Approach to Urinalysis

Madelaine Behrens, Laura Binari, Patrick Steadman

Background

  • 3 components: Gross evaluation, dipstick analysis, microscopic exam or urinary sediment
  • 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, porphyrias, myo/hemoglobinuria), white (chlyuria, phosphate crystals, propofol), green (methylene blue, amitriptyline, propofol), 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 and 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)
    • Ddx: primary glomerular dx vs secondary glomerular dx (DM, amyloid, infxn, sickle cell, etc.) vs tubular vs overflow (multiple myeloma)
    • Transient: due to volume depletion, CHF, fever, postural, exercise-induced
    • Mild albuminuria (30-300 mg/day) not detected by standard dipsticks
    • Degree of dilution affects semiquantitative measurement (1+,2+,3+)
    • Follow up with spot protein to Cr ratio or 24 hr urine collection (nephrotic range >3.5 g/day)
  • Heme (see “Hematuria” section): False (+) if semen, false (-) w/ ascorbic acid
  • WBC
    • Possibilities: False (+) due to contamination with squamous cells. If bacteria → consider UTI/pyelo with hematuria → inflammation
    • Ddx includes UTI: gram positive/negative, chlamydia, ureaplasma; TB, malignancy, viral infxn, kidney stones, GN, urethritis, steroids, cyclophosphamide use
    • Sterile pyuria (no bacteria): DDx interstitial nephritis, renal tuberculosis, nephrolithiasis, recently treated UTI, prostatitis
  • 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)
    • False (-) with ascorbic acid
  • Leukocyte esterase: enzyme released by lysed neutrophils, macrophages
    • Associated with pyuria and infections
    • False (-) from hematuria, glucosuria, or concentrated urine
  • 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, prerenal azotemia, normal subjects), RBC (GN, interstitial disease), WBC (acute interstitial nephritis, GN, pyelo, 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+2 oxalate (envelope/dumbbell shape), uric acid (rhombic/rosette shaped, classically formed in acidic urine), cystine (hexagonal, found in cystinuria), Mg+2 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
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