Casts are formed in the collecting tubules under conditions of extreme renal stasis. Waxy casts form from the degeneration of granular casts.

Broad, waxy casts are two to six times the width of ordinary casts and appear waxy and granular. Casts may vary in size as disease distorts the tubular structure (they get wider because they are a mold of the tubules). Also, as urine flow from the tubules becomes compromised, casts are more likely to form. The finding of broad, waxy casts suggests a serious prognosis—hence, the term renal failure casts.

Fatty casts are formed from the attachment of fat droplets and degenerating oval fat bodies to a protein matrix. Fatty casts are highly refractile and contain yellow-brown fat droplets, or oval fat bodies.

Negative (not seen)

1. Observe the Procedure for Microscopic Urine Examination on pages xxx through xxx of this chapter.

2. Examine urine sediment microscopically under low power.

1. Broad and waxy casts occur in:

   1. Severe kidney failure

   2. Tubular inflammation and degeneration (nephrotic syndrome)

   3. Localized nephron obstruction (extreme stasis of urine flow)

   4. Malignant hypertension

   5. Amyloidosis

   6. Diabetic nephropathy

   7. Kidney allograft rejection

2. Fatty casts are found in:

   1. Disorders causing lipiduria, such as nephrotic syndrome and lipoid nephrosis

   2. Chronic glomerulonephritis

   3. Kimmelstiel–Wilson syndrome

   4. SLE

   5. Nephrotoxicity

Pretest Patient Care

1. Explain purpose of test, procedure for random urine sample collection, and interfering factors.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Recent exposure to radiographic contrast may result in precipitation of crystals.