A 28-year-old woman with systemic lupus erythematosus (SLE) on long-term corticosteroids presents with fatigue, dyspnea, and jaundice. Laboratory findings show hemoglobin 6.8 g/dL, elevated indirect bilirubin, elevated LDH, and low haptoglobin. Peripheral blood smear shows schistocytes. She denies fava bean or drug exposure. Which investigation best identifies a secondary cause of hemolysis in this patient?

Explanation

## Clinical Scenario Analysis This patient has **autoimmune hemolytic anemia (AIHA)** secondary to SLE, NOT a primary pentose phosphate pathway disorder. The key diagnostic clue is the clinical context: SLE, corticosteroid use, and absence of oxidative triggers. ## Why Pentose Phosphate Pathway Testing Is Inappropriate **Key Point:** G6PD deficiency and other pentose phosphate pathway enzyme deficiencies cause hemolysis triggered by **oxidative stress** (fava beans, sulfonamides, antimalarials). This patient has no such trigger and has evidence of **immune-mediated hemolysis**. ### Distinguishing Features | Feature | G6PD Deficiency | AIHA (SLE-related) | |---|---|---| | **Trigger** | Oxidative agents (fava, drugs) | Autoimmune IgG/IgM antibodies | | **Clinical context** | Acute episodic hemolysis | Chronic, insidious onset | | **Peripheral smear** | Heinz bodies, bite cells | Spherocytes, schistocytes | | **Direct Coombs test** | **Negative** | **Positive** | | **Haptoglobin** | Low | Low | | **Reticulocyte response** | Brisk | Brisk | **High-Yield:** The **Direct Antiglobulin Test (DAT/Coombs)** is the gold standard for diagnosing immune-mediated hemolysis. It detects IgG, IgM, or complement bound to RBC surface. ## Correct Investigation Pathway ```mermaid flowchart TD A[Hemolytic Anemia]:::outcome --> B{Oxidative trigger<br/>present?}:::decision B -->|Yes| C[Check G6PD activity]:::action B -->|No| D{Immune-mediated<br/>suspected?}:::decision D -->|Yes| E[Direct Coombs Test]:::action D -->|No| F[Membrane/enzyme defects]:::action E -->|Positive| G[AIHA confirmed<br/>Treat underlying cause]:::outcome C -->|Deficient| H[G6PD deficiency<br/>Avoid triggers]:::outcome ``` **Clinical Pearl:** SLE is a well-known cause of AIHA; corticosteroids are the first-line treatment. The absence of an oxidative trigger and the presence of schistocytes (suggesting mechanical hemolysis or immune injury) rule out pentose phosphate pathway deficiency.