Procedure to quantify formed elements in a urine specimen
Thomas Addis developed the "Addis Count" procedure
1926
Addis Count
Quantified formed elements in a 12-hour urine specimen to monitor renal disease
Purpose of microscopic examination of urine
Detect and identify formed elements present in a urine sample
The blood, kidney, lower genitourinary tract and external contamination all contribute to the formation of urine sediments
Some urine sediments are of no clinical significance and others are considered normal constituents unless present in increased amounts
Chemical sieving
Microscopic examination of urine only done on specimens with abnormal physical/chemical results
Chemical sieving has advantages in terms of faster turnaround time
Some patient populations need microscopic examination even when physical/chemical results are normal, e.g. pregnant women with asymptomatic bacteriuria
CLSI recommends microscopic examination when requested by a physician, for specified patient populations, or when any abnormal physical/chemical result is obtained
Commercial slide systems
Offer more consistency in sediment volume, sediment examined and quantitation compared to manual methods
Despite commercial systems, medical technologists must learn the conventional method of preparing and examining urinary sediments
Microscopic examination should be done on fresh or adequately preserved urine specimens
Specimen volume
10-15 mL is centrifuged, 12 mL is frequently used to contain a representative sampling of urine formed elements
If less than 10-15 mL is available, the volume used should be noted and numeric counts doubled
Centrifugation
5 minutes at 400 RCF to concentrate the sediment with least damage to formed elements
Sediment preparation
Resuspend sediment button by gentle agitation to provide equal distribution, avoid vigorous agitation
Volume of sediment examined
Recommended 0.02 mL (15 μL) covered by 22x22 mm coverslip
Examining at least 10 low power and 10 high power fields is the minimum recommended
Reporting microscopic examination
Average number or range per 10 low/high power fields, or semiquantitative terms like rare, few, moderate, many
Microscopic results should be correlated with physical and chemical findings to ensure accuracy and reliability
Sediment stains/enhancers
Used to increase visibility and aid identification of urine sediments
Sternheimer-Malbin stain
Most frequently used stain, consisting of crystal violet and safranin O, stains leukocytes, epithelial cells, casts
Toluidine blue
Enhances nuclear detail to differentiate WBCs and renal tubular epithelial cells
Acetic acid
Enhances nuclear detail, can be used to lyse RBCs
Lipid stains
Oil Red O, Sudan III stain triglycerides and neutral fats, polarizing microscopy differentiates cholesterol
Gram stain
Used to identify bacterial casts
Hansel stain
Preferred stain for eosinophils, useful in detecting drug-induced allergic renal reactions
Prussian blue stain
Stains hemosiderin granules in renal tubular cells and casts following hemoglobinuria
Urine cytology
Preparation of permanent slides for detecting malignancies, monitoring kidney transplant rejection, inflammatory conditions, microorganisms
Brightfield microscopy
Most common, requires controlling light intensity to visualize low refractive index sediments
Phase-contrast microscopy
Provides contrast based on refractive index differences, useful for low refractive index elements like hyaline casts
Polarizing microscopy
Aids identification of birefringent crystals and lipids
Interference-contrast microscopy
Provides 3D image with fine structural detail
Dark-field microscopy
Rarely used in urinalysis, more common in microbiology for identifying spirochetes
Fluorescence microscopy
Not usually used in urinalysis
Red blood cells in urine
Indicates hematuria, often due to injury/bleeding along the genitourinary tract
Polarizing microscopy
Produces a characteristic Maltese cross pattern
Useful in differentiating some polarizing crystals from each other and from RBCs, which are non-polarizing
Interference-contrast microscopy
Provides a three-dimensional image showing very fine structural detail
Dark-field microscopy
Most often utilized for the identification of the spirochete Treponema pallidum
Object appears light against a black background or dark-field
Not usually used in urinalysis (more commonly used in microbiology)