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pa_urinalysis

urinalysis

heme

  • detects both free Hb (or myoglobin) and Hb within RBCs, although is more sensitive for the former.
  • normal individuals pass up to 1 million RBC's/24hrs in their urine which equates to ⇐ 5RBC's/hpf.
  • as little as 1ml of blood per litre urine may cause grossly appreciable haematuria
  • it may detect as few as 3 RBC's/hpf (high power field), but may fail to detect up to 15% of pts with microscopic haematuria defined as being > 5RBC's/hpf
  • see also haematuria

causes of false negative tests:

  • substances that alter the Hb molecule (eg. large amounts of urinary vitamin C)
  • dilute urine
  • large amounts of proteinuria

causes of false positive tests:

  • chlorine or other oxidising agents
  • if positive then microscopy should be done (unless it is obviously contamination - eg. menstruating females in which case consider recollection with tampon in situ & careful attention to technique):
    • if no RBC's then this suggests free Hb or myoglobin
    • if RBC's, determine whether glomerular in origin or not

protein:

  • if proteinuria is present then consider urinary protein electrophoresis to detect non-albumin proteins.

dipstick test:

  • can detect protein at 10-15mg/dL but does not reliably yield positive results until [ ] > 30mg/dL
  • correlation between color intensity & concentration is only approximate
  • 3-5x more sensitive to albumin than to globulins & Ig light chains (Bence Jones protein), an important limitation.

false positives:

  • alkaline urine
  • haematuria
  • prolonged immersion of dipstick in urine

sulphosalicylic acid (SSA) test:

  • more sensitive, can detect 5mg/dL of albumin or non-albumin protein
  • 8 drops of 20% SSA added to 2ml urine ⇒ if turbid then +ve for protein

false positives:

  • radiographic contrast agents
  • penicillin
  • sulphonylurea drugs

false negatives:

  • alkaline urine

proteinuria

  • during 24 hours, kidneys normally filter 180L of plasma containing 12kg of protein. The 1-2L of urine produced from this filtrate contains only 40-80mg of protein in normal individuals.
  • abnormal proteinuria is defined as excretion of > 150mg/24hr in adults or > 140mg/m2/24hr in children.

types:

glomerular:

  • the more common type, results from increased permeability of glomerular capillaries to plasma proteins
  • contains both albumin & globulins
  • may cause 10g protein loss per day or more & lead to the nephrotic syndrome (>3.5g/day)

tubular:

  • occurs in pts with normal glomeruli when the smaller proteins that are normally filtered at the glomerulus & then reabsorbed in the tubule appear in the urine because of tubular or interstitial abnormality.
  • eg. urinary tract obstruction, HbS, & other causes of acute or chronic interstitial nephritis
  • daily losses rarely exceed 2g

overflow:

  • urinary loss of small proteins that are present in blood in excessive concentrations which thus overload the tubular reabsorptive capacity
  • eg. light chains in multiple myeloma

miscellaneous causes of transient proteinuria:

  • exertion, stress, fever
  • pregnancy (up to 300mg protein/day in normal pregnancy - greater than this suggests pre-eclampsia)

orthostatic:

  • occurs in some pts during upright posture periods only, rarely > 2g/day

urine microscopy

  • examine as soon as possible after voiding otherwise structures such as red cell casts may disintegrate with time
  • 10ml urine placed in conical tube, spun at 2000rpm for 5 mins and supernatant discarded, sediment is resuspended in residual urine & a drop is placed on a slide, & covered with a cover slip.
  • casts tend to concentrate at the periphery of the slide & thus this section is scanned at low power
  • slide is then scanned at high power for RBC's, WBC's, renal tubular epithelial cells, oval fat bodies, bacteria & crystals
  • observations are recorded as number of cells seen per high power field - normal RBC counts:
    • 2-3 RBC's/hpf in adult men & 2-4 RBC's/hpf in adult women.

casts:

  • are formed from urinary Tamm-Horsfall protein, a product of tubular epithelial cells that gels at low pH and high concentration & when mixed with albumin, RBC's, tubular cells, or cellular debris. The composition of a cast thus reflects the contents of the tubule.

casts are described according to their appearance & contents:

hyaline:

  • devoid of contents
  • seen with dehydration, exercise, or in association with glomerular proteinuria

red-cell casts:

  • indicate glomerular haematuria as seen in glomerulonephritis

white-cell casts:

  • imply the presence of renal parenchymal inflammation:
    • interstitial nephritis (if eosinophilic cell on staining then suggests allergic aetiology)
    • papillary necrosis
    • pyelonephritis

granular casts:

  • composed of cellular remnants & debris
  • occur in ATN

fatty casts:

  • like oval fat bodies, are generally assoc. with heavy proteinuria & the nephrotic syndrome, but have been noted to occur in a substantial proportion of pts with non-glomerular renal disease too.
  • if casts are broad (> 3WBC diameters wide) then this suggests chronic renal disease with enlargement of the still functioning nephrons.

crystals:

uric acid:

  • suggest uric acid nephropathy but are extremely non-specific

oxalic acid or hippuric acid:

  • may be seen in ethylene glycol ingestion
pa_urinalysis.txt · Last modified: 2014/01/02 05:03 by gary1