Urine examination is useful to detect diseases affecting the kidneys. It may reveal diseases that would have gone unnoticed due to absence of striking signs and symptoms such as diabetes mellitus, glomerulonephritis and urinary tract infections. Urine analysis is a part of routine evaluation of a child suspected to have renal disease. Gross Inspection of the urine, dipstick analysis and microscopic examination are essential for a complete urine analysis.
METHODS OF URINE COLLECTION:
Ideally clean- catch midstream urine specimen should be collected after cleaning the external urethral meatus with soap & water. Urinary catheterization is required only in special situations such as in comatose or confused patients. In newborns and infants, urine can be collected with help of a urine bag or by suprapubic transabdominal bladder aspiration. To determine specific gravity, urine should be collected after a period of water deprivation such as an early morning sample before ingestion of any fluid. Specific gravity > 1.022 in a randomly collected specimen denotes adequate renal concentrating capacity as long as there are no abnormal solutes in the urine.
MACROSCOPIC EXAMINATION OF THE URINE:
Normally urine is pale to dark yellow or clear in color. Turbid or cloudy urine may be due to excessive cellular debris or proteins and may be seen in chyluria and UTI. Red colored urine may be seen after ingestion of a food dye, eating fresh beet, drugs such as NSAIDS or presence of hemoglobin or myoglobin. Cola colored urine is suggestive of a renal bleed such as in the nephron and is seen in glomerulonephritis.
DIPSTICK ANALYSIS OF URINE:
Dipsticks are available to qualitate and quantitate the presence of proteins, glucose, ketones, bacteria, pus cells and occult blood in urine. Multistix combines the tests for proteinuria, glycosuria, hematuria & detection of nitrites. pH reading through dipstick is not very accurate.
Dipstick methods are reliable and easy to perform. Dipstick screening should be done on whole urine but semi- quantitative tests should be done on the supernatant of centrifuged urine since the cells suspended in normal urine can produce a falsely high estimation of protein . These reagent strips are impregnated with a dye. Proteins bind with the dye and color change occurs. These dipsticks are most sensitive to albumin and detect globulins and Bence Jones proteins poorly. False positive may occur if there is delay in reading the strip. Chemical analysis with 3% sulphosalicylic acid is a very sensitive precipitation test. It can detect albumin and even globulins & Bence Jones proteins at very low concentrations.
Trace proteinuria = 10mg/dl
1+ proteinuria = 30mg/dl
2+ proteinuria =100mg/dl
3+ proteinuria =300mg/dl
4+ proteinuria = 1gm/ dl
Glycosuria is generally suggestive of diabetes mellitus. Dipsticks utilizing glucose oxidase reaction are specific for glucose and do not detect other reducing sugars such as galactose and fructose. Clinitest, a modified Benedict's copper reduction test is used to detect reducing substances in newborn and infant urines. False negative glycosuria may be seen with Vitamin C , tetracyclines or homogentistic acid.
Hematuria is presence of occult blood in urine due to glomerular damage, tumors, kidney trauma, urinary stones, acute tubular necrosis, UTI and nephrotoxins. Dipsticks to detect occult blood are sensitive to any heme group and may suggest presence of either intact RBCS, hemoglobin or myoglobin. An urine examination positive by hemostix should be examined microscopically to confirm the presence of RBCs. If no RBCS are seen, hemoglobinuria can be differentiated from myoglobin by immunodiffusion techniques.
Ketones in urine may be seen in diabetic ketoacidosis or starvation. They are detected by dipsticks or tablets containing sodium nitroprusside.
More than 90% of common urinary pathogens (Gram negative rods) are nitrite-producing bacteria. In the presence of infection urinary nitrate is converted to nitrite, which can be detected in urine using a test strip.
Leukocyte esterase test:
A positive test is seen in presence of WBCs in urine and suggestive of pyuria.
The glomerular filtrate is usually acidified by renal tubules and collective ducts. The pH of urine normally ranges from 4.5 to 7.5. Urine pH should be measured with a pH meter on a freshly voided urine sample as on standing diffusional loss of CO2 or bacterial contamination may occur changing the pH value. Urine pH is important for diagnosis of RTA.
URINE SPECIFIC GRAVITY & OSMOLALITY:
Specific gravity determines the ability of the kidney to concentrate or dilute urine. It is precisely measured with the help of a refractometer. Normal specific gravity of a urine ranges from 1.001 to 1.030. It is necessary in diagnosis of renal or nephrogenic Diabetes Insipidus and Psychogenic DI. Normally, Osmolality of urine is between 40 to 1200mosm/kg.
Osmolality = ( Specific gravity -1.000) x 40,000.
MICROSCOPIC EXAMINATION OF URINE:
Fresh-centrifuged sample should be examined microscopically for presence of WBCs & RBCs. Centrifuged sediment should be examined to identify crystals, casts, and squamous cells. If microscopy is delayed, the specimen should be refrigerated at + 40C to prevent bacterial overgrowth and dissolution of cells and casts.
Red Blood Cells: >5RBC/cu mm in a HPF in 10ml of centrifuged sample is considered abnormal. Once hematuria is established, it is important to determine the source of bleeding. RBC casts, dysmorphic crenated RBCs suggest glomerular bleed whereas normal RBC morphology suggests lower urinary tract bleed.
White Blood Cells: > 10 WBC/cu mm for body and > 50 WBC/cu mm for girls is abnormal and suggestive of pyuria that is seen in UTI or with acute glomerulonephritis.
Epithelial Cells: They arise from renal tubules and transitional cells arise from renal pelvis, ureter or bladder & squamous epithelial cells arise from outer urethra or skin surface. Increased renal tubular epithelial cells are seen in nephrotic syndrome and tubular degeneration.
Casts: They are solid and cylindrical structures formed by precipitation of debris in the renal tubules. Urinary casts are formed only in the distal convoluted tubule or the collecting duct. Hyaline casts are composed primarily of Tamm-Horsfall proteins. Hyaline casts are seen in healthy individuals. RBC casts are formed when RBCs stick together and are seen in glomerular disease. WBC casts are seen in acute pyelonephritis and glomerulonephritis & denote inflammation of the kidneys. Granular and waxy casts are derived from renal tubular cell casts and are seen in Nephrotic Syndrome and tubular damage.
Crystals: Common crystals seen even in healthy individuals are calcium oxalate, triple phosphate crystals and amorphous phosphates. They may also be seen in renal stones. Abnormal crystals seen are cystine crystals in congenital cystinuria, tyrosine crystals in congenital tyrosinosis and leucine crystals in severe liver disease or maple syrup urine disease.
Bacteria: Diagnosis is established with urine culture. A colony count of >105/ml of one organism signifies significant bacteruria. Multiple organisms reflect contamination.
Yeast: It may be a contaminant. Budding yeast suggest a true candidal infection.
Some Classical urine pictures in various kidney diseaseUrine in Nephrotic Syndrome:
|Other||Oval fat bodies occasional|
|24 hours urine albumin||>40mg/kg/day.|
Urine in Acute Glomerulonephritis:
Urine in Urinary Tract Infections:
|Urine culture||>105 bacteria/ml|
Urine collectionUrine should be collected in a sterile plastic or glass bottle. The bottle can be sterilized by washing it with soap & water and then by boiling it in water for 10 minutes. Urine sample should be mid-stream i.e. the initial small quantity of urine should be discarded and the remaining sample to be collected in the sterile bottle. First or second urine sample should be collected. Urine sample collected should be delivered to the laboratory within 1 hour of collection. If the time exceeds then the bottle should be kept in a bag containing ice cubes until delivered. Urine for fasting sugar should be collected in fasting condition.