How do you interpret a urine test result?

Interpreting Urine Test Results

Proper interpretation of urine test results requires understanding both the collection method and specific parameters, as these significantly impact diagnostic accuracy and clinical decision-making.

Collection Methods and Their Impact

• Collection method affects result reliability:

  • Suprapubic aspiration: Lowest contamination rate (1%), highest specificity 1
  • Catheterization: Moderate contamination (12%), good reliability 1
  • Clean catch: Higher contamination (26%), less reliable 1
  • Bagged specimens: Highest contamination, 85% false positive rate for cultures 1

• Sample handling considerations:

  • Protect samples from light to prevent falsely low results, especially for porphyrins 1
  • Morning or spot urine samples are preferred over 24-hour collections 1
  • Normalize urinary results to creatinine concentration 1
  • Interpret with caution when urinary creatinine is below 2 mmol/L 1

Key Parameters to Evaluate

Physical Characteristics

• Color and clarity:
  • Normal: Pale yellow to amber, clear
  • Abnormal colors suggest specific conditions (e.g., red/brown may indicate blood, dark yellow may indicate concentrated urine) 2
  • Cloudiness suggests possible infection, crystals, or cellular debris

Chemical Parameters (Dipstick)

• Leukocyte esterase:

  • Sensitivity: 88% (when combined with nitrite) 1
  • Specificity: 79% (when combined with nitrite) 1
  • Positive result suggests UTI or inflammation

• Nitrite:

  • Sensitivity: 49% (alone) 1
  • Specificity: 98% (alone) 1
  • Positive result strongly suggests bacterial infection
  • False negatives common with non-E. coli organisms 1

• Blood:

  • Positive result may indicate infection, trauma, kidney disease, or contamination
  • Sensitivity and specificity vary by cutoff threshold 1

• Protein:

  • Positive result may indicate kidney disease, infection, or physiological conditions
  • Quantify with protein-to-creatinine ratio for better accuracy

• pH:

  • Normal range: 4.5-8.0
  • Acidic pH (<7.0) may indicate metabolic acidosis, respiratory acidosis, or certain diets
  • Alkaline pH (>7.0) may indicate UTI, metabolic alkalosis, or vegetarian diet

Microscopic Examination

• WBCs:

  • 5 WBCs/hpf suggests inflammation or infection

  • Sensitivity: 74% (for counts >10/mL) 1
  • Specificity: 86% (for counts >10/mL) 1

• RBCs:

  • 3 RBCs/hpf is abnormal, suggesting bleeding, trauma, or kidney disease

• Bacteria:

  • Unstained bacteria sensitivity: 88%, specificity: 92% 1
  • Gram-stained bacteria sensitivity: 91%, specificity: 96% 1
  • Presence suggests infection, but must correlate with clinical picture

• Casts:

  • RBC casts: Glomerular disease
  • WBC casts: Pyelonephritis
  • Hyaline casts: Often normal or dehydration
  • Granular casts: Tubular damage

• Crystals:

  • May be normal or indicate metabolic disorders
  • Type depends on urine pH and metabolic state

Urine Culture Interpretation

• Definition of positive culture:

  • Suprapubic aspiration: >10² CFU/mL 1
  • Catheterized specimen: 10³-10⁵ CFU/mL 1
  • Clean-catch/bag: >10⁵ CFU/mL of a single organism 1
  • Multiple organisms typically indicate contamination 1

• Important considerations:

  • Negative dipstick/urinalysis doesn't always exclude UTI 1
  • 30% of positive urine cultures may have negative urinalysis 1
  • Obtain culture when UTI is suspected in children <2 years 1

Potential Interferences

• Medications affecting results:

  • Fluoroquinolones: False positive for opiates 3
  • Pseudoephedrine, dextromethorphan, antihistamines: False positives on drug screens 3

• Other interfering factors:

  • Poppy seeds: False positive for cocaine 3
  • Concentrated urine: May affect interpretation of quantitative results 1
  • Adulterated samples: May give false results 3

Common Pitfalls to Avoid

1. Relying solely on dipstick results without microscopic examination or culture when indicated
2. Misinterpreting bagged specimen results due to high contamination rates
3. Failing to correlate results with clinical presentation
4. Not considering medication effects on test results
5. Improper sample handling leading to degradation of components

Remember that urine testing is most valuable when interpreted in the context of clinical presentation, and no single parameter should be used in isolation for diagnosis. When results are unexpected, consider confirmatory testing with more specific methods.