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© Dirk Biddle
Urine tests are typically evaluated with a reagent strip that is briefly dipped into your urine sample. The technician reads the colours of each test and compares them with a reference chart. These tests are semi-quantitative; there can be some variation from one sample to another on how the tests are scored.
Urinalyses are conducted on “mid-stream” urine and your physician will direct you in how best to obtain this. 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.
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:
Generally, urinalysis may not reflect the findings of absolutely fresh urine if the sample is greater than 1 hour old. Therefore, get the urine to the laboratory as quickly as possible.
The first part of a urinalysis is direct visual observation. Normal, fresh urine is pale to dark yellow or amber in colour 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 turbidity.
A red or red-brown (abnormal) colour could be from a food dye, eating fresh beets, a drug, or the presence of either haemoglobin or myoglobin. If the sample contained many red blood cells, it would be cloudy as well as red.
i) pH is a measure of acidity for your urine. The glomerular filtrate of blood plasma is usually acidified by renal tubules and collecting ducts from a pH of about 6.0 to 7.4 in the final urine (7 is the point of neutrality on the pH scale). However, depending on the acid-base status, urinary pH may range from as low as 4.5 to as high as 8.0. The kidneys maintain normal acid-base balance primarily through the reabsorption of sodium and the tubular secretion of hydrogen and ammonium ions. Urine becomes increasingly acidic as the amount of sodium and excess acid retained by the body increases. Alkaline urine, usually containing bicarbonate-carbonic acid buffer, is normally excreted when there is an excess of base or alkali in the body. Secretion of an acid or alkaline urine by the kidneys is one of the most important mechanisms the body uses to maintain a constant body pH.
In people who are not vegetarians, the pH of urine tends to be acidic. A diet rich in citrus fruits, legumes, and vegetables raises the pH and produces urine that is more alkaline.
A highly acidic urine pH (low pH) occurs in Acidosis, Uncontrolled diabetes, Diarrhoea, Starvation and dehydration, and Respiratory diseases in which carbon dioxide retention occurs and acidosis develops.
A highly alkaline urine (high pH) occurs in Urinary tract obstruction, Pyloric obstruction, Salicylate intoxication, Renal tubular acidosis, Chronic renal failure, and Respiratory diseases that involve hyperventilation (blowing off carbon dioxide and the development of alkalosis) (1).
ii) Specific Gravity (SG) measures how dilute your urine is. Distilled water has a SG of 1.000. Most urine is around 1.002 to 1.035, but it can vary greatly depending on when you drank fluids last, or if you are dehydrated. Since the SG of the glomerular filtrate in the 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.
iii) Glucose: Normally there is no glucose in urine. Less than 0.1% of glucose normally filtered by the glomerulus appears in urine (< 130 mg/24 hr). A positive glucose test, indicating excess sugar in the urine (glycosuria), generally means diabetes mellitus.
There are a small number of people that have glucose in their urine with normal blood glucose levels, however any glucose in the urine would raise the possibility of diabetes or glucose intolerance.
iv) Protein: Normally there very little if any protein detectable on a urinalysis strip. 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 usually does not exceed 150 mg in 24 hours (or 10 mg/100 ml in any single specimen). More than 150 mg/24 hours is defined as proteinuria. Proteinuria greater than 3.5 gm/24 hours is severe and known as nephrotic syndrome (18).
Protein in the urine can thus indicate kidney damage, blood in the urine, or an infection. However, up to 10% of children can have protein in their urine and certain diseases require the use of a special, more sensitive (and more expensive) test for protein called a micro-albumin test. A micro-albumin test is very useful (for instance) in screening for early damage to the kidneys from diabetes.
v) Blood: Normally there is no blood in the urine. The presence of red blood cells (RBCs) in the urine (haematuria) can indicate glomerular damage, tumours which erode the urinary tract anywhere along its length, kidney trauma, urinary tract stones, renal infarcts, acute tubular necrosis, upper and lower urinary tract infections, nephrotoxins, and physical stress. The technician may also indicate whether it is haemolysed (dissolved blood) or non-haemolysed (intact red blood cells). The presence of intact but dysmorphic RBCs in urine suggests a glomerular disease such as a glomerulonephritis. Dysmorphic RBCs have odd shapes as a consequence of being distorted via passage through an abnormal glomerular structure.
Rarely, muscle injury can cause myoglobin to appear in the urine which also causes the reagent pad to falsely indicate blood.
Pyuria refers to the presence of abnormal numbers of leukocytes (white blood cells or WBCs) 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.
vi) Bilirubin: Normally there is no bilirubin or urobilinogen in the urine. These are pigments that are cleared by the liver. In liver or gallbladder disease they may appear in the urine as well. Bilirubin is formed when haemoglobin (particles of the blood) breaks down. Small amounts of bilirubin are present in blood from damaged or old red cells that have died. High levels of bilirubin can cause a condition called jaundice (a yellowing of the skin and the whites of the eyes). Further testing is needed to determine the cause. Too much bilirubin may mean too many red cells are being destroyed or that the liver is incapable of removing bilirubin from the blood.
vii) 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 nitroprusside.
viii) Nitrate/Nitrite: Normally negative, a positive test usually indicates a urinary tract infection. 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.
ix) Leukocyte Esterase: Normally negative, a positive leukocyte esterase test results from the presence of white blood cells (WBCs) either as whole cells or as lysed cells. Pyuria can be detected even if the urine sample contains damaged or lysed WBCs. 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 (2).
ix) Sediment: Here the lab tech looks under a microscope at a portion of your urine that has been spun in a centrifuge. Items such as mucous and squamous cells are commonly seen. Abnormal findings would include more than 0-2 red blood cells, more than 0-2 white blood cells, or the finding of crystals (calcium oxalate, triple phosphate 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), casts (formed in the distal convoluted tubule – DCT - or the collecting duct - distal nephron. They are composed primarily of a mucoprotein - Tamm-Horsfall protein - secreted by tubule cells, binding together groups of other cells - red, white, hyaline, etc - together in a lump), renal tubular epithelial cells (with nephrotic syndrome and in conditions leading to tubular degeneration, the number sloughed off is increased), or bacteria (Bacteria can be present if there was contamination at the time of collection, from external or internal means – this is why mid-stream urine is usually asked for).
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: a) lupus nephritis, b) malignant hypertension, c) diabetic glomerulosclerosis, and d) 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 (2).
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1 http://www.rnceus.com/ua/uaph.html
2 http://www-medlib.med.utah.edu/WebPath/TUTORIAL/URINE/URINE.html