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Glycogen storage disease type III (Forbes disease) – Glycogen branching enzyme activity (serum/plasma)

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

This test measures the activity of an enzyme involved in breaking down glycogen, a stored form of glucose in your body. It helps diagnose glycogen storage diseases, including Forbes disease.

Quick summary

["This test helps identify rare metabolic disorders affecting how your body stores and uses energy."]

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

Key facts at a glance

LOINC
82402-9
Short code
GD3 Gangl IgG Ser Ql Line blot
Synonyms
GD3 Gangl IgG Ser Ql Line blot
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
line-blot
CPT
82784
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 this test if you have symptoms of glycogen storage disease, such as muscle weakness, growth delays, or liver enlargement. It’s also used in genetic testing for families with a history of the condition.

How the Test Works

A small blood sample is taken, and the enzyme activity is measured using a specialized laboratory technique. The test checks how well the enzyme functions in breaking down glycogen.

What to Expect Before the Test

No special preparation is needed, but you may be asked to avoid certain medications that could affect results. If you’re taking insulin or other glucose-lowering drugs, let your provider know.

Possible Results and Interpretation

Results are compared to a reference range for healthy individuals. Low enzyme activity strongly suggests a glycogen storage disease, while normal levels rule it out. Exact values may require further genetic testing.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Low enzyme activity in this test confirms a diagnosis of glycogen storage disease type III, which can cause severe complications like liver disease, muscle weakness, and hypoglycemia. Early diagnosis allows for better management and treatment options.

Short name

Glycogen branching enzyme activity

Frequently asked questions

What causes abnormal results in this test?
Abnormal results are almost always due to glycogen storage disease type III (Forbes disease). Rarely, other metabolic or genetic conditions may affect enzyme activity, so further testing is needed.
How long does it take to get results?
Results are typically available within 1–3 business days, depending on lab processing times. Your provider will discuss the findings with you.
Can diet or medications affect this test?
Yes, certain medications (like insulin) or dietary changes that alter blood glucose levels may influence enzyme activity. Your provider will guide you on avoiding potential disruptions.
  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