MFF0017: Casts


Medical Fun Facts logo from Gary Lum

Now, what do I mean by casts? Do I mean the cast of a play or a movie? No, not at all.

I mean urinary casts. How do you get a cast on part of the urinary system? Haha, I know what you’re thinking but I’m not putting a cast on that.

No, urinary casts, are casts like plaster casts but a cast of the inside of tubules in the urinary system.

Casts are formed distal to the Loop of Henle in the kidney. I get a shudder whenever I think of nephrons. The physiology of the nephron bamboozled me in medical school.

Hyaline casts are formed from Tamm-Horsfall protein which is a mucoprotein secreted by tubule cells.

Things that enhance cast formation include:

  • Low flow rate,
  • High salt concentration, and
  • Low pH

Protein casts are long with thin tails and are called cylindroids.

Hyaline casts do not always mean pathology; they can be found in healthy people.

Erythrocytes or red blood corpuscles form red blood corpuscle casts and suggest glomerulonephritis or severe tubular damage.

Leucocyte or white blood cells form leucocyte casts and are found in acute pyelonephritis and sometimes in glomerulonephritis.

If cellular casts sit in a nephron for some time before they get flushed through, the cells may degenerate and become coarsely a granular cast, which later becomes a finely granular cast, and then, a waxy cast. Granular and waxy casts are thought to derive from renal tubular cell casts. Broad casts are thought to come from damaged and dilated tubules and are seen in the end-stages of chronic renal disease.

So when urine is examined microscopically not only are we looking for erythrocytes, leucocytes, parasites like Trichomonas and even cells like spermatozoa, we’re looking for crystals and casts. I’ll do crystals another time.

So when your doctor asks for a midstream urine to be collected we accept the referral to perform microscopy and culture and if we grow something, antimicrobial susceptibility testing.

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