URINALYSIS

URINALYSIS 

1. Sample Collection & Pre-Analytical Factors

• Avoid strenuous physical exercise for 24 h before collection (can cause transient proteinuria/hematuria).
• Midstream, 1st morning urine — most concentrated, most acidic, best for preserving formed particles (cells/casts).
• Preferred for pregnancy test, TB culture, and detecting orthostatic proteinuria.
• Permanent indwelling catheter → associated with bacteriuria, hematuria, WBCs, and Candida (not true infection markers by themselves).
• Analyse within 3 hours of voiding (or within 1 h at room temp / 4 h if refrigerated, per European guidance).
• Refrigerate at 4–8°C if delay is unavoidable — but this causes precipitation of phosphates/urates, which can obscure other crystals.
• Chemical preservatives used: formaldehyde-based solutions, buffered boric acid, and formate-based solutions.

Turbidity

Turbid (cloudy) urine suggests: urinary tract infection, heavy hematuria, or genital secretions contaminating the sample.

Table 1 — Abnormal Urine Colour

Colour	Causes
Pink	Uric acid crystalluria, hematuria, hemoglobinuria, myoglobinuria; Drug: desferrioxamine
Purple	UTI → “purple urine bag syndrome” (chronic catheter + Klebsiella, Proteus, E. coli, Providencia, Enterococcus)
Blue	Drug: methylene blue
Green	Drugs: triamterene, propofol; blue dyes in enteral feeds
Orange	Bilirubinuria; Drug: rifampin
Red	Hematuria, hemoglobinuria, myoglobinuria, porphyrinuria; Drugs: rifampin, phenytoin, phenazopyridine; Foods: beetroot, senna, rhubarb
Brown	Hematuria, hemoglobinuria, myoglobinuria, bilirubinuria; Drugs: chloroquine, nitrofurantoin
Black	Hematuria/hemoglobinuria/myoglobinuria/porphyrinuria; alkaptonuria (darkens on standing); Drugs: metronidazole, methyldopa, imipenem-cilastatin
White	Chyluria

Odour

Condition	Odour
Maple syrup urine disease (MSUD)	Maple syrup
Phenylketonuria	Musty
Isovaleric acidemia	Sweaty feet
Hypermethioninemia	Rancid butter / fishy

2. Urine Reagent Strip (Dipstick) Testing

Dipstick colour-change is semiquantitative; can also be read by a reflectance meter — inexpensive, needs only a single drop of urine, and is more accurate/objective than visual reading.

Table 2 — False-Negative / False-Positive Results

Constituent	False-Negative	False-Positive
Specific gravity	Urine pH > 6.5 (urine alkalosis)	Urine protein > 7.0 g/L
Hemoglobin	High SG; ascorbic acid; formaldehyde (0.5 g/L — used as preservative)	Myoglobin
Glucose	Ascorbic acid; bacteria (mechanism: glucose oxidase catalyst)	Very acidic urine pH; oxidizing detergents
Albumin	Low SG; albumin 0.25–0.30 g/L (detection limit); low urine vol; tubular proteins; monoclonal light chains (reagent: tetrabromophenol blue)	Urine SG ≥ 1.030; urine pH > 8.0; quaternary ammonium detergents; chlorhexidine; povidone
Nitrites	Bacteria that don’t reduce nitrate→nitrite  (Pseudomonas, S. albus, Enterococcus);  vegetable-poor diet; short bladder incubation	Abnormal urine colour
Ketones	Improper storage (loses acetoacetate/acetone via nitroprusside reaction; does NOT detect β-hydroxybutyrate)	Free sulfhydryl groups (e.g., captopril); levodopa; abnormal colour

Dipstick leukocyte-esterase detection limit ≈ 20 × 10⁶ leukocytes/L. False results also occur with time-expired strips.

Specific Gravity (normal 1.010–1.030)

• 1.010 (isosthenuria — same as plasma): seen in ATN and CKD with ↓ GFR.
• > 1.040: suggests radiocontrast in the urine.
• 1.000–1.003: diabetes insipidus or water intoxication.
• Osmolality is NOT influenced by urine temperature or protein concentration (unlike SG). 
• 10 g/L glucose ≈ raises osmolality by 55.5 mOsm/L — high glucose → ↑ osmolality disproportionate to SG.

pH (normal range 5.0– 8.5/9.0)

Gold standard = pH meter. Blood-gas analyzers use automated ion-selective electrodes.

Acidic urine	Alkaline urine
Metabolic acidosis; high-protein meals; volume depletion (via aldosterone stimulation)	Renal tubular acidosis (RTA); vegetarian diets; infection by urease-producing organisms

3. Proteinuria

Physiologic Limits

• Adults: < 150 mg / 24 h
• Children: < 140 mg / m² / 24 h

PCR vs ACR

• Measured by chemical assays, turbidimetric techniques, or dye-binding techniques.
• KDIGO recommends ACR (albumin:creatinine ratio) over PCR as the first measurement of proteinuria in adults — albuminuria is a reliable, specific, sensitive marker of glomerular-permeability change and CKD outcome.
• Children & glomerulonephritis (GN): PCR > ACR preferred — to avoid missing congenital disorders with non-albumin proteinuria.
• PCR is spuriously ↑ in the elderly and in females due to ↓ creatinine excretion.

