Diagnosing a seizure involves piecing together several types of evidence: a detailed account of what happened, brain wave recordings, blood tests, and often brain imaging. No single test can confirm a seizure on its own. Instead, doctors build a case by combining a witness description of the event with objective data from labs and imaging to determine whether a seizure occurred, what type it was, and what caused it.
The Witness Account Is the Most Important Clue
Most seizures are over by the time a person reaches medical care, so the description from someone who saw it happen carries enormous diagnostic weight. Doctors use two approaches to gather this information. The first is “free recall,” where the witness simply describes what they saw in their own words. The second is a systematic set of targeted questions covering specific features: Were the eyes open or closed? Did shaking start suddenly or gradually? Did the movements affect both sides of the body or just one? How long did the episode last?
These details matter because different seizure types look different, and the pattern of symptoms helps classify what happened. A seizure that starts with jerking on one side of the body before spreading suggests a focal origin in the brain, while one that begins with stiffening and shaking on both sides simultaneously points to a generalized seizure. The distinction shapes every diagnostic and treatment decision that follows.
If you witnessed someone’s seizure, try to note the timeline as precisely as possible. Even rough estimates of duration (under a minute vs. several minutes), the sequence of events (what happened first, what came next), and the person’s awareness during the episode all help doctors narrow down the type.
What Happens After the Seizure Ends
The period immediately after a seizure, called the postictal state, also provides diagnostic clues. After a generalized tonic-clonic seizure (the type most people picture, with full-body convulsions), a person typically enters a deep state of unresponsiveness with slow, heavy breathing. Recovery of consciousness is gradual and can take minutes to hours.
One particularly telling sign is temporary weakness or paralysis on one side of the body after the event, known as Todd’s paralysis. When this appears, it strongly suggests the seizure started in one specific area of the brain before spreading. Doctors check for this and other focal deficits during the physical exam because these patterns help localize where in the brain the seizure originated.
Blood Tests Rule Out Treatable Causes
Before labeling a seizure as epilepsy, doctors need to check whether something correctable triggered it. A first seizure workup typically includes blood glucose, electrolytes, calcium, and kidney and liver function tests. Women of childbearing age receive a pregnancy test. If there’s any suspicion of drug use, alcohol withdrawal, or poisoning, a toxicology screen is added.
These tests matter because seizures caused by low blood sugar, dangerously low sodium, or drug intoxication aren’t epilepsy. They’re “provoked” seizures, and fixing the underlying problem usually prevents them from happening again. Only when metabolic and toxic causes have been excluded does the workup shift toward evaluating for epilepsy itself.
Prolactin Levels Can Help in Uncertain Cases
When it’s unclear whether an episode was a true seizure or a nonepileptic event (a spell that looks like a seizure but has a psychological origin), a blood test for prolactin can help. Prolactin levels rise after genuine generalized tonic-clonic seizures, and when drawn 10 to 20 minutes after the event, an elevated level is highly predictive of a real seizure, with a specificity around 96%. A baseline comparison sample should be drawn at least 6 hours later.
There are important limits to this test. It’s better at detecting generalized convulsive seizures (60% sensitivity) than focal seizures with impaired awareness (46% sensitivity). A normal prolactin level does not rule out a seizure. And the test cannot distinguish a seizure from fainting (syncope), so it’s only useful in specific clinical scenarios.
EEG: Recording the Brain’s Electrical Activity
An electroencephalogram, or EEG, records electrical patterns from the brain through sensors placed on the scalp. Doctors look for abnormal spikes and waves that indicate a tendency toward seizures. This is the closest thing to a direct test for epilepsy, but it has a significant limitation: it only captures what the brain is doing during the recording.
A routine EEG lasts about 20 to 30 minutes and picks up epilepsy-related abnormalities in roughly 12% of cases. Sleep-deprived EEGs, where you stay awake the night before to increase the chance of capturing abnormal activity, perform similarly at about 13%. The numbers are low because the brain doesn’t produce abnormal discharges constantly. An ambulatory EEG, worn for 24 hours at home, has better sensitivity (around 63% in studies of patients whose routine EEG was normal) simply because it records for much longer.
A normal EEG does not mean you didn’t have a seizure. Many people with confirmed epilepsy have normal results on their first EEG. If suspicion remains high, doctors may repeat the test or use longer monitoring periods. In some cases, patients are admitted for continuous video-EEG monitoring over several days, which records both brain activity and a video of the patient simultaneously to catch and characterize episodes in real time.
Brain Imaging: MRI and CT Scans
Imaging the brain serves two purposes: finding a structural cause for the seizure and ruling out emergencies. Which scan you get depends on the clinical situation.
In an emergency, a CT scan without contrast is the go-to choice. It’s fast, doesn’t require the safety screening that MRI does, and can quickly identify bleeding in the brain, stroke, fluid buildup, tumors, and signs of dangerous brain swelling or shifting. If someone arrives in the emergency department actively seizing or immediately after a seizure, especially after head trauma, CT is typically the first imaging study.
MRI is the preferred scan for a thorough evaluation after the acute situation is handled. It provides far more detail and can detect subtle abnormalities that CT misses entirely. These include scarring in the hippocampus (a common finding in temporal lobe epilepsy), developmental abnormalities in the brain’s outer layer that formed before birth, small tumors, and evidence of old injuries or tiny areas of bleeding. Specialized epilepsy MRI protocols use specific sequences optimized to find these subtle changes, and they’re recommended for anyone being evaluated for epilepsy after a first unprovoked seizure.
Diagnosing Seizures in Children
Children between 6 months and 5 years old can have seizures triggered by fever, called febrile seizures, which are the most common seizure type in this age group. The diagnostic approach hinges on distinguishing simple febrile seizures from more concerning types.
A simple febrile seizure is a brief (under 15 minutes), generalized seizure that happens once during a febrile illness, with no neurologic deficits before or after. These occur during the rising phase of a high fever, and the prognosis is excellent. They do not indicate epilepsy.
Complex febrile seizures are longer (over 10 minutes), may involve only one side of the body, can recur within the same fever episode, and sometimes leave temporary weakness afterward. These carry a 10 to 15% risk of later afebrile seizures, and that risk climbs to 15 to 20% when additional factors are present: developmental delay, prior neurological problems, cerebral palsy, a family history of epilepsy, or recurrence during the same illness. Children with complex febrile seizures typically receive a more thorough workup, including EEG and sometimes MRI.
When Genetic Testing Enters the Picture
Genetic testing isn’t part of a routine seizure workup, but it becomes relevant in specific situations. The American Epilepsy Society recommends considering it for epilepsy that remains unexplained after standard evaluation, seizures that begin in childhood (especially severe forms), epilepsy that doesn’t respond to medications, cases where two or more first-degree family members have epilepsy, and when brain imaging shows developmental abnormalities in the brain’s structure.
Genetic results can change treatment in meaningful ways. Some genetic epilepsies respond better to specific medications, and others are worsened by drugs that would otherwise seem like reasonable choices. Testing typically involves a panel that screens for mutations in ion channels, metabolic pathways, and genes associated with brain development. It’s most likely to yield actionable results in children with early-onset or treatment-resistant epilepsy.