Page 231 - Medicine and Surgery

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Chapter 6: Clinical 227

Urine microscopy Table 6.2 Causes of proteinuria
This is performed on fresh mid-stream urine.
Cause Examples
White blood cells: >10 WBCs per high power field

(HPF)onuncentrifugedurineisabnormal.Causesin- Physiological (up to Fever
300mg/24h)
clude cystitis, tubulointerstitial nephritis and calculi.
Exercise
Red blood cells: >1 RBC per high power field is Orthostatic proteinuria

abnormal. Red cells from the lower urinary tract look Renal Glomerular disease
like normal, round RBCs seen on a blood film. If they Amyloidosis
are of variable size and shape, with blebs, budding or Pyelonephritis
Acute tubular necrosis
as though a ‘bite’ has been taken out of an edge, they
Lower urinary tract Cystitis
are dysmorphic RBCs, which are a sign of glomerular Obstructive uropathy
disease. Extra-renal causes Diabetes mellitus
Bacteria: Visible bacteria may be due to contamina-
(most of these cause Pre-eclampsia
tion of the specimen, or a urinary tract infection. some degree of Hypertension
intra-renal damage) Congestive cardiac failure
Whether treatment is needed depends on the number
Myeloma (or other cause of
of WBCs per HPF present. increased plasma protein)
Crystals: Uric acid crystals and calcium oxalate or cal-

ciumphosphatecrystalsarecommonlyseeninnormal
urine. Other types may signify an underlying disease. Aetiology
Casts: These are cylinders formed in the renal tubules
Causes of proteinuria include those shown in Table 6.2.
from Tamm–Horsfall protein, which is normally se-
cretedbytubularepithelium.Inglomerularortubular Pathophysiology
disease, cells in the urine become incorporated into The glomeruli normally filter 7–10 g of protein per
the casts. Red cell casts are diagnostic of glomerular 24 hours, but less than 2% of this is actually excreted
disease. White cell casts occur in tubulointerstitial because protein is actively reabsorbed in the proxi-
disease and pyelonephritis. Other sorts of casts such mal tubules. Normal urinary protein excretion is <150
as granular or epithelial cell casts exist. mg/24h,ofwhichlessthan35mgisalbumin.Proteinuria
may occur by various mechanisms.
1 Overflow: Increased plasma protein exceeding tubu-
Urinary electrolytes
lar resorptive capacity, such as occurs in multiple
Comparing urinary and serum sodium concentration
myeloma.
is useful in the assessment of fluid balance. In hypona-
2 Glomerular proteinuria is due to increased permeabil-
traemia, a low urinary sodium is physiological, whereas
ity of the glomerular basement membrane. Glomeru-
a high urinary sodium suggests renal failure. In a patient
lar proteinuria may range from mild to heavy. Heavy
with a normal serum sodium, a low urinary sodium in-
proteinuria (>3 g/day) is termed nephrotic range
dicates salt-and-water depletion (dehydration). Urinary
proteinuria which indicates glomerular pathology.
sodium is also useful in differentiating types of acute re-
Nephrotic range proteinuria with hypoalbuminaemia
nal failure. Following abdominal or pelvic surgery, it can
and oedema is termed nephrotic syndrome.
be useful to measure urea and creatinine concentrations
3 Tubular disease causes impaired reabsorption of pro-
in fluid from drains or aspirated from a collection. If
tein. Urinary β -microglobulin can be used as a mea-
2
these are similar to the urine urea and creatinine con-
sure of tubular function, because this small peptide
centrations, this indicates a urinary leak.
is completely filtered by the glomeruli and completely
reabsorbed by normal tubules. The proteinuria is usu-
ally mild in tubular disease, such as in acute tubular
Proteinuria
necrosis or pyelonephritis.
Definition 4 Increased secretion of protein (Tamm–Horsfall pro-
Agreater than normal amount of protein in the urine. tein) by the kidneys or uroepithelium, which normally