Question:
A 45-year-old man is found to have microscopic hematuria on urinalysis. Phase-contrast microscopy shows acanthocytes and RBC casts. Which of the following best describes the underlying pathology?
A 45-year-old man is found to have microscopic hematuria on urinalysis. Phase-contrast microscopy shows acanthocytes and RBC casts. Which of the following best describes the underlying pathology?
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Key differentials for urine microscopy:
Dysmorphic RBCs + RBC casts + Proteinuria = Glomerulonephritis (Glomerular source).
Isomorphic RBCs + Normal morphology = Urolithiasis, malignancy, or trauma (Extraglomerular source).
Dysmorphic RBCs + RBC casts + Proteinuria = Glomerulonephritis (Glomerular source).
Isomorphic RBCs + Normal morphology = Urolithiasis, malignancy, or trauma (Extraglomerular source).
Updated On: Sep 3, 2026
- Isomorphic RBCs - Glomerular disease
- Dysmorphic RBCs - Glomerular disease
- Isomorphic RBCs - Lower urinary tract disease
- Dysmorphic RBCs - Tubular disorder
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The Correct Option is B
Solution and Explanation
Concept:
The evaluation of asymptomatic microscopic hematuria requires distinguishing between a glomerular source of bleeding (e.g., glomerulonephritis) and an extraglomerular/lower urinary tract source (e.g., kidney stones, bladder cancer, or urinary tract infection).
Urine microscopy, specifically using phase-contrast, is a vital non-invasive tool to make this distinction based on red blood cell morphology.
Explanation:
• When red blood cells pass through the tight fenestrations of a damaged glomerular basement membrane and navigate the osmotic variations of the renal tubular system, they suffer severe mechanical and osmotic stress.
• This stress causes the RBCs to become distorted, resulting in "dysmorphic" red blood cells.
• Acanthocytes (also known as "Mickey Mouse" cells or G1 cells) are a highly specific subtype of dysmorphic RBCs characterized by ring-shaped bodies with one or more blebs or projections. The presence of acanthocytes ($>$ 5%) is virtually pathognomonic for glomerular hematuria.
• Red blood cell (RBC) casts form when these RBCs become entrapped within Tamm-Horsfall mucoprotein in the distal tubules. The presence of an RBC cast definitively proves that the bleeding originated from the nephron (specifically the glomerulus), rather than the bladder or urethra.
• Conversely, if bleeding occurs from the lower urinary tract (like a bleeding bladder tumor or a ureteric stone), the RBCs do not traverse the glomerulus or tubules. Therefore, they retain their normal biconcave shape and are termed "isomorphic" RBCs.
• Because the question specifies the presence of acanthocytes (dysmorphic cells) and RBC casts, the pathology is definitively a glomerular disease.
Final answer:
The combination of acanthocytes and RBC casts signifies Dysmorphic RBCs originating from Glomerular disease.
The evaluation of asymptomatic microscopic hematuria requires distinguishing between a glomerular source of bleeding (e.g., glomerulonephritis) and an extraglomerular/lower urinary tract source (e.g., kidney stones, bladder cancer, or urinary tract infection).
Urine microscopy, specifically using phase-contrast, is a vital non-invasive tool to make this distinction based on red blood cell morphology.
Explanation:
• When red blood cells pass through the tight fenestrations of a damaged glomerular basement membrane and navigate the osmotic variations of the renal tubular system, they suffer severe mechanical and osmotic stress.
• This stress causes the RBCs to become distorted, resulting in "dysmorphic" red blood cells.
• Acanthocytes (also known as "Mickey Mouse" cells or G1 cells) are a highly specific subtype of dysmorphic RBCs characterized by ring-shaped bodies with one or more blebs or projections. The presence of acanthocytes ($>$ 5%) is virtually pathognomonic for glomerular hematuria.
• Red blood cell (RBC) casts form when these RBCs become entrapped within Tamm-Horsfall mucoprotein in the distal tubules. The presence of an RBC cast definitively proves that the bleeding originated from the nephron (specifically the glomerulus), rather than the bladder or urethra.
• Conversely, if bleeding occurs from the lower urinary tract (like a bleeding bladder tumor or a ureteric stone), the RBCs do not traverse the glomerulus or tubules. Therefore, they retain their normal biconcave shape and are termed "isomorphic" RBCs.
• Because the question specifies the presence of acanthocytes (dysmorphic cells) and RBC casts, the pathology is definitively a glomerular disease.
Final answer:
The combination of acanthocytes and RBC casts signifies Dysmorphic RBCs originating from Glomerular disease.
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