Can a complement activation‑related pseudo‑allergy (CARPA) reaction to an infusion act as a trigger for a complement‑mediated disorder, thereby supporting the presence of an underlying complement disease in a patient with an otherwise elusive diagnosis?

Can CARPA Trigger Complement-Mediated Disease?

A CARPA reaction alone does not trigger or cause an underlying complement disorder, but it may unmask or precipitate clinical manifestations in a patient with pre-existing complement dysregulation. The key distinction is that CARPA represents acute complement activation by an external agent (the infused drug), whereas complement-mediated diseases like atypical hemolytic uremic syndrome (aHUS) involve chronic dysregulation of complement regulation.

Understanding the Relationship

CARPA as an Acute Event

CARPA is a hypersensitivity reaction caused by direct complement activation from nanoparticles, liposomes, monoclonal antibodies, and other infused agents 1, 2, 3. This represents an acute, transient complement activation triggered by the drug itself—not an underlying disease of complement regulation. The complement system activates normally in response to the foreign material, generating anaphylatoxins (C3a, C5a) that cause mast cell degranulation and the clinical symptoms of infusion reactions 4, 3.

Complement-Mediated Diseases Involve Chronic Dysregulation

In contrast, true complement disorders like aHUS involve genetic or acquired defects in complement regulatory proteins (CFH, CFI, CD46, C3, etc.) or the presence of autoantibodies against these regulators 5. These patients have baseline complement dysregulation that makes them vulnerable to thrombotic microangiopathy when exposed to triggering events.

Could CARPA Act as a Trigger in Pre-existing Disease?

Yes, but only if the patient already has underlying complement dysregulation. The 2025 Italian consensus on aHUS explicitly states that various factors can trigger aHUS episodes, including medications, infections, pregnancy, and malignancies 5. The guideline notes that "infections may act as a triggering background factor in some predisposed individuals" 5.

The Critical Distinction:

• In a normal patient: CARPA causes transient complement activation → symptoms resolve → no lasting complement disease
• In a patient with complement dysregulation: CARPA causes complement activation → may trigger aHUS episode in someone with pre-existing genetic/acquired complement defects → ongoing thrombotic microangiopathy

Diagnostic Implications for Your Patient

The occurrence of CARPA reactions does NOT, by itself, confirm complement disease. However, if your patient has:

1. Recurrent or severe CARPA reactions to multiple different agents
2. Clinical features suggesting TMA (hemolytic anemia, thrombocytopenia, renal involvement) following infusions
3. Persistent symptoms beyond the acute infusion period
4. No identified genetic mutations in standard complement testing (remember: 40-50% of aHUS patients have no identifiable mutation) 5

Then the CARPA reaction could represent a triggering event that unmasked underlying complement dysregulation rather than the cause of a complement disorder.

Recommended Diagnostic Approach

Evaluate for Complement-Mediated TMA:

• First-level tests 5: Complete blood count, peripheral smear for schistocytes, LDH, haptoglobin, indirect bilirubin, direct Coombs test (should be negative), creatinine, urinalysis for hematuria/proteinuria
• Urgent ADAMTS13 activity to exclude TTP (should be >10%) 5
• Stool culture for STEC if diarrhea present 5

If TMA criteria are met:

• Second-level complement testing 5: C3, C4, CH50, AP50, genetic testing for CFH, CFI, CD46, C3, CFB, THBD, DGKE, and anti-CFH antibodies
• Important caveat: The absence of identified mutations does not exclude aHUS, as complement dysregulation can occur without detectable genetic abnormalities 5

Monitor for TMA features after infusions:

• New or worsening anemia with schistocytes
• Thrombocytopenia
• Rising creatinine or new proteinuria/hematuria
• These would suggest the infusion triggered a TMA episode rather than isolated CARPA

Key Clinical Pitfalls

Do not assume CARPA reactions equal complement disease. CARPA is extremely common with certain drug formulations (liposomal drugs, monoclonal antibodies) and occurs in patients with normal complement regulation 1, 2. The vast majority of patients experiencing CARPA do not have underlying complement disorders.

Do not dismiss complement disease because genetic testing is negative. More than 40% of aHUS patients have no identifiable mutation 5. The diagnosis relies on clinical and laboratory criteria with exclusion of other TMAs, not solely on genetic confirmation 5.

Look for persistence beyond the acute event. True complement-mediated disease will show ongoing laboratory abnormalities (hemolysis, thrombocytopenia, renal dysfunction) that persist or worsen after the infusion, whereas isolated CARPA typically resolves within hours to days 3.

Bottom Line for Your Patient

A CARPA reaction is supportive evidence of complement disease only if:

• Clinical TMA features develop during or after the reaction
• Laboratory criteria for TMA are met (hemolysis, thrombocytopenia, renal involvement)
• Other causes of TMA are excluded (TTP, STEC-HUS, secondary causes)
• Symptoms persist beyond the acute infusion period

The CARPA reaction itself does not create a complement disorder but could serve as the triggering event that precipitates clinical disease in someone with pre-existing (possibly undetected) complement dysregulation 5. Without evidence of TMA, recurrent CARPA reactions more likely reflect the patient's response to specific drug formulations rather than underlying complement disease.