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Fasting Serum Immunoglobulin Subclass Study (IgG, IgA, IgM)

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

This test measures levels of three key antibodies—IgG, IgA, and IgM—in your blood after you’ve fasted for a set period. These antibodies help your immune system fight infections and protect against diseases.

Quick summary

["Understanding your antibody levels can help identify immune system health or deficiencies."]

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

Key facts at a glance

LOINC
97101-0
Short code
FSIG+US Study
Synonyms
FSIG+US Study
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
flexible-sigmoidoscopy-us
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 doctor may recommend this test if you have symptoms like frequent infections, unexplained fatigue, or autoimmune conditions. It can also be part of a broader evaluation for immune-related disorders or before certain treatments that affect immunity.

What to Expect Before the Test

You’ll need to fast (no food or drink except water) for 8–12 hours before the test. Bring your fasting schedule to your appointment. Bring a photo ID and any medications you’re currently taking.

How the Test Works

A blood sample will be drawn from a vein in your arm. The blood is processed to separate serum, where the antibody levels are measured using a specialized immunoassay technique. The results help determine if your immune response is functioning normally.

What the Results Mean

Abnormally low or high levels of IgG, IgA, or IgM may signal immune system issues, such as infections, allergies, or conditions like HIV, lupus, or certain cancers. Your doctor will interpret the results in the context of your health history.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

IgG/A/M Profile

Frequently asked questions

How long will I have to fast for this test?
You’ll need to fast for 8 to 12 hours before your blood draw, with only water allowed during this time.
What should I avoid before the test?
Avoid eating or drinking anything except water for the fasting period. Also, inform your doctor about any medications you’re taking, as some may affect your results.
Can I take my usual medications on the fasting day?
Yes, you can take your usual medications as long as you follow your doctor’s instructions. However, some medications may need to be adjusted, so always check with your healthcare provider.
  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