Can Prior Treatments Cause False-Negative Antibody Tests?

It is a common and stressful scenario: you or your loved one started treatment for suspected autoimmune encephalitis before the antibody test results came back, or testing was only done after treatments like steroids or plasma exchange had already begun. This leads to an important question: can these early treatments cause an antibody test to come back falsely negative?

The answer is that treatment can sometimes reduce the sensitivity of some tests, though a negative result after treatment is not automatically a false negative. Treatments that suppress or remove antibodies can reduce their levels, which may lead to a false-negative result, but the effect depends heavily on the specific treatment, the timing, and whether blood or spinal fluid was tested ^[1][2].

How Specific Treatments Affect Antibody Levels

Different treatments impact the immune system and antibody levels in different ways:

• Plasma Exchange (PLEX) and Immunoadsorption: These treatments involve filtering the blood. Their primary goal is to physically remove harmful circulating antibodies from your bloodstream ^[3][1]. Because they are highly effective at this job, blood samples (serum) drawn after these treatments are at a higher risk of testing negative. However, plasma exchange primarily affects the circulating blood, so its effect on the spinal fluid (CSF) is not equivalent.
• Intravenous Immunoglobulin (IVIG) and Corticosteroids: These treatments work to calm the immune system. Research shows that antibody levels can sometimes fall following these treatments, but the effects are highly variable and often not immediate ^[2][4]. Additionally, IVIG has complex effects—because it is made from donor antibodies, it can sometimes interfere with testing and even cause misleading positive results.

If you had testing done after these treatments, your neurologist and the testing laboratory will need to carefully review the exact treatment and collection dates to interpret the results accurately.

The Importance of Timing: Draw Before You Treat (If Safe)

Because immunotherapy can alter test results, medical guidelines strongly recommend that doctors collect both serum (the liquid part of the blood) and CSF (the fluid surrounding the brain and spinal cord) before starting any immune treatments, whenever it is safely possible ^[5].

However, suspected autoimmune encephalitis can require urgent hospital evaluation and treatment, and urgent care should never be dangerously delayed just to wait for an antibody test result or a lumbar puncture ^[6][7]. Under urgent specialist care, and after appropriate evaluation and treatment for potential infections (like the herpes simplex virus), many patients are appropriately started on initial (empiric) immune treatments before all testing is complete. You should not start, stop, or delay treatment based on an antibody result alone.

Other Reasons for False Negatives

Beyond prior treatment, several other factors can lead to a negative test result when autoimmune encephalitis is actually present:

• Testing Only Blood or Only Spinal Fluid: Some autoantibodies are easily found in the blood, while others are heavily concentrated in the spinal fluid ^[5]. For instance, NMDA receptor antibodies are generally more sensitive in CSF, while LGI1 antibodies might show up better in serum ^[8]. Testing both is essential to avoid missing the diagnosis ^[9].
• Disease Stage: Antibody levels naturally fluctuate depending on how long you have been sick ^[10].
• Assay Limitations: The standard laboratory methods used to detect a target (commercial cell-based assays) in many hospitals can sometimes miss antibodies ^[11]. If clinical suspicion remains high despite a negative test, doctors can consult with a laboratory experienced in autoimmune neurology. They may suggest a tissue-based assay, which is a broader screening method, though it is not automatically more accurate and usually requires confirmation ^[12].

Seronegative Autoimmune Encephalitis

Ultimately, a negative antibody test does not definitively rule out autoimmune encephalitis, nor does it mean your symptoms are imagined. Doctors diagnose “seronegative” (antibody-negative) autoimmune encephalitis by carefully applying clinical criteria. This involves evaluating your symptoms, MRI scans, EEG (brain wave) results, and signs of inflammation in the spinal fluid ^[13].

It is important to note that MRI, EEG, and CSF tests can sometimes be normal, so their absence does not automatically rule out the disease either. A seronegative diagnosis also strictly requires excluding other causes, such as infections, seizures, toxic-metabolic disease, and malignancy ^[13].