A 28-year-old male presents with recurrent meningococcal infections despite appropriate antibiotic therapy. Complement deficiency is suspected. To confirm a deficiency in complement-fixing antibodies and assess the functional integrity of the classical complement pathway, which investigation is most appropriate?

Explanation

## Investigation of Choice for Assessing Classical Complement Pathway Function **Key Point:** The **Total Hemolytic Complement (CH50) assay** is the gold-standard functional test for the classical complement pathway. It measures the ability of patient serum to lyse antibody-coated sheep red blood cells (RBCs), integrating all components of the cascade. ### Why CH50 Assay? 1. **Functional assessment:** CH50 measures the integrated activity of C1, C2, C3, C4, C5, C6, C7, C8, and C9 — a single deficiency anywhere in the cascade will reduce CH50. 2. **Detects complement-fixing antibodies:** IgG and IgM antibodies bind to antigen-coated RBCs and activate C1q, initiating the classical pathway. A low CH50 indicates defective complement activation. 3. **Clinical relevance:** Recurrent meningococcal infections are classically associated with terminal complement deficiencies (C5–C9) or early component deficiencies (C1, C2, C4). ### Comparison of Complement Investigations | Investigation | Measures | Detects | Clinical Use | |---|---|---|---| | CH50 (Total hemolytic complement) | Functional activity of classical pathway | Any deficiency in C1–C9 | **Screening for complement deficiency** | | C3 and C4 levels (radial immunodiffusion) | Protein concentration only | Quantitative deficiency; consumption | Identifies which component is low | | IgG subclass quantification | Antibody protein levels | IgG1, IgG2, IgG3, IgG4 separately | Assesses opsonizing capacity | | Antigen-specific Ab titers (ELISA) | Antibody response to specific antigens | Functional antibody production | Vaccine response assessment | **High-Yield:** A **low or absent CH50 with normal C3/C4 levels** suggests early complement deficiency (C1, C2, C4) or rare deficiencies. **Low CH50 + low C3/C4** suggests consumption or deficiency of these abundant components. **Mnemonic:** **CH50 = Complete Hemolytic 50** — the dilution of serum that causes 50% lysis of antibody-coated RBCs. It is a **functional assay**, not a protein quantification. **Clinical Pearl:** Meningococcal infections are a hallmark of **terminal complement deficiency (C5–C9 deficiency)** — these patients have normal CH50 at diagnosis but develop recurrent Neisseria meningitidis. CH50 screening followed by specific component assays (C5, C6, C7, C8, C9) confirms the diagnosis.