Monday, July 9, 2012

urine analysis Part 2: D.D. & urine sediments


Differential Diagnosis for Routine Urine Analysis
  • Appearance:  amber yellow (other colors)
  • pH:
    • Acidic: High-protein (meat) diet, ammonium chloride, mandelic acid and other medications, acidosis (due to ketoacidosis [starvation, diabetes], chronic obstructive pulmonary disease [COPD])
    • Basic: Urinary tract infections (UTIs), renal tubular acidosis, diet (high-vegetable, milk, immediately after meals), sodium bicarbonate therapy, vomiting, metabolic alkalosis, diuretic therapy
  • Specific gravity:
    • Usually corresponds to osmolarity, except with osmotic diuresis. A value >1.023 indicates normal renal concentrating ability:
      • Increased: Volume depletion, congestive heart failure (CHF), adrenal insufficiency, diabetes mellitus, inappropriate antidiuretic hormone (ADH), increased proteins (nephrosis); if markedly increased (1.040–1.050), suspect artifact or excretion of radiographic contrast media.
      • Decreased: Diabetes insipidus, pyelonephritis, glomerulonephritis, water load with normal renal function
  • Bilirubin:
    • Positive: Obstructive jaundice (intrahepatic and extrahepatic), hepatitis (Note: False positive with stool contamination)
  • Blood:
    • Positive: See Section I: “Hematuria”
    • Note: If the dipstick is positive for blood, but no RBCs are seen, free hemoglobin may be present from trauma, from a transfusion reaction, or from lysis of RBCs (RBCs will lyse if the pH is <5 or >8), or there may be myoglobin present because of a crush injury, burn, or tissue ischemia.
  • Glucose:
    • Positive: Diabetes mellitus, pancreatitis, pancreatic carcinoma, pheochromocytoma, Cushing syndrome, shock, burns, pain, steroids, hyperthyroidism, renal tubular disease, iatrogenic causes
(Note: The glucose oxidase technique in many kits is specific for glucose and will not react with lactose, fructose, or galactose.)
  • Ketones:
    • Detects primarily acetone and acetoacetic acid and not β-hydroxybutyric acid:
    • Positive: Starvation, high-fat/low-carbohydrate diet, diabetic ketoacidosis, vomiting, diarrhea, hyperthyroidism, pregnancy, febrile states (especially in children)
  • Nitrite:
    • Many bacteria will convert nitrates to nitrite. (See also the section on “Leukocyte Esterase,” below.)
      • Positive: Infection (A negative test does not rule out infection, because some organisms, such as Streptococcus faecalis and other gram-positive cocci, will not produce nitrite, and the urine must also be retained in the bladder for several hours to allow the reaction to take place.)
  • Protein:
    • Indication by dipstick of persistent proteinuria should be quantified by 24-hr urine studies:
      • Positive: Pyelonephritis, glomerulonephritis, Kimmelstiel-Wilson syndrome (diabetes), nephrotic syndrome, myeloma, postural causes, preeclampsia, inflammation, and malignancies of the lower tract, functional causes (fever, stress, heavy exercise), malignant hypertension, congestive heart failure
  • Leukocyte esterase (see Section I: “Pyuria”.):
    • This test detects ≥5 WBCs/HPF or lysed WBCs. When combined with the nitrite test, it has a predictive value for UTI of 74% if both tests are positive, and >97% if both tests are negative:
      • Positive: Infection (false-positive with vaginal contamination)
  • Reducing substance:
    • Positive: Glucose, fructose, galactose, false-positives (vitamin C, salicylates, antibiotics, etc.)
  • Urobilinogen:
    • Positive: Cirrhosis, CHF with hepatic congestion, hepatitis, hyperthyroidism, suppression of gut flora with antibiotics (Note: With obstructive jaundice, urobilinogen is usually normal, but bilirubin is elevated.)
Urine Sediment

Many labs no longer do microscopic examinations unless specifically requested or if the dipstick test shows evidence of an abnormal finding (such as positive leukocyte esterase):
  • RBCs: Trauma, pyelonephritis, genitourinary tuberculosis (TB), cystitis, prostatitis, stones, tumors (malignant and benign), coagulopathy, and any cause of blood on dipstick test (see above on hemoglobin)
  • WBCs: Infection anywhere in the urinary tract, TB, renal tumors, acute glomerulonephritis, radiation, interstitial nephritis (analgesic abuse)
  • Epithelial cells: Acute tubular necrosis (ATN), necrotizing papillitis (most epithelial cells are from an otherwise unremarkable urethra)
  • Parasites: Trichomonas vaginalis, Schistosoma haematobium infections
  • Yeast: Candida albicans infection (especially in diabetics, immunosuppressed patients, or if a vaginal yeast infection is present)
  • Spermatozoa: Normal in males immediately after intercourse or nocturnal emission
  • Crystals: Note that urine should be examined fresh and warm because clouding due to phosphate precipitation may be observed when urine cools:
    • Abnormal: Cystine, sulfonamide, leucine, tyrosine, cholesterol
    • Normal in acidic urine: Oxalate (small square crystals with a central cross), uric acid
    • Normal in alkaline urine: Calcium carbonate, triple phosphate (resemble coffin lids)
  • Contaminants: Cotton threads, hair, wood fibers, amorphous substances (all usually unimportant)
  • Mucus: Large amounts suggest urethral disease (normal from ileal conduit or other forms of urinary diversion).
  • Glitter cells: WBCs are lysed in hypotonic solution.
    P.723

  • Casts: The presence of casts in a urine sample localizes some or all of the disease process to the kidney itself:
    • Hyaline casts (occasionally acceptable, unless they are numerous), benign hypertension, nephrotic syndrome, after exercise:
    • RBC casts: Acute glomerulonephritis, lupus nephritis, subacute bacterial endocarditis (SBE), Goodpasture disease, after a streptococcal infection, vasculitis, malignant hypertension
    • WBC casts: Pyelonephritis
    • Epithelial (tubular) casts: Tubular damage, nephrotoxin, virus
    • Granular casts: Breakdown of cellular casts leads to waxy casts; dirty brown granular casts typical for ATN
    • Waxy casts (end stage of granular casts): Severe, chronic renal disease; amyloidosis
    • Fatty casts: Nephrotic syndrome, diabetes mellitus, damaged renal tubular epithelial cells
    • Broad casts: Chronic renal disease

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