CM
CLINICAL MICROSCOPY — EXAM REVIEWER NOTES
URINALYSIS: Physical Examination & Macroscopic Screening
Reference: Strasinger & Lorenzo, Urinalysis and Body Fluids, 6th & 7th ed.
PART 1: PHYSICAL EXAMINATION
The physical examination of urine involves evaluating color, pigments, and clarity/turbidity — all done macroscopically (by naked eye).
I. URINE COLOR
Urine color is an indicator of the state or degree of hydration.
Normal: Pale yellow → Dark yellow
Abnormal (most common): Red / Pink / Brown
Color variations may be due to: normal metabolic functions, physical activity, ingested materials, or pathologic conditions
Note: A change in urine color is often the reason a patient seeks medical advice.
URINE PIGMENTS
Pigment | Color | Notes |
|---|---|---|
Urochrome | Yellow | Major urinary pigment; contributes to normal urine color; production is directly proportional to metabolic rate |
Uroerythrin | Pink | Most evident in refrigerated specimens; normal urinary constituent |
Urobilin | Orange-brown | Imparts orange-brown color to non-fresh urine; normal urinary constituent |
URINE COLOR REFERENCE TABLE
(Slide 5–6 visual content — key chart from the presentation)
Color | Possible Cause |
|---|---|
Colorless / Very pale | Very dilute urine, diabetes insipidus, overhydration |
Pale to dark yellow | Normal (urochrome) |
Orange | Urobilin, bilirubin, certain drugs (pyridium/phenazopyridine), concentrated urine |
Pink / Red | Hematuria (blood), hemoglobinuria, myoglobinuria, beets/berries (diet), certain drugs |
Brown / Amber | Concentrated urine, bilirubin (oxidized = biliverdin), methemoglobin, melanin |
Green / Blue-green | Biliverdin, Pseudomonas infection, certain drugs (methylene blue, amitriptyline) |
Black / Dark brown | Melanin, homogentisic acid (alkaptonuria), methemoglobin |
White / Milky | Chyluria (lymph), pyuria (heavy WBCs), phosphate/carbonate crystals |
KEY POINTS ON RED/PINK/BROWN URINE (Most Common Abnormal Color)
Three main substances that turn urine red/pink:
Hemoglobin — from lysed RBCs; positive dipstick blood; no RBCs microscopically
Myoglobin — from muscle destruction (rhabdomyolysis); also positive dipstick blood; no RBCs
Intact RBCs (Hematuria) — positive dipstick blood; RBCs visible microscopically
These three can be distinguished by microscopy + dipstick correlation.
II. URINE CLARITY / TURBIDITY
Along with color, clarity describes the overall visual appearance of the specimen.
Refers to the transparency or turbidity of urine
Provides a key to microscopic examination results
Clarity Grading / Terminology
Term | Description |
|---|---|
Clear | No visible particulates; transparent |
Hazy | Slight turbidity; print easily seen through specimen |
Cloudy | Turbid; print blurred through specimen |
Turbid | Very turbid; print cannot be seen through specimen |
Milky | May precipitate or clot |
NONPATHOLOGIC CAUSES OF TURBIDITY
(From Slide 10 — these are NORMAL and NOT disease-related)
Cause | Details |
|---|---|
Squamous epithelial cells | Common contaminant from skin; more common in females |
Mucus | From urethral/vaginal secretions; common in females |
Amorphous phosphates & carbonates | Precipitate in alkaline urine; white/gray precipitate; dissolve with acetic acid |
Amorphous urates | Precipitate in acid urine; pink/"brick dust" precipitate; dissolve when warmed to 37°C |
Spermatozoa | From sexual activity or retrograde ejaculation |
Fecal contamination | Collection error |
Radiographic contrast media |
PATHOLOGIC CAUSES OF TURBIDITY
(From Slide 11)
Cause | Details |
|---|---|
WBCs (pyuria) | Infection/inflammation; white turbidity |
RBCs (hematuria) | Smoky, red, or brown turbidity |
Bacteria | Infection; may have accompanying pyuria |
Yeast | Candida; common in diabetics or immunocompromised |
Fat / Lipids | Nephrotic syndrome; milky appearance; fat droplets float on top |
Lymph (Chyluria) | Obstruction of lymphatics; milky white |
PART 2: MACROSCOPIC SCREENING
WHAT IS MACROSCOPIC SCREENING?
Macroscopic screening refers to the cross-reference and validation of chemical dipstick results using microscopic examination.
