Complement C4 optimal range and reference range

Compiled by Alina Radu, founder of Sera, from the published sources below · Last reviewed November 9, 2026

Sera's bands for Complement C4:

  • Optimal 15 – 40 mg/dL (0.15 – 0.4 g/L); sufficient 10 – 40 mg/dL (0.1 – 0.4 g/L); out of range below 10 or above 40 mg/dL.

Evidence grade: Published reference interval. A band that primarily reflects a published adult laboratory or population reference interval. It is not necessarily applicable to every assay or laboratory.

All units

UnitOptimalSufficientOut of range
mg/dL15 – 4010 – 40below 10 or above 40
g/L0.15 – 0.40.1 – 0.4below 0.1 or above 0.4

What the evidence says

Falls earlier than C3 in classical-pathway activation, so an isolated low C4 points at lupus, cryoglobulinaemia or hereditary angioedema. Partial C4 gene deletions are common, giving a persistently low level in a healthy person, so a stable low C4 is not necessarily disease. The band, 10 to 40 mg/dL (0.10 to 0.40 g/L), is a UK NHS laboratory reference interval; Mayo and Labcorp set the lower limit at 14 and 12 mg/dL, so a result of 10 to 13 may be printed as low on the report, and the report's interval takes precedence. The narrower optimal band is a functional target; no published evidence supports it.

Sources

  • Complement (C3, C4 Levels) — Gloucestershire Hospitals NHS Foundation Trust, 2026 · Laboratory / reference-interval sourceUK laboratory interval for C4 of 0.10 to 0.40 g/L (updated January 2026): Sera's sufficient lower edge of 10 mg/dL and the proposed upper edge of 40. The page adds that a very low C4 in isolation can go with C1 inhibitor deficiency.
  • Complement C4, Serum — Mayo Clinic Laboratories, 2026 · Laboratory / reference-interval sourceReference interval 14 to 40 mg/dL. Confirms 40 as the upper limit and shows a stricter lower limit of 14. An undetectable C4 with a normal C3 suggests congenital C4 deficiency.
  • Complement C4, Serum — Labcorp, 2026 · Laboratory / reference-interval sourceReference interval over 17 years: 12 to 38 mg/dL for men and women. A third lower limit (12) between 10 and 14, and an upper limit below 45.

What Complement C4 measures

An early component of the classical complement pathway, ordered alongside C3.

Falls earlier than C3 in classical-pathway activation, so an isolated low C4 with a normal C3 points that way — lupus, cryoglobulinaemia, or hereditary angioedema. Partial C4 gene deletions are common and give a persistently low level in a healthy person, so a stable low C4 is not necessarily disease.

Related autoimmune markers

See Complement C4 in the full list of biomarker ranges

About the author

Alina Radu is the founder of Sera and a software engineer. The idea started with her own Hashimoto's tests, scattered across years without the big picture. She is not a clinician: these ranges are compiled from the published sources linked on each page, with every source named. Alina Radu on LinkedIn

This page is a reference for practitioners, not medical advice. Sera's bands are practitioner targets for structured review, not diagnostic thresholds, and Sera does not diagnose or recommend treatment. Read a result alongside the interval printed by the issuing laboratory and the client's history and symptoms. Range set: Sera Production, version 1.0, last reviewed November 9, 2026.