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Diagnostic Tests: Sucrose Lysis Test
Overview — What This Test Is
The sucrose lysis test is a screening test for paroxysmal nocturnal hemoglobinuria (PNH).
In this test, the patient’s red blood cells are placed in a sucrose-containing solution, which promotes complement activation. PNH RBCs are unusually sensitive to complement-mediated destruction, so the presence of hemolysis in this solution suggests PNH.
The mechanism:
When to Answer “Sucrose Lysis Test”
Choose the sucrose lysis test in a clinical scenario that describes the classic features of PNH, such as:
When these features appear together, the sucrose lysis test is the classic screening test.
Most Accurate Diagnostic Test
The most accurate and modern diagnostic test for PNH is:
Flow Cytometry for CD55 and CD59 Deficiency
Flow cytometry is the gold standard because it directly demonstrates the absence of these protective markers on RBCs and WBCs.
Overview — What This Test Is
The sucrose lysis test is a screening test for paroxysmal nocturnal hemoglobinuria (PNH).
In this test, the patient’s red blood cells are placed in a sucrose-containing solution, which promotes complement activation. PNH RBCs are unusually sensitive to complement-mediated destruction, so the presence of hemolysis in this solution suggests PNH.
The mechanism:
- PNH cells lack protective surface proteins (CD55, CD59) that normally prevent complement-mediated lysis.
- Sucrose enhances complement binding → RBC destruction → hemolysis detected.
When to Answer “Sucrose Lysis Test”
Choose the sucrose lysis test in a clinical scenario that describes the classic features of PNH, such as:
- Dark urine in the morning (from overnight hemoglobinuria)
- Episodic hemolysis
- Pancytopenia
- Venous thrombosis, especially in unusual sites such as hepatic, portal, or cerebral veins
- Negative Coombs test (helps distinguish PNH from autoimmune hemolysis)
When these features appear together, the sucrose lysis test is the classic screening test.
Most Accurate Diagnostic Test
The most accurate and modern diagnostic test for PNH is:
Flow Cytometry for CD55 and CD59 Deficiency
- These proteins are also called decay-accelerating factor (DAF) and membrane inhibitor of reactive lysis (MIRL).
- PNH cells are missing CD55 and CD59 due to a mutation in the PIGA gene affecting the GPI anchor that normally attaches these protective proteins to the cell surface.
Flow cytometry is the gold standard because it directly demonstrates the absence of these protective markers on RBCs and WBCs.
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