A 45-year-old man with a 10-year history of hemolytic anemia presents with acute jaundice, dark urine, and severe anemia (hemoglobin 5.2 g/dL). Flow cytometry shows absence of CD55 and CD59 on blood cells. Which investigation is most specific for confirming paroxysmal nocturnal hemoglobinuria (PNH)?

Question

Accepted Answer

D. Flow cytometry with anti-CD55 and anti-CD59 antibodies. ## Diagnosis: Paroxysmal Nocturnal Hemoglobinuria (PNH)

The clinical presentation of chronic hemolytic anemia with thrombosis risk, combined with **absence of CD55 and CD59** (complement regulatory proteins), is pathognomonic for PNH.

### Why Flow Cytometry with Anti-CD55 and Anti-CD59 is Most Specific

**Key Point:** PNH results from a somatic mutation in the **PIGA gene**, which encodes phosphatidylinositol glycan anchor (PIG-A). This prevents synthesis of GPI-anchored proteins, including:
- **CD55** (decay-accelerating factor, DAF)
- **CD59** (membrane inhibitor of reactive lysis, MIRL)

**High-Yield:** Flow cytometry detecting **absence of CD55 and CD59** is the **gold standard** and most specific test for PNH because:
- It directly detects the molecular defect (GPI-anchor deficiency)
- Has **100% specificity** for PNH
- Quantifies the proportion of abnormal clones
- Can detect small populations (>1% of cells)
- Is **flow cytometric confirmation** of the diagnosis

**Clinical Pearl:** PNH is characterized by the triad of hemolytic anemia, thrombosis, and cytopenias. The absence of GPI-anchored proteins on blood cells is the pathologic hallmark.

### Investigation Comparison for PNH Diagnosis

| Investigation | Principle | Sensitivity | Specificity | Current Status |
| --- | --- | --- | --- | --- |
| **Flow cytometry (CD55/CD59)** | Detects GPI-anchor deficiency | >99% | 100% | **Gold standard** |
| Sucrose lysis test | Osmotic lysis in hypotonic sucrose | 80–90% | 95% | Obsolete; replaced by flow |
| Acidified serum lysis (Ham test) | Complement-mediated lysis in acidified serum | 70–80% | 90% | Historical; rarely used now |
| DAT with anti-C3d | Detects C3 on RBC surface | Moderate | Low | Non-specific; may be positive in PNH |

**Mnemonic:** **GPI = Gone (absent) in PNH** — loss of GPI-anchored proteins (CD55, CD59) is the defining feature.

Answer

A. Acidified serum lysis test (Ham test)

Answer

B. Direct antiglobulin test (DAT) with anti-C3d

Answer

C. Sucrose lysis test (sugar water test)

A 45-year-old man with a 10-year history of hemolytic anemia presents with acute jaundice, dark urine, and severe anemia (hemoglobin 5.2 g/dL). Flow cytometry shows absence of CD55 and CD59 on blood cells. Which investigation is most specific for confirming paroxysmal nocturnal hemoglobinuria (PNH)?

Explanation

Diagnosis: Paroxysmal Nocturnal Hemoglobinuria (PNH)

Why Flow Cytometry with Anti-CD55 and Anti-CD59 is Most Specific

Investigation Comparison for PNH Diagnosis

Practice similar questions

Join our NEET PG community

Investigation	Principle	Sensitivity	Specificity	Current Status
Flow cytometry (CD55/CD59)	Detects GPI-anchor deficiency	>99%	100%	Gold standard
Sucrose lysis test	Osmotic lysis in hypotonic sucrose	80–90%	95%	Obsolete; replaced by flow
Acidified serum lysis (Ham test)	Complement-mediated lysis in acidified serum	70–80%	90%	Historical; rarely used now
DAT with anti-C3d	Detects C3 on RBC surface	Moderate	Low	Non-specific; may be positive in PNH