Summary
| Direction in Long COVID | ↑ Elevated — higher than in the comparison group |
|---|---|
| Category | Proteomic |
| Also described as | C3, C4, factor B |
| Compared against | HC (healthy controls) |
| Associated symptoms | Complement-mediated inflammation |
| Test type | Blood test (serum / plasma / whole blood) |
| Source | Baalbaki et al. 2025 — doi:10.1111/all.16526 |
| Condition atlas | Long COVID Biomarker Atlas |
Trials, interventions and tests
In clinical-trial registrations this marker maps to the outcome term Brain-derived neurotrophic factor (BDNF) (2 registered trials use a matching outcome measure).
Clinical trials using Complement cascade proteins as an outcome
- NCT06171152: Study of Liraglutide (A Weight Loss Drug) in High Risk Obese Participants With Cognitive and Memory Issues (Suspended)
Outcome measure: Change from Baseline in Serum Brain Derived Neurotrophic Factor (BDNF) after 8 Weeks - NCT07278206: Brain Stimulation in Long COVID (Recruiting)
Outcome measure: Brain-derived neurotrophic factor (BDNF)
Related Proteomic markers in Long COVID
Full list: Long COVID Biomarker Atlas.
Frequently asked questions
Is Complement cascade proteins high or low in Long COVID?
Elevated. Studies summarised in the OSMF atlas report higher Complement cascade proteins in Long COVID than in healthy controls. Source: Baalbaki et al. 2025.
What symptoms is Complement cascade proteins associated with in Long COVID?
The cited literature associates this marker with Complement-mediated inflammation. These are population-level associations and do not mean the marker causes the symptoms.
Which test measures Complement cascade proteins?
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 cascade proteins result diagnostic of Long COVID?
No. No single biomarker is diagnostic of Long COVID. Complement cascade proteins 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.