A 28-year-old man of North Indian descent presents to the emergency department with acute hemolytic anemia (hemoglobin 7.2 g/dL, reticulocyte count 12%), jaundice, and dark urine after taking sulfonamide antibiotics for a urinary tract infection. Peripheral blood smear shows Heinz bodies. He reports a family history of similar episodes triggered by infections and certain drugs. Which investigation is most specific for confirming the underlying glycolytic enzyme deficiency?

Explanation

## Clinical Presentation Analysis This patient presents with: - **Acute hemolytic anemia** triggered by **sulfonamide antibiotic** exposure - **Heinz bodies** on peripheral smear (denatured hemoglobin precipitates from oxidative stress) - **Dark urine** (hemoglobinuria) - **Family history** of similar episodes triggered by infections and drugs - **North Indian descent** (G6PD deficiency is prevalent in this population) This is a **classic presentation of G6PD deficiency**. ## Why G6PD Enzyme Assay? **Key Point:** Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the **hexose monophosphate (pentose phosphate) shunt**, which generates NADPH. NADPH is essential for maintaining glutathione in its reduced form, protecting RBCs from oxidative damage. **Pathophysiology of G6PD Deficiency:** - G6PD deficiency → ↓ NADPH → ↓ reduced glutathione → inability to neutralize oxidative stress - Oxidative agents (sulfonamides, dapsone, primaquine, infections) → hemoglobin oxidation → **Heinz body formation** → intravascular hemolysis - Episodes are **episodic/acute**, with patients being completely asymptomatic between attacks **High-Yield:** The **G6PD enzyme assay** (spectrophotometric measurement of G6PD activity in RBCs) is the **gold standard confirmatory test** for G6PD deficiency. It directly quantifies enzyme activity and is the most specific investigation for this condition. **Clinical Pearl:** During an acute hemolytic crisis, G6PD levels may appear falsely normal because older, G6PD-deficient RBCs have been destroyed and the remaining reticulocytes (which are younger and have higher G6PD activity) predominate. Therefore, the assay should ideally be repeated **4–6 weeks after the acute episode** for definitive diagnosis. (Harrison's Principles of Internal Medicine, 21e, Ch. 164) ## G6PD vs. PK Deficiency — Key Differentiators | Feature | G6PD Deficiency | PK Deficiency | |---|---|---| | **Pathway** | Pentose phosphate shunt | Glycolysis (Embden-Meyerhof) | | **Trigger** | Oxidative drugs, infections | Infections (not typically drugs) | | **Baseline hemolysis** | Absent (silent between attacks) | Present (chronic, compensated) | | **Heinz bodies** | **Classic finding** | Rare | | **Splenomegaly** | Absent | Often present | | **Neonatal jaundice** | Can occur | Common | | **Confirmatory test** | **G6PD enzyme assay** | PK enzyme assay | ## Why Not the Other Options? | Investigation | Why Not Optimal | |---|---| | **Osmotic fragility + DAT** | Used for hereditary spherocytosis and immune hemolytic anemia, respectively. Both are normal in G6PD deficiency. | | **Hemoglobin electrophoresis/HPLC** | Identifies hemoglobinopathies (sickle cell, thalassemia). Hemoglobin structure is normal in G6PD deficiency. | | **Pyruvate kinase (PK) enzyme assay** | PK deficiency causes **chronic non-spherocytic hemolytic anemia** with splenomegaly and neonatal jaundice. It is NOT typically triggered by sulfonamides, and Heinz bodies are not a classic feature. This presentation does not fit PK deficiency. | **Mnemonic:** **G6PD = "Gone 6 Phosphate Dehydrogenase"** — triggered by oxidative stress → Heinz bodies → hemolysis → dark urine. [cite: Harrison 21e Ch 164; Robbins & Cotran Pathologic Basis of Disease 10e Ch 14; KD Tripathi Essentials of Medical Pharmacology 8e]