A 28-year-old woman of Indian origin presents with fatigue, jaundice, and dark urine. Hemoglobin is 9.2 g/dL with elevated reticulocyte count (6.5%), elevated indirect bilirubin (3.8 mg/dL), and elevated LDH (580 U/L). Peripheral blood smear shows spherocytes and polychromasia. Which investigation is most appropriate to confirm the diagnosis of hereditary spherocytosis?

## Clinical Context The patient presents with hemolytic anemia (elevated reticulocyte count, elevated indirect bilirubin, elevated LDH, jaundice) with **spherocytes on peripheral smear**. This morphologic finding, combined with hemolysis, strongly suggests **hereditary spherocytosis (HS)**. The osmotic fragility test is the gold standard confirmatory test for HS. ## Why Osmotic Fragility Test is Correct **Key Point:** Hereditary spherocytosis is characterized by spherical RBCs with reduced surface area-to-volume ratio and increased membrane osmotic fragility. The osmotic fragility test is the **gold standard and most specific test** for confirming HS. **High-Yield:** The osmotic fragility test principle: - RBCs are exposed to hypotonic solutions of decreasing osmolarity - Normal RBCs swell and lyse at osmolarity ~0.36% NaCl - **Spherocytes lyse at higher osmolarity** (e.g., 0.48% NaCl) because they are already maximally spherical and cannot swell further - **Incubated osmotic fragility test** (RBCs incubated at 37°C for 24 hours) is even more sensitive, as it increases the osmotic stress **Mnemonic:** **SOFF** — Spherocytes Osmotic Fragility is Fast (i.e., they lyse at higher osmolarity). **Clinical Pearl:** The osmotic fragility test is **specific for HS** and can differentiate it from other causes of hemolytic anemia with spherocytes (e.g., autoimmune hemolytic anemia, which would have a positive DAT). ## Diagnostic Algorithm for Hemolytic Anemia with Spherocytes ```mermaid flowchart TD A[Hemolytic Anemia + Spherocytes]:::outcome --> B{DAT Positive?}:::decision B -->|Yes| C[Autoimmune Hemolytic Anemia]:::outcome B -->|No| D[Hereditary Spherocytosis likely]:::outcome D --> E[Osmotic Fragility Test]:::action E --> F{Increased Fragility?}:::decision F -->|Yes| G[HS Confirmed]:::outcome F -->|No| H[Consider other causes]:::outcome ``` ## Comparison of Investigations in Hemolytic Anemia | Investigation | Purpose | HS Finding | AIHA Finding | |---|---|---|---| | **Osmotic Fragility Test** | Assess RBC membrane deformability | **Increased fragility** | Normal fragility | | **DAT (Coombs)** | Detect RBC-bound antibodies | **Negative** | Positive | | **Flow cytometry** | Detect complement/antibody deficiency | Normal (not indicated) | Not indicated | | **Hemoglobin electrophoresis** | Identify hemoglobinopathies | Normal | Normal | ## Why DAT Must Be Negative The negative DAT (Coombs test) is crucial to rule out **autoimmune hemolytic anemia (AIHA)**, which can also present with spherocytes and hemolysis. Once DAT is negative, osmotic fragility test becomes the confirmatory test for HS.