Urine Analysis (1/3): Sediment and Dipstick Examination
Urine Sediment: Microscopic Examination
Preparing the Specimen
12 ml of midstream urine is centrifuged with 400 g for 5 minutes. The supernatant is discarded (around 11.5 ml) and the rest is resuspended by manual shaking of the centrifuge tube. One drop of the fluid is placed on a microscope slide and covered with a slip. The urine sediment specimen is viewed with 400× magnification.
Differential Diagnosis of the Urine Sediment
Abnormal urine color:
- Cloudy urine: pyuria
- Red color of urine: hematuria, food (beetroot, blueberry, blackberry), myoglobinuria, hemoglobinuria, chronic lead poisoning
- Dark yellow to orange color of urine: bilirubinuria (urobilinogen), medication with rifampicin
- Brown color of urine: old hemorrhage, myoglobinuria, medication with nitrofurantoin, metronidazole or L-Doa, porphyria
- Green color or urine: bilirubinuria (biliverdin)
Hematuria and Microhematuria
Hematuria is defined as visible blood in urine; microhematuria is defined with more than 5 erythrocytes per high power field. More than 20 erythrocytes per high power field is a strong microhematuria. Please see section hematuria for differential diagnosis.
Leukocyturia is more than 15 leukocytes per high power field. Differential diagnoses include urinary tract infections, urogenital tuberculosis (sterile leucocyturia), nephrolithiasis and foreign bodies.
Urine casts are cylindrical mucoprotein structures, which are formed in the distal nephron and passed into the urine. Depending on the luminal content of the distal nephron, casts are acellular or contain cells (erythrocytes, leukocytes or tubular cells). Cellular casts in the urine sediment suggest a renal inflammatory disease (e.g. glomerulonephritis, pyelonephritis). Degeneration of cellular casts results in granular casts and waxy casts. Acellular casts (e.g. hyaline casts) may have no pathologic significance or are an indication for a reduced fluid intake.
Many different crystal types can be identified in patients with stone disease and in normal urine. Only the identification of cystine crystals (hexagonal crystals) is important to establish the diagnosis of cystinuria.
Visible bacteria in the freshly processed midstream urine are diagnostic for a urinary tract infection. 5 bacteria per high power field correspond to a colony count of 100000/ml.
Urine Analysis with Dipsticks
Standard values for urine dipstick analysis are shown in Table dipstick standard values. In screening for urological diseases, the urine analysis with dipsticks is superior to the urine sediment. The dipstick analysis provides important information about the urinary pH, specific gravity, proteinuria, bilirubinuria, glucosuria, ketonuria and nitrite test.
Urine dipstick standard values with test principle and possible influencing and confounding factors (Roche 2014).
||Influencing and confounding factors
||Hemoglobin peroxidase activity
||False negative due to Vitamin C. False-positive due to myoglobinuria, strong oxidizing detergents, menstrual blood contamination, marked leukocyturia, high physical activity.
||Esterase activity of the granulocytes
||False-positive: contamination with vaginal leukocytes, urine staining, antibiotics. False-negative: strong proteinuria, glucosuria, drugs such as cephalexin, gentamycin or ACE inhibitors.
||morning urine 5--6
||Combination of indicator dyes
||Incorrectly high values in old urine. Acidic pH with a high-protein diet, basic pH with a vegetarian diet. Disinfectants strongly influence pH measurement.
||Protein error of pH indicators
||No detection of microalbuminuria. Wrong-positive due to semen contamination, disinfectants, physical activity, pregnancy or nitrofurantoin.
||<20 mg/dl (<1,1 mmol/l) in fasting morning urine
||Glucose oxidase peroxidase reaction, coupled with dye
||False negative: Vitamin C, reducing agents (e.g., nitrofurantoin) or bacteriuria. Wrong-positive: detergents, fever, MESNA.
||Urine ions and complexing reagent release protons
||False-low: urinary pH >7. False-high values: proteinuria and ketonuria.
||Dye reaction with nitrite
||False-positive in old urine. False-negative for non-nitrate-degrading bacteria, high urine dilution and short residence time of urine in the urinary bladder, antibiotic therapy, vegetable-free diet.
||Dye reaction with acetoacetic acid and acetone
||False-positive:e.g. Captopril, imipenem, MESNA or fever.
||Negative (<1 mg/dl)
||Dye reaction with Urobilinogen
||False negative in posthepatic jaundice.
Differential Diagnosis of Dipstick Urine Analysis
Please see section hematuria for differential diagnosis.
Indicates the urine concentration to monitor fluid intake. Specific gravity is an important aid for interpretation of urine dipstick analysis. For highly concentrated urine, false-positive test results are possible. For highly diluted urine, only slightly elevated test results are also relevant.
Differential diagnoses include urinary tract infections, urogenital tuberculosis (sterile leukocyturia), nephrolithiasis and foreign bodies.
Alkaline urine is a sign for infection with urea-splitting bacteria (e.g. Proteus), renal tubular acidosis, after a big meal, old urine.
Acid urine is a sign for uric acid or cystine urolithiasis.
Glucosuria is always suspicious of diabetes mellitus, since the dipstick reacts only >50 mg/dl glucose in urine. Seldom differential diagnoses: renal glucosuria.
The urinary protein concentration should be <30 mg/ml: differential diagnosis of proteinuria.
Detection of liver diseases (hepatitis, liver cirrhosis, toxic liver damage) or hemolysis. Posthepatic jaundice does not lead to an increase in urobilinogen.
The nitrite test should be negative. A positive test corresponds to a colony count of >100000/ml. False negative nitrite tests are obtained with non-nitrite-producing bacteria such as Enterococcus, high vitamin C intake or urine storage time in the bladder <4 h.
Index: 1–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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