DIP Episode 85 - USMLE Step 1 Hematology Review Part 6 (Final)

Source / episode info

**Episode:**85
**Title:**Divine Intervention Episode 85 – USMLE Step 1 Hematology Review Part 6 (Final)
**Published:**2019-03-10
Source:Episode page

Hereditary Spherocytosis: Characterized by red blood cells lacking central pallor due to membrane protein defects (Spectrin, Band 3, Ankyrin). Diagnosis involves osmotic fragility testing; MCHC is typically elevated.
Coombs Test Distinction: The Direct Coombs test detects antibodies already bound to the RBC surface (used for autoimmune hemolytic anemia); the Indirect Coombs test detects free antibodies in the serum that could bind to a donor's blood (screening).
Hemolysis Types: Intravascular hemolysis is associated with complement activation, leading to low haptoglobin and schistocytes; Extravascular hemolysis occurs primarily in macrophages of the reticuloendothelial system.
Paroxysmal Nocturnal Hemoglobinuria (PNH): An acquired disorder caused by a mutation in the {PIGA} gene, resulting in deficiency of GPI-anchored proteins ({CD}55 and {CD}59), leading to uncontrolled complement lysis.

Differentiate the pathophysiology and diagnostic tests for hereditary spherocytosis versus other hemolytic anemias.
Interpret the results of Direct vs. Indirect Coombs tests to determine the source and type of autoantibody.
Classify different types of hemolysis (intravascular vs. extravascular) based on laboratory findings (\text{haptoglobin}, schistocytes, bilirubin).
Understand the complement cascade deficiencies associated with PNH and its management.

Condition	Key Finding	Association	Board Exam Tip
Hereditary Spherocytosis	Red cells lacking central pallor; Elevated MCHC	Spectrin, Ankyrin, Band 3 defects	Think osmotic fragility test and pigment gallstones.
Paroxysmal Nocturnal Hemoglobinuria (PNH)	Thrombosis, Hemolysis, Low {CD}55/{CD}59	{PIGA} gene mutation; Complement deficiency	Treatment involves C5 blockade ({Eculizumab}).
Sickle Cell Disease (SCD)	Vaso-occlusion; Dactylitis in infants	-globin chain Glu -> Val substitution	Remember the severity order: {HbSS} is worst.
Coombs Test	Direct vs. Indirect testing	Autoimmune Hemolytic Anemia (AIHA)	Direct = on RBCs; Indirect = in serum.

Topic	Key Point	Context	Exam Relevance
Hereditary Spherocytosis	Membrane protein defects ({Spectrin}, {Ankyrin}) cause loss of membrane surface area.	Leads to spherocytes and increased MCHC; susceptible to osmotic lysis.	High yield for physical exam/blood smear questions.
Direct Coombs Test	Detects antibodies bound to the RBC membrane.	Used in Autoimmune Hemolytic Anemia (AIHA). Positive result confirms antibody binding.	Crucial distinction from Indirect test.
Intravascular Hemolysis	Occurs within the circulation; involves complement activation.	Characterized by low {haptoglobin} and schistocytes ({DIC}, PNH, {PTG}).	Low haptoglobin is a hallmark of intravascular destruction.
PNH Pathophysiology	Deficiency of GPI-anchored proteins ({CD}55, {CD}59) due to {PIGA} mutation.	Allows uncontrolled complement attack on RBCs, leading to hemolysis and thrombosis.	Remember the treatment is C5 blockade (Eculizumab).