Table 3 — Quantitative Methods for Proteinuria

Method	Sensitivity (mg/L)	Linearity (mg/L)	Notes
Turbidimetric (trichloroacetic acid)	20	20–2400	Same sensitivity for albumin & globulins; many drug interferences
Dye-binding (benzethonium chloride)	10	10–1600	Albumin over/under-estimation at higher conc.
Dye-binding (Coomassie blue)	2.5	5–1500	High sensitivity; underestimates tubular proteins
Ponceau / Biuret	20	100–1600	Similar sensitivity for albumin/globulin; interferes with aminoglycosides

Types of Proteinuria

Type	Key Feature
Glomerular	Typically > 1 g/day
Tubular	Associated with tubular / interstitial disease
Overflow	Excess filtered low-MW proteins overwhelm reabsorption (e.g., light chains)
Post-renal	IgG, IgM in urine (from urinary-tract source, not filtration)

Table 4 — UACR / UPCR / 24-h Excretion Correlation

Category	UACR	UPCR	Albumin/24h	Protein/24h
Moderately ↑ albuminuria	300 mg/g	~500 mg/g	~300 mg	~500 mg
Severely ↑ albuminuria	700 mg/g	~1000 mg/g	~700 mg	~1000 mg
Nephrotic-range	2200 mg/g	~3500 mg/g	~2200 mg	~3500 mg

Urine Albumin Testing Methods

• Dipstick (indicator dye): tetrabromophenol blue
• Sulfosalicylic acid (SSA) test — a precipitation method
• Heat & acetic acid test
• Immunochemical methods: anti-human albumin antibody — immunoturbidimetry, immunonephelometry, ELISA, HPLC, albumin-specific test strips (micro/macroalbuminuria)
• Serum albumin assays use bromocresol green or bromocresol blue dyes.

4. Urine Sediment — Cells, Casts & Crystals

Red Cells — Glomerular vs Non-glomerular Hematuria

• Isomorphic (nonglomerular) RBCs: uniform, ~6 µm, often crenated.
• Dysmorphic (glomerular) RBCs: irregular membranes; Acanthocytes (ring body with blebs) = pathognomonic subtype.
• Glomerular hematuria defined by: > 40% dysmorphic RBCs OR > 5% acanthocytes OR ≥ 1 RBC cast.
• In hypotonic urine RBCs swell, lose Hb → “ghost/shadow cells.”

Other Cells

• Neutrophils: ~10 µm, lobulated nucleus, granular cytoplasm.
• Macrophages: 13–95 µm; with lipid droplets → “oval fat bodies” (not diagnostic on their own); 
• Eosinophils identified with Hansel’s stain.
• Uroepithelial (transitional) cells: deep layer (~18 µm, “tail-like”/caudate) — ≥ 1/hpf suggests severe damage (neoplasia, stones, obstruction, long-standing catheter). Superficial layer (~25 µm) — mild damage, e.g. cystitis.
• Squamous epithelial cells: ~50 µm (usually contamination).
•  

Fat Particles

• Cholesterol/lipid: birefringent, symmetric “Maltese cross” under polarized light; typical of glomerular disease with marked proteinuria (cholesterol crystals themselves are non-birefringent).
• Fabry disease: fat particles with asymmetric/truncated Maltese cross, external protrusions — contain glycosphingolipids (globotriaosylceramide-3); may occur even without proteinuria. Similar particles seen in TMA (α-Gal A related).

Table 5 — Casts & Main Clinical Associations

Cast	Association
Hyaline / Hyaline-granular	Normal individuals (dehydration); also kidney disease
Granular	Kidney disease; AKI with ATN
Waxy	Kidney disease with functional impairment (CKD/AKI/RPGN, ↓GFR)
Fat	Marked proteinuria (usually nephrotic range)
Erythrocyte	Glomerular hematuria — usually active proliferative GN; AIN
Leukocyte	AIN; acute pyelonephritis; active proliferative GN
Renal tubular epithelial cell	AKI/ATN; active/nonproliferative GN with heavy proteinuria; AIN
Hemoglobin	Glomerular hematuria; hemoglobinuria from intravascular hemolysis
Myoglobin	AKI due to rhabdomyolysis
Bilirubin	Jaundice with ↑ direct bilirubin
Bacterial / Fungal	Bacterial or fungal kidney infection
Crystalline	Crystalline nephropathies
Mixed	Per components present

Special stains: Sternheimer-Malbin stain (×1000) helps identify degenerating WBCs in pyelonephritis (“glitter cells” show Brownian/granular motility); Kova stain also used.

