11. Urinalysis Interpretation

Urinalysis and urine sediment exam is most helpful with fresh urine whether voided or collected from clamped foley catheter tubing and collected from valve with syringe (within 30-60 minutes). Foley catheters can introduce hematuria due to insertion and urethra/bladder trauma.

• Specific gravity: in absence of diuretics, may aid determination of intravascular volume. A specific gravity >1.010 (iso-osmotic) suggests that the kidney’s concentrating ability is preserved as in pre-renal AKI. May not correlate when large particles such as protein, glucose, or contrast dye are present. Dilute urine can lead to osmotic swelling, cell lysis and inaccurate cell counts. Can estimate Uosm: for each 0.001 above 1 of SpG is ~30osm (i.e. SpG 1.010 = Uosm ~300).
• Urine pH: normal range is 4.5-8. Alkaline (>7) with urea splitting organisms such as Proteus (usually not E-Coli) infection, metabolic alkalosis and calcium phosphate stones. Acidic ( <5) with metabolic acidosis. Acid urine can be associated with uric acid, cystine and calcium oxalate stones.
• Leukocyte esterase: screens for >10 WBC/hpf. Positive with UTIs, pyelonephritis, and interstitial nephritis. False positive with vaginal contamination. False negative with high specific gravity, glucosuria. Correlate with microscopy.
• Nitrites: produced by gram-negative bacteria (converts nitrates to nitrites). Negative with Enterococcus and Staph saprophyticus (gram +). In order for bacteria to convert dietary nitrates to nitrites, urine must incubate in the bladder for >4 hours.
• Blood: dipstick hematuria can be caused by hemoglobin from hemolyzed red blood cells, myoglobin from rhabdomyolysis, or RBC’s. Correlate with microscopy. Hematuria is 3 or more RBC’s per high power field in at least 2 of 3 specimens. See section Hematuria.
• Ketones: seen in diabetic ketoacidosis, alcoholic ketoacidosis, or starvation. Detects acetoacetate, misses acetone and β-hydroxybutyrate. False positive with levodopa metabolites.
• Bilirubin/urobilinogen: usually only conjugated (water soluble) bilirubin is found in urine; however, when serum bilirubin is high, unconjugated bilirubin (usually bound to albumin) can be detected.
• Protein: 1+ ~30mg/dL, 2+ ~100mg/dL, 3+ ~300mg/dL, 4+ ~>2000mg/dL. A qualitative test, dipstick proteinuria is not a sensitive test for microalbuminuria and misses Bence-Jones proteins. Spot UProt/UCr and 24-hour urine collection are the two methods for quantifying proteinuria. False positive with contrast. Etiologies include losing albumin through disrupted filtration (GN), decreased reabsorption of freely filtered proteins (ATN), increased production of proteins (MM).
• Glucose: seen when serum glucose >180mg/dL (the concentration that exceeds the proximal tubule’s reabsorptive capacity for glucose), in pregnancy, or in Fanconi syndrome (proximal tubular dysfunction).
• Urine microscopy (or urine sediment examination):
  • Dysmorphic RBCs or RBC casts: glomerulonephritis.
  • WBCs: PMNs=UTI; eosinophils (AIN with special Wright stain).
  • WBC casts: pyelonephritis, interstitial nephritis, +/- GN.
  • “Muddy brown” granular casts, renal tubular epithelial (RTE) cells, RTE cell casts: ATN.
  • Crystalluria: uric acid, ethylene glycol intoxication, drug-induced (sulfa, acyclovir, indinavir); struvite (coffin lid shaped, seen in chronic UTI with urea splitting organisms).
  • Hyaline casts: pre-renal, non-specific. Can be seen with diuretics and after exercise.
  • Waxy casts: most commonly seen in chronic kidney disease.

  Fogazzi GB, Fogazzi SV, Garigali G. Giovanni B. Urinalysis: core curriculum 2008. American Journal of Kidney Diseases, Vol. 51, Issue 6, p1052–1067

  Simerville JA, Maxted WC, Pahira JJ. Urinalysis: a comprehensive review. Am Fam Physician 2005;71:1153-1162.