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Complement Component 4 (C4) Serum

Published: August 9, 2026Updated: August 16, 2026

This test measures the level of C4, a protein in your blood that is part of the complement system, which helps your immune system fight infections and inflammation.

Quick summary

["C4 helps identify immune system function and potential inflammatory conditions."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
57949-0
Short code
Complement C4c SerPl-mCnc
Synonyms
Complement C4c SerPl-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
86160
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your healthcare provider may order a C4 test to check for conditions affecting your immune system, such as lupus, autoimmune diseases, or infections. It can also help monitor treatment progress for these conditions.

What to Expect Before the Test

No special preparation is usually needed. You may have a small needle inserted into your arm to draw blood, which is then sent to a lab for testing.

What the Results Mean

Lower than expected levels of C4 might suggest an overactive immune response or certain autoimmune diseases. Higher levels could indicate conditions like chronic inflammation or certain infections.

Common Conditions Monitored

This test is often used to diagnose or monitor conditions like systemic lupus erythematosus (SLE), rheumatoid arthritis, or other autoimmune disorders where the immune system attacks healthy tissues.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

C4 levels can be low in autoimmune diseases where the immune system mistakenly attacks the body’s own tissues. It may also be reduced in conditions causing chronic inflammation or certain infections. Elevated levels can sometimes be seen in chronic inflammatory states or during pregnancy.

Short name

C4 Blood Test

Frequently asked questions

What does a low C4 level mean?
A low C4 level may indicate an autoimmune condition, such as lupus or another immune-related disorder, or could be due to an infection. Your doctor will consider your full medical history and other test results to interpret the result.
How often should I get this test?
Your doctor will determine how often you need this test based on your health condition, treatment plan, and symptoms. Regular monitoring may be needed if you have an ongoing autoimmune or inflammatory disease.
Can diet or lifestyle affect C4 levels?
Diet and lifestyle factors like stress, diet, or exercise generally do not directly affect C4 levels. However, managing conditions like autoimmune diseases through lifestyle changes may indirectly support overall health.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

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 16, 2026. · Read our editorial policy