Acetylcholine Receptor Antibody Test

The acetylcholine receptor (AChR) antibody test is a blood test used to help diagnose myasthenia gravis, an autoimmune disease that causes muscle weakness. These antibodies attack the connection between nerves and muscles.

What the Test Measures

At the neuromuscular junction, the nerve releases acetylcholine (ACh) which binds to receptors on the muscle, triggering contraction. In myasthenia gravis, antibodies interfere with this process.

Types of AChR Antibodies

Binding antibodies

• Most commonly measured
• Bind directly to ACh receptors

Blocking antibodies

• Prevent acetylcholine from binding
• Less commonly ordered

Modulating antibodies

• Cause receptors to be internalized
• May be positive when binding antibodies are negative

When This Test Is Ordered

• Suspected myasthenia gravis
• Ocular myasthenia
• Unexplained ptosis (drooping eyelid)
• Unexplained double vision
• Fatigable weakness

Interpreting Results

Positive Result

• Strongly supports diagnosis of myasthenia gravis
• Higher levels may (but don't always) correlate with severity
• Does not correlate well with disease course

Negative Result

• Does not rule out myasthenia gravis
• About 15% of generalized MG patients are "seronegative"
• About 50% of ocular myasthenia patients are negative
• Consider other tests if clinical suspicion high

Sensitivity by Disease Type

Type of Myasthenia	AChR Antibody Positive
Generalized MG	~85%
Ocular MG	~50%
MuSK-positive MG	Usually negative
Seronegative MG	Negative by definition

Related Tests

Anti-MuSK Antibodies

• Muscle-specific kinase antibodies
• Found in some AChR-negative patients
• Different clinical features
• About 40% of "seronegative" MG

Anti-LRP4 Antibodies

• Less commonly available
• May be positive in some seronegative cases

Other Myasthenia Tests

• Ice test
• Repetitive nerve stimulation
• Single-fiber EMG
• See Autoantibody Panels

Test Details

Preparation

• No special preparation needed
• Fasting not required

Sample

• Blood draw from vein
• Sent to specialized laboratory

Timing

• Results typically in 1-2 weeks
• May be longer depending on laboratory

False Positives

Rarely, positive results can occur in:

• Patients with thymoma (without MG)
• Some autoimmune conditions
• Family members of MG patients (low levels)

Clinical Correlation

A positive test supports the diagnosis, but:

• Clinical picture is most important
• Antibody level doesn't guide treatment
• Serial levels not typically useful for monitoring
• Some patients improve but remain antibody positive

What Happens If Positive

If AChR antibodies are detected:

• Confirms myasthenia gravis diagnosis
• CT chest to look for thymoma
• Pulmonary function testing
• Discussion of treatment options
• See Myasthenia Gravis