Summary
| Direction in Long COVID | ↑ Elevated — higher than in the comparison group |
|---|---|
| Category | Immune |
| Also described as | Anaphylatoxins |
| Compared against | HC (healthy controls) |
| Associated symptoms | Complement activation |
| Test type | Blood test (serum / plasma / whole blood) |
| Source | Baalbaki et al. 2025 — doi:10.1111/all.16526 |
| Condition atlas | Long COVID Biomarker Atlas |
Related Immune markers in Long COVID
- β2-microglobulin ↑
- T cell exhaustion markers ↑
- Anti-GPCR autoantibodies ↑
- Low naive T cells / High effector memory ↕
- Anti-IFN-ω autoantibodies ↑
- Anti-IFN-α autoantibodies ↑
Full list: Long COVID Biomarker Atlas.
Frequently asked questions
Is Complement C3a / C5a high or low in Long COVID?
Elevated. Studies summarised in the OSMF atlas report higher Complement C3a / C5a in Long COVID than in healthy controls. Source: Baalbaki et al. 2025.
What symptoms is Complement C3a / C5a associated with in Long COVID?
The cited literature associates this marker with Complement activation. These are population-level associations and do not mean the marker causes the symptoms.
Which test measures Complement C3a / C5a?
It is measured as a blood test (serum / plasma / whole blood). Many of the cited studies used research-grade assays, so clinical availability may vary.
Is one abnormal Complement C3a / C5a result diagnostic of Long COVID?
No. No single biomarker is diagnostic of Long COVID. Complement C3a / C5a findings come from group comparisons, overlap considerably between patients and controls, and vary with study design, timing and comparison group. A result should be interpreted by a clinician alongside symptoms, history and other tests.
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Last reviewed: 2026-06-29 · Page generated from long-covid.json · Browse: Long COVID atlas · Biomarker Index · All marker pages
Disclaimer: This page is an educational synthesis of published, peer-reviewed research and is not medical advice. Biomarker findings vary across studies with case definitions, comparison groups, timing and assay methods. Direction of change reflects the predominant finding in the cited source and does not establish a diagnostic threshold. Discuss any test result with a qualified clinician.