.
However, the presence of any organism in catheterized or suprapubic tap
specimens should be considered significant.
Yeast cells may be contaminants or represent a true yeast infection. They
are often difficult to distinguish from red cells and amorphous crystals but are
distinguished by their tendency to bud. Most often they are Candida, which may
colonize bladder, urethra, or vagina.
Common crystals seen even in healthy patients include calcium oxalate,
triple phosphate crystals and amorphous phosphates.
Very uncommon crystals include: cystine crystals in urine of neonates with
congenital cystinuria or severe liver disease, tyrosine crystals with congenital
tyrosinosis or marked liver impairment, or leucine crystals in patients with
severe liver disease or with maple syrup urine disease.
Oxalate crystals in urine
Triple phosphate crystals in urine
Cystine crystals in urine
General "crud" or unidentifiable objects may find their way into a specimen, particularly those that patients bring from home.
Spermatozoa can sometimes be seen. Rarely, pinworm ova may contaminate the
urine. In Egypt, ova from bladder infestations with schistosomiasis may be seen.
METHODS OF URINE COLLECTION
Random collection taken at any time of day with no precautions regarding
contamination. The sample may be dilute, isotonic, or hypertonic and may
contain white cells, bacteria, and squamous epithelium as contaminants. In
females, the specimen may cont contain vaginal contaminants such as
trichomonads, yeast, and during menses, red cells.
Early morning collection of the sample before ingestion of any fluid. This is usually hypertonic and reflects the ability of the kidney to concentrate
urine during dehydration which occurs overnight. If all fluid ingestion has
been avoided since 6 p.m. the previous day, the specific gravity usually exceeds
1.022 in healthy individuals.
Clean-catch, midstream urine specimen collected after cleansing the
external urethral meatus. A cotton sponge soaked with benzalkonium
hydrochloride is useful and non-irritating for this purpose. A midstream urine
is one in which the first half of the bladder urine is discarded and the
collection vessel is introduced into the urinary stream to catch the last half.
The first half of the stream serves to flush contaminating cells and microbes
from the outer urethra prior to collection. This sounds easy, but it isn't (try
it yourself before criticizing the patient).
Catherization of the bladder through the urethra for urine collection is
carried out only in special circumstances, i.e., in a comatose or confused
patient. This procedure risks introducing infection and traumatizing the
urethra and bladder, thus producing iatrogenic infection or hematuria.
Suprapubic transabdominal needle aspiration of the bladder. When done
under ideal conditions, this provides the purest sampling of bladder urine.
This is a good method for infants and small children.
To summarize, a properly collected clean-catch, midstream urine after
cleansing of the urethral meatus is adequate for complete urinalysis. In fact,
these specimens generally suffice even for urine culture. A period of
dehydration may precede urine collection if testing of renal concentration is
desired, but any specific gravity > 1.022 measured in a randomly collected
specimen denotes adequate renal concentration so long as there are no abnormal
solutes in the urine.
Another important factor is the interval of time which elapses from
collection to examination in the laboratory. Changes which occur with time
after collection include: 1) decreased clarity due to crystallization of
solutes, 2) rising pH, 3) loss of ketone bodies, 4) loss of bilirubin, 5)
dissolution of cells and casts, and 6) overgrowth of contaminating
microorganisms. Generally, urinalysis may not reflect the findings of absolutely fresh urine if the sample is > 1 hour old. Therefore, get the
urine to the laboratory as quickly as possible.