What are the components and uses of urinalysis (urine analysis)?

Components and Clinical Uses of Urinalysis

Urinalysis is a comprehensive diagnostic test consisting of three main components—physical examination, chemical analysis (dipstick), and microscopic evaluation—that serves as the second most commonly used biofluid test after blood for diagnosing urinary tract infections, kidney disease, metabolic disorders, and monitoring disease progression. 1, 2

Three Core Components of Urinalysis

Physical Examination

Physical assessment evaluates the following characteristics:

• Color and clarity: Visual inspection provides initial screening information, with clear urine having a 97% negative predictive value for urinary tract infection 1
• Volume: Normal daily urine production ranges from 1-2 liters 1
• Specific gravity: Indicates urine concentration and kidney function 3, 4
• Odor: Can suggest metabolic abnormalities or infection 4

Chemical Analysis (Dipstick Testing)

The dipstick provides rapid assessment of multiple parameters simultaneously:

• Leukocyte esterase: Detects white blood cells with 83% sensitivity (range 67-94%) and 78% specificity (range 64-92%) for UTI 2, 5
• Nitrite: Highly specific (92-100%) but poorly sensitive (19-48%) for detecting gram-negative bacteria that convert urinary nitrates to nitrites 2, 5
• Protein: Identifies proteinuria suggesting kidney disease 3, 4
• Blood: Detects hematuria from hemoglobin or myoglobin 3, 4
• Glucose: Screens for diabetes mellitus and renal glycosuria 3, 4
• Ketones: Identifies diabetic ketoacidosis or starvation states 3, 4
• pH: Ranges normally from 4.5-8.0, influences crystal formation and infection risk 3, 4
• Bilirubin and urobilinogen: Evaluate hepatobiliary function 3, 4

When leukocyte esterase and nitrite are both positive, specificity for UTI increases to 96% with combined sensitivity of 93%. 1, 2

Microscopic Examination

Microscopic sediment analysis identifies cellular and crystalline elements:

• White blood cells (WBCs): Pyuria defined as ≥10 WBCs per high-power field suggests inflammation or infection, with sensitivity of 73% and specificity of 81% 5
• Red blood cells (RBCs): Hematuria requires further evaluation for malignancy, stones, or glomerular disease 3, 4
• Bacteria: Presence correlates with ≥10⁵ CFU/mL on culture when visualized in fresh, Gram-stained uncentrifuged urine 2
• Epithelial cells: High numbers suggest contamination requiring repeat specimen collection 2
• Casts: Cylindrical structures indicating kidney pathology—hyaline, granular, cellular, or pathological casts each have distinct clinical significance 3, 4, 6
• Crystals: Type depends on urine pH and can indicate stone risk or metabolic disorders 3, 4
• Microorganisms: Yeast, parasites, or other pathogens 3, 4

Clinical Uses and Applications

Urinary Tract Infection Diagnosis

The primary utility of urinalysis in UTI diagnosis is ruling out infection when negative, not confirming it when positive—diagnosis requires both clinical symptoms AND urinalysis findings, never urinalysis alone. 2

• Negative predictive value: Absence of both leukocyte esterase and nitrite effectively rules out UTI in most populations with 82-91% negative predictive value 2, 5
• Positive predictive value: Pyuria alone has exceedingly low positive predictive value as it indicates genitourinary inflammation from many noninfectious causes 1, 2
• Culture indication: Obtain urine culture when pyuria (≥10 WBCs/HPF or positive leukocyte esterase) is present AND symptoms suggest UTI (dysuria, frequency, urgency, fever, hematuria) 2
• Pediatric criteria: Children aged 2-24 months require both urinalysis suggesting infection (pyuria and/or bacteriuria) AND ≥50,000 CFU/mL on culture for definitive diagnosis 2, 5

Kidney Disease Evaluation

Urinalysis provides critical information for diagnosing and monitoring renal pathology:

• Proteinuria detection: Persistent protein in urine indicates glomerular or tubular dysfunction requiring quantification with protein-to-creatinine ratio 3
• Hematuria with casts: Red blood cell casts specifically indicate glomerulonephritis or other intrinsic kidney disease 3, 4
• Tubular markers: Granular casts and renal tubular epithelial cells suggest acute tubular necrosis or interstitial nephritis 3, 4

Metabolic and Systemic Disease Screening

• Diabetes monitoring: Glucose and ketone detection screens for hyperglycemia and diabetic ketoacidosis 3, 4
• Liver disease: Bilirubin and urobilinogen abnormalities suggest hepatobiliary pathology 3, 4
• Acid-base disorders: Urine pH helps evaluate renal tubular acidosis and metabolic disturbances 3, 4

Emerging Applications

Urinary extracellular vesicles (uEVs) represent a rapidly growing field, as these vesicles mirror molecular processes and pathological conditions in kidney, urothelial, and prostate tissue, enabling noninvasive liquid biopsy for biomarker discovery. 1

Critical Timing and Specimen Handling

Urine composition begins changing immediately after voiding, requiring processing within 1 hour at room temperature or 4 hours if refrigerated to ensure accurate results. 2, 6

Delayed analysis causes significant changes:

• Bacteria levels increase over time, leading to false-positive results 6
• Red blood cells and pathological casts decrease due to cellular degradation 6
• Crystal formation fluctuates with temperature and pH changes 6

Common Pitfalls and How to Avoid Them

Specimen Collection Errors

• Contamination: High epithelial cell counts indicate poor collection technique; if repeat specimen remains contaminated with strong clinical suspicion, perform catheterization for definitive specimen 2
• Bag specimens in children: While negative bag specimens may be useful, positive results require confirmation with catheterization or suprapubic aspiration due to only 15% positive predictive value 5

Interpretation Mistakes

• Do not diagnose UTI on urinalysis alone: Asymptomatic bacteriuria with pyuria is common (prevalence 15-50% in long-term care residents) and should not be treated 2
• Do not rule out UTI based solely on negative nitrite: This would miss many true infections given the poor sensitivity of 19-48% 2, 5
• Do not order urinalysis for asymptomatic patients: Routine screening in asymptomatic individuals leads to unnecessary testing and overtreatment of colonization 1, 2

Special Population Considerations

• Infants with frequent voiding: Shorter bladder dwell time results in negative nitrite tests despite true infection, as bacteria require extended urine exposure for nitrate conversion 1, 2
• Elderly patients: Non-specific symptoms like confusion or falls alone should not trigger UTI evaluation without specific urinary symptoms 2
• Catheterized patients: Do not screen for or treat asymptomatic bacteriuria; reserve testing only for symptomatic patients with fever, hypotension, or specific urinary symptoms 2

Diagnostic Algorithm for UTI Evaluation

Step 1: Assess for specific urinary symptoms (dysuria, frequency, urgency, fever, gross hematuria) 2

• If absent: Do not order urinalysis or culture 2
• If present: Proceed to Step 2

Step 2: Obtain proper specimen using appropriate collection method 2

• Infants/young children: Catheterization or suprapubic aspiration 2, 5
• Cooperative adults: Midstream clean-catch 2
• Women with contamination: In-and-out catheterization 2

Step 3: Perform urinalysis checking leukocyte esterase, nitrite, and microscopic WBCs 2

• Both leukocyte esterase AND nitrite negative: UTI effectively ruled out 2, 5
• Either positive with typical symptoms: Treat as uncomplicated cystitis in healthy nonpregnant patients without culture 2
• Complicated cases or recurrent UTIs: Obtain culture for targeted therapy 1, 2

1, 2, 5, 3, 4, 7, 6, 8