Testing for a stroke starts the moment symptoms appear and involves a fast sequence of physical checks, brain imaging, and blood work, all racing against the clock. Every minute matters because clot-dissolving treatment works best within the first 4.5 hours of symptom onset, and its benefit shrinks the longer you wait. Here’s what happens at each stage, from the first signs at home to the full hospital workup.
Recognizing a Stroke With BE FAST
Before any medical test, the first “test” is recognizing that a stroke is happening. The BE FAST acronym covers the six things to check:
- Balance: Sudden loss of balance or coordination.
- Eyes: Vision changes, including loss of sight in one or both eyes or sudden double vision.
- Face: Ask the person to smile. One side of the face will droop if a stroke is occurring.
- Arms: Ask them to raise both arms. If one arm drifts downward, that signals weakness on one side of the body.
- Speech: Listen for slurred words or difficulty understanding what you’re saying.
- Time: Call 911 immediately. Don’t wait to see if symptoms improve.
These aren’t just guidelines for bystanders. Paramedics run similar rapid neurological checks in the ambulance, and the results help the hospital prepare the right team and equipment before you even arrive.
What Happens in the First 25 Minutes at the Hospital
The emergency team’s first priority is brain imaging, and they aim to get it done within 25 minutes of arrival. The standard first scan is a non-contrast CT, a type of X-ray image of the brain that takes only a few minutes. Its main job is to answer one critical question: is the stroke caused by a blood clot blocking an artery (ischemic) or by bleeding in the brain (hemorrhagic)? This distinction changes the treatment entirely, because clot-dissolving medication would be dangerous in a bleeding stroke.
CT scans are fast and available in virtually every emergency department, which is why they come first. The trade-off is that CT is not as sensitive at detecting very early ischemic strokes. MRI is better at picking up fresh clots, pinpointing the exact location of damage, and ruling out conditions that mimic a stroke (like seizures or migraines). Some hospitals now use MRI as the initial scan, aiming for the same 25-minute target. In practice, though, CT remains the workhorse because speed is everything.
Blood Tests During Stroke Triage
While imaging is being arranged, the team draws blood and performs a rapid fingerstick glucose test. This is done first because low blood sugar can cause symptoms nearly identical to a stroke, including slurred speech, confusion, and one-sided weakness. National stroke guidelines require a blood glucose level before clot-dissolving medication can be given.
Additional blood work checks clotting function. If you take blood thinners, the team needs to know how thinned your blood already is before adding a clot-dissolving drug. These coagulation tests measure how quickly your blood forms clots and help determine whether thrombolysis is safe.
Advanced Imaging for Larger Clots and Late Arrivals
If the initial scan confirms an ischemic stroke, the team often moves to CT angiography, which uses contrast dye to map the blood vessels in the brain and neck. This reveals exactly where a clot is lodged and how large it is. For large clots blocking a major artery, a mechanical clot-retrieval procedure (thrombectomy) may be needed, and the angiogram tells surgeons precisely where to go.
For patients who arrive later, or who woke up with stroke symptoms and don’t know when they started, perfusion imaging becomes essential. This technique measures blood flow across different regions of the brain in real time. Software automatically calculates two zones: brain tissue that is already irreversibly damaged (the core) and tissue that is starved of blood but still salvageable (the penumbra). The penumbra is calculated by subtracting the dead core from the total area of reduced blood flow. If a significant amount of brain tissue is still salvageable, treatment can be worthwhile even up to 9 hours from symptom onset for clot-dissolving drugs, or up to 24 hours for mechanical clot retrieval.
Testing for the Cause: Heart and Rhythm Checks
Once the immediate crisis is managed, the diagnostic work shifts to figuring out why the stroke happened. Heart problems cause 15 to 30 percent of strokes in the United States, so cardiac testing is a standard part of every stroke workup.
An electrocardiogram (EKG) is typically done in the emergency department. It serves two purposes: identifying heart rhythms that could have sent a clot to the brain, and checking for signs of heart disease that may have been previously undiagnosed. Atrial fibrillation, an irregular heart rhythm that allows blood to pool and clot in the heart, shows up on EKG in up to 25 percent of patients with a first-ever ischemic stroke. Some people have intermittent atrial fibrillation that only occurs in episodes, so extended heart monitoring over days or weeks may be needed if the initial EKG looks normal.
An echocardiogram, an ultrasound of the heart, looks for structural problems that can launch clots into the bloodstream. These include blood clots sitting inside the heart chambers, a small hole between the upper chambers of the heart (patent foramen ovale), heart valve infections, and tumors. Identifying the specific cardiac source directly shapes the long-term prevention plan, whether that means blood thinners, a heart procedure, or a different medication strategy.
When a Spinal Tap Is Needed
In most stroke cases, a spinal tap (lumbar puncture) is not part of the workup. The exception is when doctors suspect a subarachnoid hemorrhage, a type of bleeding around the surface of the brain often caused by a ruptured aneurysm. The hallmark symptom is a sudden, explosive headache sometimes described as the worst headache of your life.
If a CT scan comes back negative but suspicion for this type of bleed remains high, a lumbar puncture can detect blood breakdown products in the spinal fluid. This test is especially important when more than six hours have passed since the headache started, because by that point the bleeding may be too subtle for CT to catch. The fluid is checked for a yellowish discoloration called xanthochromia, which confirms that blood has been present in the spinal fluid.
The Neurological Exam at the Bedside
Throughout this process, doctors perform a structured neurological exam called the NIH Stroke Scale. It evaluates 11 categories of brain function, including level of consciousness, eye movements, visual fields, facial muscle control, arm and leg strength, coordination, sensation, language ability, and speech clarity. Each category is scored, and the total gives a standardized measure of how severe the stroke is.
This score isn’t just academic. It guides treatment decisions, helps predict recovery, and provides a baseline that doctors compare against over the following hours and days. A worsening score can signal a new clot or expanding bleed, triggering a return to imaging. An improving score confirms that treatment is working. The scale is repeated multiple times during a hospital stay, making it one of the most frequently used tools in stroke care.
Why Speed Changes Outcomes
The entire diagnostic sequence is designed around one principle: faster treatment produces better recovery. Data from large registries shows that the benefit of clot-dissolving drugs is greatest in the first hour and declines steadily after that. For mechanical clot retrieval, a pooled analysis of five major trials found that the odds of reduced disability at 90 days dropped with every additional minute between symptom onset and the start of the procedure.
This is why stroke centers drill relentlessly on time targets. The 25-minute door-to-imaging goal, the push to start clot-dissolving treatment as fast as safely possible, and the rapid deployment of clot-retrieval teams all exist because the brain loses roughly 1.9 million neurons per minute during a large vessel blockage. The tests themselves are not complicated. The challenge is performing all of them fast enough to save brain tissue that is still alive but fading.