Table 6 — Key Crystals

Crystal	Appearance / Notes
Uric acid	Rhomboid (most common) or needle/grenade-like; birefringent
Calcium oxalate	Bihydrate = bipyramidal “envelope”; monohydrate = variable shapes
Struvite (triple phosphate)	“Coffin-lid” appearance; forms in alkaline urine
Cystine	Hexagonal, stacked; pathognomonic of cystinuria
2,8-Dihydroxyadenine (2,8-DHA)	Central umbilicus; homozygous APRT (adenine phosphoribosyltransferase) deficiency
Tyrosine / Leucine	Rare; severe liver disease (tyrosine also in tyrosinemia); leucine = yellow-brown spheres, concentric striations, birefringent; tyrosine = needle-shaped, aggregates

Decoy Cells (Polyomavirus BK infection): 

Seen on phase-contrast microscopy

5. Assessment of GFR

• GFR = sum of the filtration rate of each nephron in both kidneys — the best overall index of kidney function in health & disease.
• Normal: Men ~130 mL/min/1.73m²; Women ~120 mL/min/1.73m² (standardized to BSA 1.73 m² since GFR depends on age, sex, body size, diet, exercise).
• ~8% higher in young men than women. Diurnal variation: ~10% lower at midnight vs. afternoon.
• Mean rate of decline: 0.75 mL/min/1.73m² per year after age 40.
• ↓ Nephron number → compensatory ↑ single-nephron GFR — so overall GFR does NOT fall until late-stage disease.
• Renal functional reserve (RFR) ≈ 20–50%.

Gold Standard & Exogenous Markers

Marker	Administration	Key Comment
Inulin	Continuous IV infusion	GOLD STANDARD
Iothalamate	Bolus IV / SC injection	Secreted → overestimates GFR
⁹⁹ᵐTc-DTPA	Bolus IV	Plasma protein binding → underestimates GFR
⁵¹Cr-EDTA	Bolus IV	~10% lower clearance than inulin
Iohexol	Plasma clearance after bolus IV	Low adverse effects; may overestimate (extrarenal clearance)
Relmapirazin & VFI (novel)	Plasma / transdermal fluorescence	Faster than plasma clearance markers; not yet FDA-approved

Clearance (Cx) = Ax (rate of elimination) / Px (average plasma conc.) = volume of plasma cleared of a marker per unit time. Classic Smith protocol: continuous IV infusion + bladder catheterization with timed collections; alternatives use bolus dosing + spontaneous voiding.

Endogenous Markers

Overall clearance of endogenous low-MW proteins can estimate GFR after correcting for non-renal excretion (using age, sex, race, weight in equations).

Variable	Creatinine	Cystatin C	Urea
MW (Da)	113 (amino-acid derivative)	13,000 (non-glycosylated protein)	60
Generation	Varies with muscle mass & dietary protein; ↓ in elderly, women	Constant — produced by all nucleated cells; ↑ in hyperthyroid state, steroid use	Varies with dietary protein intake
Extrarenal elimination	No	Preliminary evidence of ↑ at reduced GFR	Yes — ↑ at reduced GFR
Advantages	Low cost, widely available	Less affected by muscle mass; more accurate in low-muscle-mass / liver disease / diabetic patients	—

• Other endogenous markers (less accurate than creatinine/cystatin C): urea, β₂-microglobulin, β-trace protein.
• Non-GFR determinants affecting creatinine clearance: generation (diet, muscle cells), tubular secretion, and extrarenal elimination — overall Excretion = Renal + Extrarenal clearance.

Estimating Equations

Equation	Key Points
Cockcroft-Gault (1976)	Uses age, weight; needs BSA adjustment to 1.73 m²; several limitations — not accurate at GFR > 60, overestimates in edema/obesity, under/overestimates in elderly Preferred for drug dosing
MDRD	More accurate; derived from CKD population — not for GFR ≥ 60, or non-CKD groups (donors, diabetics, transplant recipients)
CKD-EPI (2009)	Race-based
CKD-EPI (2021)	Currently recommended by ASN & NKF; removes race.  In Blacks → underestimates by ~3 mL/min/1.73m²; in others → overestimates by ~3.
CKD-EPI Cystatin-C / Combined (2012)	Cystatin-based or creatinine+cystatin combined; KDIGO 2012 recommends for confirmatory testing

• Cystatin C rises earlier than creatinine in kidney injury.

When to Confirm / Measure GFR Directly

• Prominent non-GFR determinants of creatinine: high/low muscle mass, creatine supplements, high animal-protein or vegetarian diet, liver disease, extreme frailty.
• Creatinine-based eGFR 45–60 mL/min/1.73m² with no other CKD features (e.g., albuminuria, imaging changes).
• Living kidney donor evaluation (US practice requires a clearance measurement).
• Clinical applications of GFR: CKD & AKI diagnosis/staging, donor evaluation, and drug dosing (Cockcroft-Gault still widely used for dosing; KDIGO recommends CKD-EPI for other purposes). GFR should be reported per 1.73 m² (BSA adjustment) — though its removal has been debated).