Overview
This lecture covers the types, pathophysiology, and disease associations of urinary casts, focusing on high-yield facts for medical exams.
Urinary Casts Overview
- Urinary casts are cylindrical structures formed in the distal convoluted tubule and collecting duct of the kidney.
- All casts are made on a base of Tamm-Horsfall protein (uromodulin), which is produced in the ascending loop of Henle.
- Casts form when urinary stasis increases osmolality and decreases pH, causing proteins and cellular material to coagulate.
Types of Urinary Casts & Associations
Hyaline Casts
- Most common cast; almost entirely Tamm-Horsfall protein.
- Seen in healthy individuals, especially with dehydration or after exercise.
- Appears clear and cylindrical under the microscope.
Granular Casts
- Result from breakdown of cellular casts.
- Seen in various renal diseases, but specific disease associations are less important for exams.
Muddy Brown Casts
- Subtype of granular cast; dark, muddy appearance.
- Highly associated with acute tubular necrosis (ATN).
Fatty Casts
- High Tamm-Horsfall protein with fat droplets (oval fat bodies, Maltese cross on polarized light).
- Specific for nephrotic syndrome.
Waxy Casts
- Late-stage degeneration of granular casts.
- Indicative of long-standing or end-stage renal disease.
- Appears homogeneous with sharp, broken-off edges.
Red Blood Cell (RBC) Casts
- Indicate glomerular nephritis or vasculitis.
- Associated with diseases like lupus nephritis, post-streptococcal glomerulonephritis, and Goodpasture’s syndrome.
White Blood Cell (WBC) Casts
- Suggest intrarenal inflammation.
- Associated with pyelonephritis and interstitial nephritis.
Epithelial Cell Casts
- Clumps or sheets of tubular epithelial cells.
- Seen in acute tubular necrosis (especially from toxic or ischemic injury), and interstitial nephritis.
Key Terms & Definitions
- Urinary Cast — Cylindrical structures formed in renal tubules from protein and/or cells, visible in urine.
- Tamm-Horsfall protein (Uromodulin) — Main protein component of urinary casts, produced in the nephron.
- Muddy Brown Cast — Pigmented granular cast, pathognomonic for acute tubular necrosis.
- Oval Fat Body — Lipid-laden cell seen in fatty casts, associated with nephrotic syndrome.
- Maltese Cross — Shape seen under polarized light in fatty casts.
Action Items / Next Steps
- Memorize key cast-disease associations (especially muddy brown = ATN, fatty = nephrotic, waxy = CKD).
- Review the characteristics and disease links for each type of cast.
- Practice exam questions on urinary casts to reinforce recognition and associations.