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Approach to Urinalysis

Laura Binari, Patrick Steadman

Background

  • 3 components: Gross Evaluation, Dipstick Analysis, Microscopic Exam

  • Indications: dysuria, gross hematuria, fever + GU symptoms, AKI, volume overload

  • If Foley, obtain sample from catheter, not the urine bag

  • Spinning Urine

    • At VUMC, take sample to lab on 4th floor to centrifuge the sample at 1500 rpm for 5 minutes, remove supernatant and then resuspend sediment, place drops of urine on the slide, examine with microscope

    • At the VA, there is a microscopy room where you can spin urine as well

    • Ideally, the specimen should be a fresh catch (\<2-4 hours old); Beware: casts like to migrate to the edges of the coverslip!

Gross Evaluation

  • Turbid: Infection, precipitated crystals, or chyluria

  • Color: Red Urine (broad DDx, see Hematuria section, includes certain meds such as rifampin/phenytoin), White (polyuria, phosphate crystals), Green (methylene blue), Pink (uric acid crystals, post-propofol infusion), Black (hemoglobinuria/myoglobinuria)

Dipstick Analysis

  • Quality of sample: should have zero squamous epithelial cells

    • Specific gravity: normal = 1.010

    • Surrogate for urine osmolality & hydration: can have falsely high specific gravity if large particles (contrast, glucose) present

    • Trick: Last 2 digits of S.G. x 30 = Uosm. For example: S.G. is 1.013; 13 x 30 = 390 mOsm/L

  • Urinary pH: normal pH is 5.5-6.5

    • Alkaline pH: bicarb suppl, vegan diet, urease producing organisms (staghorn calculi)

    • Acidic pH: uric acid stones, appropriate response to acidemia

  • Proteinuria: dipstick detects albumin ONLY (not paraproteins)

    • Mild albuminuria (30-300 mg/day) not detected by standard dipsticks

    • F/u with spot protein to Cr ratio or 24 hr urine collection (nephrotic range >3.5 g/day)

    • Transient: due to volume depletion, CHF, fever, postural, exercise-induced

    • Ddx: primary glomerular dx, secondary glomerular dx (DM, amyloid, infxn, sickle cell, etc.) vs tubular vs overflow (multiple myeloma)

  • Heme (see hematuria section): False(+) if semen, false(-) w/ ascorbic acid

  • WBC:

    • False(+) 2/2 contamination with squamous cells. If bacteria -> consider UTI/pyelo w/hematuria -> inflammation; May have sterile pyuria

    • Ddx includes chlamydia, ureaplasma, TB, malignancy, viral infxn, kidney stones, GN, urethritis, steroid, cyclophosphamide use

  • Ketones:

    • Never normal in urine; only detects acetic acid

    • Ddx: DKA, starvation ketoacidosis, pregnancy, keto diet

  • Glucose: max threshold at proximal tubule exceeded (~serum glucose 180 mg/dL)

    • DM, Cushing’s, liver/pancreatic dx, SGLT2i use; or a primary defect of proximal reabsorption (w/phosphaturia, uricosuria, amino aciduria think Fanconi syndrome)

  • Leukocyte esterase: enzyme released by lysed neutrophils, macrophages

    • Associated with pyuria and infections; false(-) from hematuria or glucosuria

  • Nitrites: reduction of urinary nitrates by nitrate reductase

    • Certain bacteria (e.g. Enterobacteriaceae) express, others (e.g. Enterococci) do not

  • Bilirubin: conjugated = water soluble (passes through glomerulus), unlike unconjugated

    • Liver dysfunction and biliary obstruction

  • Urobilinogen: end product of conjugated bilirubin, normally ~1.0mg/dL is normal

    • Can be elevated due to hepatocellular dx or hemolysis

Microscopic Examination of the Urine Sediment

  • Cells:

    • Dysmorphic RBCs (sign of GN), squamous epithelial cells (contamination), tubular cells (abnormal, indicates renal dx), neutrophils (UTI, AIN, TB, sterile pyuria), eosinophils (think AIN, not sensitive thus cannot exclude diagnosis)

  • Casts:

    • Hyaline (pyelo; CKD, normal subjects), RBC (GN), WBC (GN, pyelo, interstitial dx, inflammation), epithelial - renal tubular cells (ATN, interstitial nephritis, nephritic sx, heavy metal ingestion), granular or waxy (presence of kidney disease, but nonspecific), muddy brown casts (ATN); fatty (nephrotic syndrome)

  • Crystals:

    • Ca++ oxalate (envelope/dumbbell shape), uric acid (rhombic/rosette shaped, classically formed in acidic urine), cystine (hexagonal, found in cystinuria), Mg ++ ammonium phosphate (aka struvite stones, from increased ammonia production, in setting of urease producing bacteria such as Proteus or Klebsiella UTIs)

    • Calcium oxalate crystals + AKI, consider ethylene glycol intoxication

    • Uric Acid crystals + AKI, consider tumor lysis syndrome