6 Microscopy of urine (CASTS)

Where are casts made

in the nephron (loop of henle, distal tubule, collecting duct)

Cast composition

• MAIN: tamm-horsfall protein (uromodulin)

  • Urinary stasis causes geling of uromodulin

• Other contents of glomerular filterate that get trapped in gel matrix

Uromodulin

Tamm-horsfall protein

• glycoprotein made by RTE at a constant rate

  • increased rate of production during stress or exercise → hyaline casts increase with stress/exercise

• acts as a lubricant

• mucus and cast component

• Not detected by protein reagent strip test

Cast size and shape

Reflect where cast was formed

• Broad casts may result from tubular distension or, in the case of extreme urine stasis, from formation in the collecting ducts.

Cylinduria

Casts in urine

Cast formation encouraging conditions

• urinary stasis/ decreased flow

• acidic pH

• Increased electrolytes (Na, Ca)

Cast counting

10X (lpf)

Hyaline Cast

• most commonly seen

• composed mostly of uromodulin

• normal 0-2/lpf

Hyaline cast: nonpathological cause

strenuous exercise (increased uromodulin production), dehydration (more stasis), heat exposure

Hyaline cast: pathological cause

acute glomerulonephritis, pyelonephritis, chronic renal disease, congestive heart failure

RBC cast

RBC inside cast

Accompanied by

• free floating RBC

• positive blood strip test

RBC cast: Pathological cause

Bleeding in nephron

RBC passing though glomerulus

• glomerulonephtitsis

RBC cast: nonpathological cause

strenuous exercise (rare)

Orange, red, brown RBC cast

RBC cast with hemoglobinuria

Brown RBC cast

RBC cast with hemoglobinuria. Hemoglobin oxidized to methemoglobin

Hemoglobin RBC casts are associated with __________

Acute tubular necrosis (large amounts of hemoglobinuria toxic to kidneys)

WBC cast

• mostly PMN inside cast

• granular

• easy to confuse with WBC clumps

• accompanied by free floating WBC

WBC cast cause

ALWAYS PATHOLOGICAL (inflammation or infection)

• pyelonephritis - upper UTI

• acute interstitial nephritis - kidney inflammation with not infection

• glomerular nephritis

________ does not produce WBC casts

cystitis (lower UTI)

Bacterial casts

• accompanied by WBCs and free floating bacteria

• rare

• looks like granular cast

• confirm with gram stain

• in pyelonephritis

Renal Tubule Epithelial casts

• ALWAYS PATHOLOGICAL (BUT RARE)

• tubular destruction, differentiate with WBC cast by looking at size of cell and nucleus (mononucleated)

Fatty (oval fat body) cast

• confirm with polarized light or fat stain

  • wont stain with sternheimer malbin

• Cause:

  • nephrotic syndrome

  • tubular necrosis

  • crush injury

Mixed cellular cast

cast with multiple cell types

RBC and WBC mixed cellular cast

Common in glomerulonephtitis

WBC and RTE mixed cellular cast

Common in pyelonephritis

Granular cast

• degradation of material in cast

  • Crystal, RBC, WBC, RTE

• Begins as coarse and becomes fine during urinary stasis

Waxy cast

• further breakdown of granular cast with urinary stasis → chronic renal failure

• usually seen with other casts

• stains dark pink with usually squared ends

Broad casts

• aka RENAL FAILURE CAST

• 2-6x wider than regular casts

• can be any type but granular and waxy most common

Broad cast cause (2)

• dilation or destruction of distal convoluted tubule

• formation in collecting duct from SEVERE urinary stasis