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Diagnostic Tests: Decay-Accelerating Factor (DAF) / CD55–CD59 Assay
Overview
The CD55/CD59 assay—also known as the decay-accelerating factor (DAF) assay—is a laboratory test used to evaluate the presence of GPI-anchored protective proteins on the surface of red blood cells. In paroxysmal nocturnal hemoglobinuria (PNH), a mutation in the PIGA gene prevents the attachment of these proteins, particularly CD55 and CD59, leaving RBCs vulnerable to complement-mediated lysis. Measuring the presence or absence of CD55/CD59 via flow cytometry allows clinicians to directly identify PNH cells.
When This Is the Most Accurate Diagnostic Test
The CD55/CD59 flow cytometry assay is the most accurate and definitive test for diagnosing paroxysmal nocturnal hemoglobinuria. Unlike older screening tests such as the sucrose lysis test or Ham’s test, flow cytometry precisely quantifies the proportion of blood cells lacking complement-regulating proteins. Because it identifies the exact molecular defect, it remains the gold standard for diagnosing PNH.
When to Choose DAF (CD55/CD59) as the Correct Answer
You should select the CD55/CD59 assay when a clinical presentation strongly suggests PNH, especially in cases involving:
Whenever these features cluster together, particularly dark morning urine and thrombosis, the DAF (CD55/CD59) assay is the diagnostic test of choice.
Overview
The CD55/CD59 assay—also known as the decay-accelerating factor (DAF) assay—is a laboratory test used to evaluate the presence of GPI-anchored protective proteins on the surface of red blood cells. In paroxysmal nocturnal hemoglobinuria (PNH), a mutation in the PIGA gene prevents the attachment of these proteins, particularly CD55 and CD59, leaving RBCs vulnerable to complement-mediated lysis. Measuring the presence or absence of CD55/CD59 via flow cytometry allows clinicians to directly identify PNH cells.
When This Is the Most Accurate Diagnostic Test
The CD55/CD59 flow cytometry assay is the most accurate and definitive test for diagnosing paroxysmal nocturnal hemoglobinuria. Unlike older screening tests such as the sucrose lysis test or Ham’s test, flow cytometry precisely quantifies the proportion of blood cells lacking complement-regulating proteins. Because it identifies the exact molecular defect, it remains the gold standard for diagnosing PNH.
When to Choose DAF (CD55/CD59) as the Correct Answer
You should select the CD55/CD59 assay when a clinical presentation strongly suggests PNH, especially in cases involving:
- Intermittent dark urine, often most noticeable in the morning due to overnight complement activation
- Pancytopenia, reflecting bone marrow failure that often accompanies PNH
- Unexplained large-vessel thrombosis, particularly in unusual locations such as hepatic, portal, or cerebral veins
- Hemolysis with a negative Coombs test, consistent with complement-mediated destruction rather than autoimmune hemolysis
Whenever these features cluster together, particularly dark morning urine and thrombosis, the DAF (CD55/CD59) assay is the diagnostic test of choice.
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