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Complement Factor C3 in Serum (Quantitative)

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

This test measures the amount of C3 protein in your blood, which is part of the immune system’s complement pathway. It helps identify issues with your body’s ability to fight infections or inflammation.

Quick summary

["C3 protein levels can signal immune system problems or chronic conditions."]

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

Key facts at a glance

LOINC
69029-7
Short code
LPT TT max/CD3 Bld FC-NFr
Synonyms
LPT TT max/CD3 Bld FC-NFr
Category
chemistry
Sample
whole-blood
Container
Lavender-top (EDTA)
Units
N/A
Ref low
0.8
Ref high
1.2
Critical range
— – 5 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
flow-cytometry
CPT
85610
SNOMED
396252006
Specimen volume
3 mL
Stability
Room temperature: 4h, Refrigerated: 24h
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

What Does This Test Measure?

C3 is a key protein in your immune system that helps tag harmful substances (like bacteria) for destruction. Low levels may indicate conditions like immune deficiencies, chronic infections, or certain autoimmune diseases. High levels could be linked to inflammation or certain medical treatments.

Why Might You Need This Test?

Your healthcare provider may order this test if you have symptoms like frequent infections, unexplained fatigue, or signs of inflammation. It’s also used to monitor conditions like lupus, kidney disease, or after treatments that affect the immune system.

What to Expect Before the Test

No special preparation is needed. You’ll have a small needle inserted into your arm, and blood will be drawn into a tube. It usually takes a few minutes, and you can return to normal activities afterward.

What to Expect During the Test

You’ll sit comfortably while the phlebotomist cleans the area and inserts a needle. Blood will drain into a tube, and you may feel a slight pinch. The test is quick, and the site will be covered with a bandage afterward.

Reference range

INR: 0.8-1.2 (warfarin: 2.0-3.0)N/A

Specimen: whole-blood

Inform your healthcare provider if you are taking blood thinners. No special preparation required.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

C3 Protein Level

Frequently asked questions

How long does it take to get my results?
Results are typically available within 1 to 3 business days, depending on your lab. Your doctor will contact you with the final report.
What does it mean if my C3 level is low?
A low C3 level may mean your immune system isn’t working as well as it should, which could increase your risk of infections. Your doctor may recommend further testing or treatments based on your symptoms and overall health.
Can I eat or drink before this test?
You can eat and drink normally before this test, as no special diet or fasting is required.
  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