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Serum or Plasma Opsonin Activity (Functional Complement Component Measurement)

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

This test measures how well your immune system’s complement proteins help white blood cells identify and destroy bacteria or other invaders in your blood. It’s used to check for immune system function, especially in conditions where the body struggles to fight infections.

Quick summary

["Assesses immune defense against bacterial infections by evaluating complement protein functionality."]

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

Key facts at a glance

LOINC
31970-7
Short code
S pyo Ag Ser Ql
Synonyms
S pyo Ag Ser Ql
Category
coagulation
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
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

What Is This Test Measuring?

The test evaluates the ability of your blood to activate complement proteins—a group of proteins that tag pathogens (like bacteria) for destruction by immune cells. This process is critical for clearing infections. The result helps determine if your immune system’s complement pathway is working as expected.

Why Might Your Doctor Order This Test?

Your doctor may recommend this test if you have frequent or severe infections, particularly bacterial ones, or if you have conditions like chronic granulomatous disease, HIV/AIDS, or certain autoimmune disorders that weaken immune function. It can also be part of a broader evaluation for unexplained immune deficiencies.

How Is This Test Performed?

A blood sample is drawn from a vein, usually in your arm. The sample is then tested in a lab to measure how effectively your complement proteins activate in response to specific bacterial components. The process may involve mixing the blood with standardized bacterial particles to simulate infection conditions.

What Does the Test Result Mean?

A lower-than-expected result suggests impaired complement function, which may indicate a weakened immune response to bacteria. A normal result means your complement system is functioning as intended. Your doctor will interpret results in the context of your overall health and symptoms.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Low opsonin activity can signal an underlying immune deficiency, increasing your risk of recurrent or severe bacterial infections. It may prompt further testing for conditions like complement deficiencies (e.g., C3 deficiency) or other immune disorders. Highly elevated results are less common but could indicate acute infection or certain inflammatory states. Always discuss results with your healthcare provider to tailor next steps to your needs.

Short name

Opsonin Function Test

Frequently asked questions

What causes low opsonin activity?
Low opsonin activity can result from genetic complement deficiencies, chronic infections, autoimmune diseases (such as lupus), or conditions that impair immune function, like HIV/AIDS or chemotherapy-related immunosuppression. It may also appear in people with certain medications that affect immune response.
Symptoms may include frequent or severe bacterial infections (e.g., pneumonia, skin infections, or urinary tract infections), slow wound healing, or recurrent sinus or ear infections. Some people may also experience fatigue or unexplained fevers without clear cause.
This test specifically measures the *functional* activity of complement proteins, not just their levels. While blood tests for individual complement proteins (like C3 or C4) can indicate deficiencies, this test assesses whether those proteins are working properly to tag and destroy pathogens. Both types of tests can complement each other in diagnosing immune dysfunction.
While lifestyle changes alone won’t directly improve opsonin activity, they can support overall immune health. Maintaining a balanced diet rich in vitamins (like C and D), staying hydrated, managing stress, and avoiding smoking can help your immune system function better. If you have an underlying condition, your doctor may recommend specific treatments to address the root cause of low complement function.
If your result is low, your doctor may recommend additional tests (such as genetic screening for complement deficiencies) or treatments to boost immune function. If your result is normal but you still have frequent infections, discuss other potential causes with your doctor. Always follow up to address concerns and explore next steps tailored to your health needs.
  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