When is it performed?
CLSI recommendation
Requested by physician
Laboratory population testing
When abnormal physical or chemical results are obtained
Purpose:
Cross-reference and validate the chemical dipstick
Enhance cost-effectiveness of urinalysis
ROUTINE URINALYSIS WORKFLOW (Slide 14 summary)
The standard urinalysis consists of three parts performed in sequence:
Physical Examination — color, clarity, odor, volume, foam
Chemical Examination — dipstick testing (glucose, protein, blood, pH, etc.)
Microscopic Examination — sediment analysis
Macroscopic screening determines whether step 3 (microscopy) is indicated based on steps 1 and 2.
PART 3: MICROSCOPIC EXAMINATION OF URINE SEDIMENT
A. SPECIMEN PREPARATION & VOLUME
Collect in a wide-mouth screw cap sterile container
Examine specimens fresh or properly preserved
Formed elements (RBCs, WBCs, hyaline casts) disintegrate rapidly, especially in dilute alkaline urine
Refrigeration may cause crystal precipitation
Warm specimen to 37°C to dissolve nonpathologic crystals
Use midstream clean-catch to reduce contamination
Mix specimen thoroughly before centrifugation
Standard volume: 10–15 mL (commonly 12 mL)
Adjust reporting if less than 12 mL is collected
B. CENTRIFUGATION
Parameter | Recommendation |
|---|---|
Recommended setting | 400 RCF for 5 minutes |
Use | RCF (Relative Centrifugal Force) not RPM |
Why RCF over RPM | Accounts for centrifuge size |
Braking mechanism | Do NOT use — disrupts sediment |
Tube caps | Always use — prevents aerosols |
Calibration | Perform routine calibration |
RCF Formula:
RCF = 1.118 × 10⁻⁵ × radius (cm) × RPM²
C. VOLUME OF SEDIMENT EXAMINED
Recommended sediment volume: 20 µL (0.02 mL)
Coverslip: 22 × 22 mm glass coverslip
Examine: 10 fields under the microscope
Get the average and report results
D. REPORTING THE MICROSCOPIC EXAMINATION
Element | Reported As | Objective Used |
|---|---|---|
Casts | Average number per Low Power Field (LPF) | Low power (10×) |
Mucus threads | Average number per Low Power Field (LPF) | Low power (10×) |
WBCs | Average number per High Power Field (HPF) | High power (40×) |
RBCs | Average number per High Power Field (HPF) | High power (40×) |
Squamous epithelial cells | Semi-quantitative per LPF (rare/few/moderate/many) | Low power (10×) |
Transitional epithelial cells | Semi-quantitative per LPF | Low power (10×) |
E. CORRELATING RESULTS
Microscopic results must be correlated with physical and chemical findings to ensure accuracy.
Key correlations to remember:
Physical/Chemical Finding | Expected Microscopic Correlate |
|---|---|
Red/smoky urine | RBCs |
Positive dipstick blood, no RBCs | Hemoglobinuria or myoglobinuria |
Positive dipstick nitrite + leukocyte esterase | Bacteria + WBCs |
Turbid urine | WBCs, bacteria, crystals |
Positive protein (heavy) | Casts (especially granular/waxy) |
Positive glucose | Yeast possible (especially if also has WBCs) |
QUICK REVIEW: HIGH-YIELD FACTS FOR EXAM
Urochrome = major yellow pigment, proportional to metabolic rate
Uroerythrin = pink, most evident after refrigeration
Urobilin = orange-brown, seen in non-fresh urine
RCF = 1.118 × 10⁻⁵ × radius × RPM² — memorize this formula
400 RCF × 5 minutes = standard centrifugation
20 µL sediment under 22×22 mm coverslip
Do NOT use braking when centrifuging
Casts & mucus → LPF; WBC & RBC → HPF
Squamous & transitional epithelial cells → semi-quantitative terms (rare/few/moderate/many)
Amorphous phosphates (alkaline) dissolve with acetic acid; Amorphous urates (acid) dissolve at 37°C
Formed elements disintegrate fastest in dilute, alkaline urine
Midstream clean-catch is the proper collection method
Standard volume = 12 mL; examine 10 fields, get average
Macroscopic screening is done when physical or chemical findings are abnormal
Physical → Chemical → Microscopic is the standard urinalysis order
Sources: Strasinger, S.K. & Lorenzo, M.S.D. (2014). Urinalysis and Body Fluids, 6th ed. | (2020) 7th ed.