If you've suffered a stroke or have had symptoms of a stroke, you need to be fully evaluated by a doctor to get the right diagnosis and treatment. In the course of evaluating a patient who is suspected of having a stroke, doctors will conduct a physical and neurological exam and imaging tests, as well as a number of different types of blood tests.

“If we know what caused a stroke, we can prevent immediate progression as well as recurrent stroke,” says Aneesh Singhal, MD, the director of the Stroke and Neuro-ICU Observership Program at Massachusetts General Hospital in Boston.

When a stroke is suspected, a doctor will first use imaging, such as an MRI or CT scan of the patient's brain and blood vessels of the neck and head. But, Dr. Singhal notes, imaging tests can really only show an irregularity or blockage of the artery. That’s when doctors need to perform additional imaging, for example of the heart, as well as blood tests.

“There are about 150 ways you can get a stroke. Based on what these combined tests reveal, a big part of that being blood tests, we can determine and treat the cause of a stroke,” says Singhai.

Doctors typically start with four primary blood tests, which may rule out other factors that can cause stroke-like symptoms. These initial tests include:

Complete blood count (CBC) test One of the first things a doctor will want to do is get a picture of the main components of blood. A CBC test measures the number of platelets and red and white cells in your blood. Test results can suggest certain conditions that may or may not be associated with a stroke, such as leukemia — cancer of the blood or bone marrow — anemia, or an infection.

By themselves, high red blood cell or platelet levels can cause the kind of clumping that results in clots that trigger an ischemic stroke, Singhal says. “Very low platelet levels could mean that your blood can’t clot and can bleed into the brain, causing a hemorrhagic stroke,” he says.

Blood lipid test Blood lipid tests measure good cholesterol levels (high-density lipoprotein, or HDL) and bad cholesterol levels (low-density lipoprotein, or LDL) that may be a contributing factor leading to a stroke. Lipids include fats and cholesterol, both good and bad, that build up in body tissues, including the blood. Cholesterol is a waxy substance in the blood, and having too much of it is a major risk factor for stroke, says Singhal.

Markers-of-inflammation tests Several blood tests, including erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) tests, look for markers of inflammation. Inflammation can be an indication of a bacterial, fungal, or viral infection, or a rheumatological condition, according to Singhal, who says that abnormal inflammatory markers may also be present in vasculitis, in which the body mistakenly attacks blood vessels, or endocarditis, a heart valve infection.

Blood chemistry tests A series of blood chemistry tests can also help doctors understand whether a stroke is the issue, or if something else is wrong. For example, Singhal says abnormally high or low blood glucose (sugar) levels may actually cause symptoms that are similar to stroke. Increased glucose levels worsen stroke outcome, and are an indication of diabetes, an underlying condition that increases a person’s risk of both stroke and heart attack. Knowing a patient's blood chemistry can also help doctors assess the risk that a condition, such as abnormal glucose levels, will get worse following a stroke. Blood chemistry tests can also check levels of electrolytes such as sodium and potassium, and assess kidney and liver function, says Singhal.

Eliminating Other Potential Causes of Stroke

According to Singhal, based on the results of these first-line tests, doctors may order additional blood tests that can help confirm what they suspect the cause of the stroke to be. These tests include:

Toxicology tests Doctors will want to rule out recreational drugs including amphetamines and cocaine, which can cause stroke, as well as alcohol, says Singhal. “If drug use was the cause, we can then counsel the patient on this to reduce the chance they will have another stroke,” he says.

D-Dimer test D-Dimer is part of a protein that is left over after the body breaks down a blood clot. “If a patient has a positive D-Dimer test, that tells you the patient is more prone to developing blood clots and that’s what caused the stroke. We can thin the blood using medications to prevent strokes from occurring in the future,” says Singhal, who notes that the test can also detect a stroke associated with COVID-19. “Very high levels of D-Dimer have been found in people with COVID-19 as well as in people with cancer increasing the chance of blood clots. These blood clots can travel to the brain and cause stroke,” he says.

Another blood test, called serum protein electrophoresis (SPEP), can also detect cancer if that is the suspected cause of the stroke, adds Singhal.

Antinuclear antibody (ANA) test An ANA test looks at ANA antibodies and double-stranded DNA for signs of autoimmune and rheumatology disorders including systemic lupus erythematosus (SLE), which can double a person’s risk of stroke, according to the Mayo Clinic. Singhal notes that while many of his patients worry they may have lupus, it’s a very rare disease that affects around 0.4 percent of Americans, according to the Lupus Foundation of America.

Antiphospholipid antibody (APLA) panel tests Antiphospholipid antibodies include anti-cardiolipin antibodies (ACL) and lupus anticoagulant (LA), and, like the ANA test, the presence of these antibodies can also help screen for SLE. According to Singhal, roughly 30 percent of patients with SLE test positive for APLA. Patients who have SLE and who have elevated levels of APLA have a much higher risk for stroke than SLE patients who do not.

Coagulation tests These may include prothrombin time (PT), partial thromboplastin time (PTT), and international normalized ratio (INR), which test for markers of hyper- and hypocoagulation, meaning how well — or too well — your blood is able to form clots. “If you have high levels of certain proteins you can develop blood clots, and blood clots cause ischemic stroke. Low levels can also result in bleeding that causes hemorrhagic stroke,” says Singhal.

Homocysteine tests These tests check the level of the amino acid homocysteine in the blood, which is thought to contribute to increased stroke risk and atherosclerosis, or plaque buildup in the artery wall, which kills one in seven Americans and is a known risk factor for stroke. “Markedly elevated levels of homocysteine may indicate a genetic disorder, which is associated with a high risk for stroke,” explains Singhal.

Cardiac enzyme tests Certain enzyme tests, including LDH isoenzymes, creatine kinase, and, the most commonly used, troponin, can determine whether or not a patient has had a heart attack. “Approximately 10 to 15 percent of patients with stroke develop a concurrent heart attack, which is why it is important to check for cardiac enzymes in stroke patients,” he says. “After a heart attack, clots can develop within the chambers of the heart. These clots can break off and travel into the brain, resulting in stroke.” Patients with stroke also have a high risk of future heart attack, and vice versa, says Singhal.

Infectious disease tests Certain diseases including bacterial infections, herpes virus, syphilis, and malaria can lead to stroke. According to Singhal, treating an infection with proper antibiotic medication can drastically reduce the toll a stroke takes on a person.

Genetic testing Singhal says genetic testing is not routinely done on stroke patients, but these tests can be a useful tool if a doctor suspects a gene mutation may be at fault. “A classic example is CADASIL, a condition caused by an inherited gene mutation that causes stroke,” he explains.

Treating Stroke Risk

Though blood tests cannot definitively diagnose a stroke, they do help give doctors a picture of what may have caused it. They can predict major stroke risk, show the mechanism of stroke, and predict the outcome of recovery after stroke, says Singhal.

Blood tests can also help determine which interventions may prevent a person from having another stroke in the future. Interventions like: