The Diagnostic Journey and Essential Lab Tests

The road to a diagnosis for Complement Factor H (CFH)-related disease often begins in a moment of crisis. Because the symptoms of kidney failure and blood vessel damage can look similar across many different diseases, doctors must work quickly to rule out other common causes before they can confirm a rare complement-mediated condition ^[1][2].

This process is often described as a “diagnosis of exclusion,” meaning doctors must systematically evaluate other possibilities while running specialized tests to find the root cause ^[3].

The Urgent First Steps: Ruling Out Mimics

When a patient arrives with signs of Thrombotic Microangiopathy (TMA)—the tiny blood clots and red blood cell destruction seen in aHUS—several other conditions must be evaluated immediately because they require very different treatments ^[2].

1. TTP (Thrombotic Thrombocytopenic Purpura)

TTP is a life-threatening emergency caused by a deficiency in an enzyme called ADAMTS13.

• Immediate Specialist Management: Because suspected TTP requires immediate specialist-directed management (typically plasma exchange and corticosteroids), doctors will initiate treatment while the ADAMTS13 result is pending ^[2].
• The Pre-Treatment Sample: It is critical that this blood is drawn before you receive plasma, as donor plasma can “hide” the deficiency. A result below 10% strongly supports TTP, but is not the only factor in the emergency decision ^[3][4].

2. STEC-HUS (Shiga Toxin HUS)

This is the “typical” HUS caused by bacteria, like E. coli. Doctors test for this using a Shiga toxin test ^[5]. Identifying this is vital because STEC-HUS usually resolves with supportive care ^[6].

3. Secondary TMAs

The team must also evaluate for other important secondary TMA causes, including malignant hypertension, pregnancy/HELLP syndrome, certain drugs, autoimmune diseases, infection, and severe cobalamin deficiency, all of which require specific treatments ^[2].

Evaluating the “Brakes”: Complement Lab Tests

The diagnostic work-up for suspected CFH disease may include Factor H antigen and functional testing, Factor I, anti-Factor H antibodies, and other autoantibodies ^[7].

• C3 and C4: Low levels of C3 provide supportive evidence that your alternative pathway is active ^[8]. However, a normal C3 level does not rule out the disease.
• CH50 and AH50: These functional assays measure complement pathways. It is important to note that CH50 is expected to be intentionally suppressed during effective C5 inhibitor blockade ^[7].
• sC5b-9 (Soluble Membrane Attack Complex): Elevated levels suggest terminal complement activation, but it is nonspecific and not a reliable standalone measure of active tissue damage ^[7].
• Anti-CFH Autoantibodies: Checks for the acquired/autoimmune subtype ^[9].

Note: Specimen timing is vital, because plasma exchange, transfusions, and complement inhibitors can drastically alter these results.

The Role of Genetic Testing

Genetic testing provides the “blueprints” to see if you have an inherited susceptibility. However, these results take weeks, and time-critical treatment must not be delayed waiting for them ^[10].

• Beyond Sequencing: The test should include copy-number variant (CNV) and hybrid-gene analysis, interpreted by a specialist ^[11].
• Variants of Uncertain Significance (VUS): A VUS is a change that hasn’t been proven to cause disease. A VUS alone does not establish a diagnosis; it must be combined with your clinical symptoms ^[12].

The Kidney Biopsy: Diagnosing C3G

While a kidney biopsy is central for diagnosing C3 Glomerulopathy (C3G), it carries bleeding risks and is not required to diagnose every episode of complement-mediated aHUS ^[13].

1. Immunofluorescence (IF): Shows “dominant C3 staining” in C3G ^[13].
2. Light Microscopy: Shows the general pattern of inflammation ^[14].
3. Electron Microscopy (EM): Extremely high-powered imaging used to tell the difference between Dense Deposit Disease (DDD) and C3 Glomerulonephritis (C3GN) ^[15].

Diagnostic Tools Summary