This isn't a one-test answer. The diagnosis of complement deficiencies relies on a strategic, two-tiered approach. Functional assays like CH50 and AH50 are the broad screening tools that tell you if a pathway is broken, while specific component immunoassays are the precise tools that tell you exactly where the break is.
The core diagnostic logic is simple: A functional assay is your screening net, catching both complete absences and non-functional but present proteins. Once you know which pathway is compromised, you use quantitative immunoassays to pinpoint the specific missing or depleted protein. You cannot use one without the other for a definitive diagnosis.
Understanding the Two-Tiered Diagnostic Strategy
The evaluation of the complement system is fundamentally an investigation of a proteolytic cascade. A defect anywhere in the chain can shut down the entire pathway. The diagnostic workflow must account for two very different types of defects: a missing protein or a protein that is present but functionally dead.
The Role of Functional Screening: CH50 and AH50
Functional assays are the critical first line of questioning because they answer the most important question: "Does the pathway work?"
A CH50 assay measures the functional integrity of the entire classical pathway, from C1 through C9. It's a bioassay that asks, "Can your serum lyse 50% of these antibody-coated red blood cells?" A low or absent result signals a blockage somewhere in that cascade.
An AH50 assay performs the same role but for the alternative pathway. It uses a different activating surface and a specialized buffer containing EGTA and magnesium. The EGTA chelates calcium, which the classical pathway C1 complex requires, effectively silencing it. This isolates the alternative pathway from Factor B through C9.
How Specific Component Immunoassays Pinpoint the Defect
If a functional screen is abnormal, you know that there's a problem but not what the problem is. A man might be dead because his heart is missing or because his heart is present but completely non-functional. The functional assay shows you the dead man; the immunoassay shows you the missing heart.
Quantitative immunoassays, using methods like nephelometry, turbidimetry, or ELISA, use high-affinity antibodies to measure the precise serum concentration of a single protein, such as C3 or C4. This is essential for differentiating a consumption problem from a genetic deficiency. Low C4 on an immunoassay confirms the protein is physically missing, validating and explaining the low CH50 result.
The Diagnostic Logic: Interpreting the Results
The real power of this system lies in the patterns the two functional tests create together. The comparison acts as a built-in differential diagnosis before any specific immunoassay is even run.
Pattern 1: Isolated Classical Pathway Defect (Low CH50, Normal AH50)
This pattern immediately isolates the defect to components unique to the classical pathway's activation arm, before it converges with the alternative pathway.
The proteins responsible here are early classical components that form the C3 convertase (C4b2a). The suspects are C1q, C1r, C1s, C4, and C2.
A follow-up quantitative immunoassay for C4 is run first. A normal C4 level in the face of an undetectable CH50 strongly suggests a C1 or C2 deficiency, which can then be confirmed with specific assays.
Pattern 2: Isolated Alternative Pathway Defect (Normal CH50, Low AH50)
A normal CH50 proves the classical and terminal pathways are intact. The functional defect is therefore isolated to the components unique to the alternative pathway's amplification loop.
This points directly to alternative pathway-specific factors, primarily Factor B, Factor D, and Properdin. Quantitative immunoassays targeting these specific proteins are used next to identify which one is deficient.
Pattern 3: Terminal Pathway or Common Component Defect (Low CH50, Low AH50)
When both screening assays are depressed, the problem lies in a component both pathways share, or in the terminal lytic machinery itself.
This most commonly indicates a deficiency in C3, the central convergence point, or components of the membrane attack complex (C5, C6, C7, C8, C9). It can also signal a massive systemic consumption, such as in a severe autoimmune flare. Specific immunoassays for C3 and C4 are critical here to distinguish consumption (low C3, low C4) from an isolated terminal pathway deficiency (normal C3, normal C4, low CH50/AH50).
Understanding the Trade-offs
There's no tension here, but a hard dependency. A common pitfall is to rely on component quantification alone.
If you only measure C3 and C4 antigenic levels, you will completely miss a patient with a normal level of a non-functional C3 protein. Their CH50 would be zero, their immunoassay normal, and their deficiency invisible. Functional assays detect non-functional proteins; antigenic assays do not. The screening bioassay is non-negotiable as the first step.
The opposite pitfall is stopping at the screening level. An undetectable CH50 tells you the cascade is broken but doesn't tell you if it's a lifelong genetic absence of C2 or a fleeting consumption of C4. Quantification provides the context and the definitive molecular identity.
Making the Right Choice for Your Diagnostic Goal
The application of these reagents is entirely dependent on whether you are building a broad screen or solving a specific clinical puzzle.
- If your primary focus is initial screening for immunodeficiency: Your panel must begin with both a CH50 and an AH50 functional assay. Without functional data, you are blind to one of the most critical types of defects.
- If your primary focus is monitoring autoimmune disease activity like lupus: Antigenic quantification of C3 and C4 via nephelometry or turbidimetry is paramount. Falling levels correlate with immune complex consumption and disease flares, providing a direct, quantitative biomarker.
- If your primary focus is confirming a specific, genetically suspected deficiency: Reflex testing with an immunoassay for that single protein (e.g., an ELISA for C2 or Factor H) is the definitive endpoint, but it should always be placed in the context of the functional pathway result.
Every definitive complement deficiency diagnosis stands on two pillars: a functional pathway must be broken, and a specific component must be identified as the cause.
Summary Table:
| CH50 Assay | AH50 Assay | Suspected Pathway / Defect | Recommended Follow-Up Immunoassays |
|---|---|---|---|
| Low / Absent | Normal | Classical Pathway (C1q, C1r, C1s, C4, C2) | Quantitative C4, C2, C1-INH |
| Normal | Low / Absent | Alternative Pathway (Factor B, Factor D, Properdin) | Quantitative Factor B, Factor D, Properdin |
| Low / Absent | Low / Absent | Central C3, Terminal Components (C5–C9), or Consumption | Quantitative C3, C4, C5–C9, Factor H/I |
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