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PubMed This is a summary of 22 peer-reviewed journal articles Updated
Neurology · Autoimmune Encephalitis

The Complete Workup and Excluding Mimics

At a Glance

When autoimmune encephalitis antibody tests are negative, diagnosis depends on the clinical picture plus blood and spinal-fluid testing, MRI, EEG, and sometimes FDG-PET. Doctors must also exclude infections, tumors, seizures, psychiatric disorders, and metabolic problems.

Diagnosing autoimmune encephalitis (AE) when antibody tests are negative is a process of pattern recognition and exclusion. Because there is no single “yes/no” test, your doctors must perform a comprehensive “workup”—a battery of tests designed to find evidence of brain inflammation and to rule out every other possible cause [1][2].

Testing Blood and Spinal Fluid

To evaluate a “seronegative” status, your medical team will typically test both your serum (blood) and your CSF (cerebrospinal fluid) [3]. Paired testing is preferred when feasible because different antibodies are more detectable in different specimens [4][5]. Ideally, samples should be collected before starting immunotherapy, as treatment can affect sensitivity.

It is also vital that the lab uses different methods:

  • Cell-Based Assays (CBA): These are “targeted” tests. The lab checks your samples against a specific “menu” of known antibodies.
  • Tissue-Based Assays (TBA): These are “broad” screens. The lab applies your sample to actual brain tissue to see if an immune reaction occurs. While these can be helpful, they are screening tools that may show nonspecific reactivity and do not by themselves prove AE [6][7].

Tools for Seeing Inflammation

Since the antibodies are missing, doctors look for “objective” signs that the immune system is active in the brain. Each tool has limitations, and a normal MRI or CSF does not completely exclude AE.

MRI: The Structural Map

A brain MRI looks for physical changes. In AE, doctors specifically look for T2 or FLAIR hyperintensities—areas that appear bright on the scan [8][9]. However, these changes are nonspecific and can result from seizure activity, infection, tumor, or vascular disease. Up to 66% of children and some adults with AE have a completely normal initial MRI [10][11].

CSF Analysis: The Chemical Signature

A lumbar puncture (spinal tap) checks for signs of an active immune response:

  • Pleocytosis: An abnormally high number of white blood cells in the spinal fluid [12].
  • Oligoclonal Bands (OCBs): Specific proteins that show the immune system is producing IgG in the central nervous system. However, OCBs are not specific to AE and can occur in infections or multiple sclerosis [13][14].

EEG: The Electrical Record

An EEG measures brain waves. While it cannot diagnose AE on its own, it is essential for finding “hidden” seizures or non-convulsive status epilepticus [15][16].

FDG-PET: The Metabolic View

If the MRI is normal, an FDG-PET scan is a specialized adjunct a specialist might use. It looks at how the brain uses energy, sometimes showing patterns like frontal hypometabolism or medial temporal hypermetabolism, though these patterns are not universal [10][12].

Excluding the “Mimics”

Before a clinical diagnosis can be made, doctors must rule out conditions that look identical [1]. These “mimics” include:

  • Infections: Viruses like Herpes Simplex (HSV) or Varicella-Zoster (VZV) [17]. An early negative PCR may not completely exclude infection, and doctors may start empiric antiviral treatment while results are pending [18].
  • Psychiatric Disorders: Conditions like schizophrenia or bipolar disorder [19].
  • Malignancy: Brain tumors or “paraneoplastic” syndromes caused by a hidden cancer. Cancer screening is guided by age, symptoms, and risk factors, rather than a mandatory scan for everyone [20][21].
  • Metabolic Issues: Severe vitamin deficiencies or organ failure [19].

Urgent treatment of suspected AE or infection should not be delayed simply to complete a checklist; testing requires individualized expert interpretation [22].

Common questions in this guide

Can autoimmune encephalitis be diagnosed if antibody tests are negative?
Yes. A negative blood or spinal-fluid antibody test does not by itself rule out autoimmune encephalitis. Doctors combine the clinical pattern with MRI, spinal-fluid, EEG, and sometimes PET findings while excluding infections, tumors, seizures, and other mimics.
Why are both blood and spinal fluid tested for autoimmune encephalitis?
Antibodies may be easier to detect in blood or spinal fluid depending on the antibody, so paired samples provide a more complete evaluation. When possible, samples are collected before immunotherapy because treatment can reduce test sensitivity.
What does a normal brain MRI mean when autoimmune encephalitis is suspected?
A normal initial MRI does not completely exclude autoimmune encephalitis because some children and adults have no visible changes. Doctors may use spinal-fluid analysis, EEG, clinical findings, and sometimes FDG-PET to look for additional evidence.
What do T2 or FLAIR bright spots mean on an MRI?
T2 or FLAIR hyperintensities are bright areas on an MRI that can indicate a brain abnormality, but they are not specific for autoimmune encephalitis. Seizures, infection, tumors, or vascular disease can cause similar findings, so doctors interpret them with the rest of the evaluation.
What infections need to be ruled out before autoimmune encephalitis is diagnosed?
Doctors commonly evaluate for infections such as herpes simplex virus and varicella-zoster virus using spinal-fluid testing. An early negative PCR may not fully exclude infection, so testing may be repeated or antiviral treatment started while results are pending.
What can an EEG show when autoimmune encephalitis is suspected?
An EEG records electrical brain activity and can detect hidden seizures or non-convulsive status epilepticus. It cannot diagnose autoimmune encephalitis by itself, but it can identify a time-sensitive alternative or complication.
When might an FDG-PET scan be considered?
If the brain MRI is normal, a specialist may consider FDG-PET as an additional test to assess how the brain uses energy. PET may show patterns such as frontal hypometabolism or medial temporal hypermetabolism, but these findings are not universal and do not prove autoimmune encephalitis.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Were both my blood (serum) and spinal fluid (CSF) tested for antibodies, and ideally before immunotherapy began?
  2. 2.Does my MRI show any 'T2 or FLAIR hyperintensities' in the temporal lobes or other cortical regions, and what else could cause them?
  3. 3.Since my MRI was normal, should we consider an FDG-PET scan to look for metabolic changes in my brain?
  4. 4.What specific infections, like HSV or VZV, were ruled out during my spinal fluid analysis, and do any tests need repeating?
  5. 5.Can you explain how you ruled out other 'mimics,' such as primary psychiatric disorders or non-inflammatory epilepsy?
  6. 6.Based on my EEG, is there any evidence of 'non-convulsive' seizures or status epilepticus?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

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This page explains the evaluation of antibody-negative autoimmune encephalitis for informational purposes only and does not constitute medical advice. Discuss your test results and urgent treatment decisions with your neurologist and other treating specialists.

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