Diagnostic Criteria and Clinical Evaluation

When standard lab tests do not find a specific “smoking gun” antibody, doctors do not simply guess at a diagnosis. Instead, they follow a rigorous set of international standards known as the Graus criteria ^[1]. Published in 2016, these guidelines allow neurologists to diagnose and treat autoimmune encephalitis (AE) based on a pattern of symptoms and objective evidence, even when the antibody panel comes back empty ^[2].

How the Immune System Affects the Brain

Under normal circumstances, your brain is protected by the blood-brain barrier (BBB), which selectively regulates immune traffic into the central nervous system ^[3]. In seronegative AE, this regulation is disrupted. While we are still learning exactly why this happens, a proposed model suggests that T-cells (white blood cells) may cross the barrier and infiltrate the supratentorial regions—causing inflammation and tissue swelling (edema) ^[4][5]. This T-cell explanation is a hypothesis for some cases, not an established universal mechanism for every antibody-negative patient ^[6].

Level 1: Possible Autoimmune Encephalitis

The first step in the diagnostic process is determining if a patient has possible autoimmune encephalitis. This category is used to quickly identify people who might need urgent care. To meet this level, all of the following must be true ^[1]:

1. Subacute Onset: Symptoms must progress rapidly, reaching their peak in less than three months ^[1].
2. Core Symptoms: You must experience working-memory deficits, altered mental status, or sudden psychiatric symptoms ^[1].
3. Supportive Evidence: You must have at least one objective finding: a new focal CNS finding, an unexplained new seizure, elevated white blood cells in the CSF (pleocytosis), or an MRI with features suggestive of encephalitis ^[1][7].
4. Exclusion: Reasonable exclusion of alternative causes.

Level 2: Probable Antibody-Negative AE

For patients who test negative for antibodies, doctors look for probable antibody-negative autoimmune encephalitis ^[2]. This is a much stricter category. To reach this level, doctors must ensure the following ^[1][2]:

• Absence of Antibodies: Well-substantiated absence of well-characterized neural antibodies in both serum and CSF ^[2][8].
• Objective Markers: The patient must have at least two of the following supportive findings: MRI abnormalities suggestive of AE, CSF pleocytosis, or CSF-specific oligoclonal bands/elevated CSF IgG index ^[7][9].
• Exclusion of Syndromes: Exclusion of well-defined AE syndromes (like typical limbic encephalitis).
• Exclusion of Mimics: Doctors must carefully rule out “mimics”—conditions that look like AE but aren’t, such as brain infections, metabolic disorders, or primary psychiatric illnesses ^[10][11].

Diagnostic Certainty

Diagnostic certainty is syndrome- and evidence-dependent. Some antibody-negative syndromic forms (like classic limbic encephalitis) can actually meet definite clinical criteria, and a brain biopsy is not routinely required ^[1].

While the “possible” category is broad, the probable antibody-negative criteria have high specificity (one referral-center validation reported 99% specificity, though this varies by the cohort studied) ^[10]. This strictness prevents patients from receiving powerful immune-suppressing treatments for conditions they don’t actually have ^[12]. Even so, meeting the probable criteria requires ongoing reassessment to ensure no other diagnosis has been missed ^[2].