The Diagnostic Process: EMG and Antibodies

Because LEMS is so rare, diagnosing it is like assembling a complex puzzle. No single test—not even a positive antibody test—is enough to confirm the diagnosis on its own ^[1]. Instead, your medical team must carefully correlate three things: your physical symptoms, the electrical “signals” in your muscles, and the presence of specific markers in your blood ^[2][3].

The Electrical Evidence: EMG and RNS

The most definitive way to identify LEMS is through Electrodiagnostic Testing, which includes Electromyography (EMG) and Repetitive Nerve Stimulation (RNS). This test measures how well your nerves communicate with your muscles.

In LEMS, the communication “switch” is stuck. The test typically looks for three specific electrical patterns:

1. Low Resting Signal: When your muscle is at rest, the initial electrical response (called the Compound Muscle Action Potential or CMAP) is usually very small or “low” ^[4][5].
2. The Drop-Off: When the nerve is stimulated slowly (2–5 times per second), the signal may get even weaker, a pattern called decrement ^[6].
3. The “Big Jump” (Increment): This is the hallmark of LEMS. After you exercise the muscle for 10 seconds—or when the nerve is stimulated very rapidly—the muscle signal suddenly “wakes up” and grows much larger. A classic diagnostic criterion is an increase (increment) of greater than 100%. However, many modern protocols consider an increment of 60% or more to be strongly supportive of LEMS, depending on the baseline CMAP and the specific muscle tested ^[7][8].

It is important to know that an increment alone does not “confidently diagnose” LEMS; it must be interpreted alongside your clinical pattern and antibody results.

Blood Tests: Searching for Antibodies

Your doctor will order blood work to look for antibodies, which are the “attackers” in your immune system. Laboratory reports may use concentration units and specific reference ranges rather than a conventional “titer,” so you should review the actual value with your clinician.

• P/Q-type VGCC Antibodies: These are found in approximately 85% to 90% of people with LEMS ^[9][2]. They are highly specific, meaning they aren’t usually found in healthy people. However, a minority of people with LEMS are seronegative, meaning they have the disease but don’t have these specific antibodies in their blood ^[10][11].
• SOX1 Antibodies (Adjunctive Test): This is an optional, specialized test used to help estimate the risk of an underlying cancer. A positive result is strongly associated with an underlying small-cell lung cancer (SCLC) and should intensify the cancer search. However, a negative result does not rule out SCLC and does not replace the need for chest imaging ^[9][12].

Why Context Matters

Having a “positive” antibody result does not always mean you have LEMS. Low-level or moderate elevations of VGCC antibodies can sometimes appear in other neurological conditions or even without any clear disease at all ^[1][3].

Conversely, if your antibodies are negative but your EMG shows a classic low baseline CMAP with a massive increment after exercise, and you have classic leg weakness, your doctor can still diagnose seronegative LEMS ^[11][13].

Your Diagnostic Completeness Checklist

When reviewing your test results with your doctor, ensure your report includes these three key required elements:

• [ ] Resting CMAP Amplitudes: Were they recorded as “low” before any exercise? ^[4]
• [ ] Percentage of Increment: Did the signal increase significantly (e.g., >60% or >100%) after 10 seconds of exercise or high-frequency stimulation? ^[8]
• [ ] P/Q-type VGCC Antibodies: Is the result positive based on the lab’s reference range? ^[14]