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?

Question

Accepted Answer

D. Osmotic fragility test. ## 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.

Answer

A. Hemoglobin electrophoresis

Answer

B. Flow cytometry for CD55 and CD59 deficiency

Answer

C. Direct antiglobulin test (Coombs test)

Explanation

Clinical Context

Why Osmotic Fragility Test is Correct

Diagnostic Algorithm for Hemolytic Anemia with Spherocytes

Comparison of Investigations in Hemolytic Anemia

Why DAT Must Be Negative

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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