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###1: Complement C4d [Mass/volume] in Plasma by immunoassay

Published: August 9, 2026Updated: August 15, 2026

###3: This blood test measures complement C4d, a fragment produced when the complement system is activated. It can help assess immune-system activity in certain inflammatory, autoimmune, kidney, or transplant-related conditions.

Quick summary

["###4: An elevated complement C4d level may indicate increased complement activation, but results must be interpreted with symptoms, other laboratory findings, and the clinical context."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 15, 2026Editorial policy

Key facts at a glance

LOINC
39565-7
Short code
C4d
Synonyms
C4d Plas IA-mCnc
Category
chemistry
Sample
plasma
Container
EDTA plasma tube (lavender-top) or citrate plasma tube (light blue-top); SST/serum separator tube acceptable for some assays
Units
N/A
Ref low
0.1
Ref high
0.4
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
Immunoassay
CPT
86161
SNOMED
259118002
Specimen volume
1 mL plasma (minimum 0.5 mL)
Stability
Plasma stable for 8 hours at room temperature; 2 days at 2–8°C; up to 3 months at -20°C or below. Avoid repeated freeze-thaw cycles.
Hemolysis max
50
Lipemia max
100
Icterus max
25
TAT
24–72 hours
Order
routine

###5: What the test measures

###6: Complement is part of the immune system. When complement C4 is activated, a fragment called C4d is released and can be detected in plasma. The C4d measurement is therefore used as an indicator of complement activation.

###7: Why the test is performed

###8: Clinicians may order this test when evaluating suspected immune-complex conditions, autoimmune disease, certain forms of kidney inflammation, vasculitis, or possible antibody-mediated transplant rejection. It is usually interpreted alongside other tests rather than by itself.

###9: How the test is performed

###10: A blood sample is collected, and the plasma portion is analyzed using an immunoassay. No special preparation is generally required unless the laboratory or your clinician gives specific instructions.

###11: Understanding the result

###11: Results are reported as the mass concentration of C4d in plasma. Higher values can suggest increased complement activation, while lower values may occur when complement is consumed; reference limits vary by laboratory and assay.

Reference range

Reference range established by individual laboratory; typically 0.1–0.4 mg/L in healthy adults. Values vary by assay platform and reference population.N/A

Specimen: plasma

No special patient preparation required. Fasting is not necessary. Avoid strenuous exercise prior to sample collection as it may transiently affect complement levels.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

###13: Elevated C4d can support evidence of active immune-complex or antibody-mediated inflammation, but it is not specific to one diagnosis. Low C4d may reflect complement consumption and is generally interpreted together with total complement levels, C4, symptoms, examination findings, and other laboratory results.

Short name

###2: Plasma complement C4d

Frequently asked questions

###14: What does an elevated complement C4d result mean?
###15: It may indicate that the complement system has been activated, which can occur with certain autoimmune, inflammatory, kidney, or transplant-rejection disorders. It does not identify a specific disease on its own.
###16: Is complement C4d the same as a total complement C4 test?
###17: No. C4d is a fragment released during complement activation, whereas a total complement C4 test measures the amount of intact C4. The two tests can provide different information and may be ordered together.
###18: Do medications or other conditions affect the result?
###19: Yes. Complement levels can be influenced by active inflammation, infection, autoimmune disease, kidney disease, pregnancy, and immunosuppressive or complement-directed treatments. Tell your clinician about relevant medications and conditions before interpreting the result.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 15, 2026. · Read our editorial policy