Spinal fluid (lumbar puncture) test

A spinal tap, also called a lumbar puncture, collects a small amount of the fluid that flows around your brain and spinal cord. This fluid is called cerebrospinal fluid, or CSF. A lab can check it for proteins that point to Alzheimer's disease and for other problems that affect thinking.^1,2

Many people feel nervous when a doctor suggests a spinal tap. It helps to know what happens, what it feels like, and how often problems occur. This page walks you through it, step by step.

What spinal fluid can show

CSF is a clear fluid that cushions the brain and spinal cord. Because it touches the brain, its contents change when brain diseases are present.³

For Alzheimer's disease, the lab measures these proteins:^2,3

• Amyloid (a sticky protein that builds up between brain cells as plaques). When plaques form in the brain, less amyloid floats free in the spinal fluid. So a low level is a sign of plaques.
• Phospho-tau (a changed form of tau, a protein that forms tangles inside brain cells). A high level points to Alzheimer's tangles.
• Total tau. A high level suggests brain cells are being damaged.

Labs often combine two numbers into a ratio. A ratio is more reliable than a single number. Examples include amyloid 42 compared with amyloid 40, or phospho-tau compared with amyloid 42.^6,7,8

Researchers are also studying other markers, such as neurofilament light (NfL), a protein released when nerve cells are hurt. Results can differ between labs and test brands, though this has improved.³

For other conditions, the same fluid can be tested for:

• Infection or inflammation in the brain, by checking cells, sugar and protein levels.¹
• Creutzfeldt-Jakob disease (CJD), a rare, fast-moving brain disease. A test called RT-QuIC can support a diagnosis when it is positive.¹⁰ See Creutzfeldt-Jakob disease and dementia that gets worse fast.
• Normal pressure hydrocephalus (NPH), a condition with too much fluid in the brain. See the section on the "tap test" below.⁹

FDA-cleared spinal fluid tests for Alzheimer's

As of October 2026, these spinal fluid tests are FDA-cleared to help evaluate Alzheimer's. All are for adults 55 and older who have memory or thinking problems.^6,7,8

How accurate are they? In the FDA review of the Lumipulse test, a positive result matched a positive amyloid PET scan about 97 times out of 100. Some people had a result in a gray zone. In that group, only about 6 in 10 had a positive PET scan.⁶ A PET scan is a brain scan that shows plaques. See brain scans.

The FDA says these tests are not for screening people without symptoms. They must be read together with the rest of a person's medical picture.⁶

Who might be offered a spinal tap

A spinal tap is usually ordered by a dementia specialist, not as a first test. The Alzheimer's Association published expert guidance on when it makes sense. The experts reviewed 14 situations. They rated 6 as good reasons to test and 8 as poor reasons.⁴

Common reasons a doctor may suggest one include:

• Mild cognitive impairment or early dementia where the cause is unclear. See mild cognitive impairment.
• Symptoms that started young, often before age 65, or that look unusual.
• Checking for amyloid before an anti-amyloid medicine. The FDA label for lecanemab says doctors must confirm amyloid before starting.¹¹ A spinal fluid test is one way to do that. See are anti-amyloid treatments right for us?
• Looking for other causes, such as infection, inflammation or CJD.^1,10

Spinal fluid tests for Alzheimer's are not meant for screening people who have no memory or thinking symptoms.⁶

Spinal tap, PET scan or blood test?

There are now three main ways to look for Alzheimer's changes in a living person. Each has trade-offs.

In 2025, the Alzheimer's Association released its first guideline on blood tests. It says some blood tests are accurate enough to replace a spinal tap or PET scan. Others are only good for sorting people. With those, a positive result should be confirmed with spinal fluid or PET.¹² See blood tests for Alzheimer's.

This means some people who once needed a spinal tap may not need one now. Ask your doctor which test fits you best.

Before the test

Tell your doctor if you:¹

• Take a blood thinner, aspirin, or another medicine that affects bleeding.
• Have a bleeding problem or low platelets (blood cells that help clotting).
• Are allergic to numbing medicine.
• Often get headaches. A history of headaches was the strongest risk factor for a headache after the tap in one large study.⁵

Doctors avoid a spinal tap when there may be a mass in the brain, because it can be dangerous. That is one reason you may have a brain scan first.¹

Plan for someone to drive you home. Bring your medicine list. If the person being tested has dementia, a family member can stay close, explain each step and offer calm reassurance.

What happens during a spinal tap

1. You get into position. Most often you lie on your side with your knees pulled toward your chest and your chin tucked down. Sometimes you sit and lean forward over a table. This opens the spaces between the bones of your lower back.¹
2. Your back is cleaned and numbed. The numbing shot may sting or burn for a moment.¹
3. A thin needle goes into the lower back. It goes in between two bones, below where the spinal cord ends. You will feel pressure. You may feel a short sharp feeling that passes in seconds.¹
4. Fluid drips into small tubes. Usually 1 to 10 mL is collected (up to about 2 teaspoons). The doctor may also measure the fluid pressure.¹
5. The needle comes out and a small bandage is put on.¹

The whole visit takes about 30 minutes. Collecting the fluid takes only a few minutes.¹ Staying still is important, so tell the doctor if you need a break. Sometimes doctors use an X-ray or ultrasound to guide the needle.¹

What it feels like afterward

Many people have no problems. In a study of 3,868 people tested at 23 memory clinics, here is what happened after the tap:⁵

Good news for older adults. People over 65 were less likely to get a spinal tap headache. So was using a thinner, pencil-point needle (called an atraumatic needle).⁵ You can ask what type of needle will be used.

The spinal tap headache. This headache happens when fluid leaks a little from the needle hole. It is often worse when you sit or stand and better when you lie down. It can last hours or days.¹ Caffeine drinks like coffee, tea or soda may help.¹ If it does not go away, a doctor can do a blood patch. That means putting a small amount of your own blood near the hole to seal it. Few people need one.⁵

Rest. Many clinics ask you to rest for a few hours after the tap. You do not have to lie flat the whole time.¹ In the large memory clinic study, resting did not change the headache risk.⁵

Rare but serious problems include infection, bleeding near the spine or brain, and nerve injury if a person moves suddenly during the test.¹

Understanding your results

Results usually come back as positive, negative, or in-between. Your doctor reviews them with your history, exam and memory tests.

• Negative (normal): Alzheimer's is less likely to be the cause. Your doctor will look for other causes.⁶
• Positive: Amyloid plaques are likely present. This supports Alzheimer's, but it does not prove it on its own. More tests may be needed to learn what is causing the memory problems.⁶
• In-between or "likely positive": The answer is less certain. More testing, such as a PET scan, may help.⁶

No single test can diagnose Alzheimer's or another dementia.³ A spinal fluid result is one piece of a larger picture. See getting a dementia diagnosis and after the diagnosis.

A result can bring strong feelings, whether it is good news or hard news. It is normal to feel shock, relief, fear or grief. Take time with it, and lean on people you trust. If you feel hopeless or have thoughts of suicide, call or text 988 any time.

The "tap test" for normal pressure hydrocephalus

Normal pressure hydrocephalus (NPH) causes trouble walking, bladder control problems and thinking changes. A spinal tap can help show whether surgery may help.⁹

For this "tap test," your walking is measured first, often on video. Then about 1 to 2 ounces of fluid is drained, which is more than for an Alzheimer's test. Your walking is checked again within about an hour.⁹

If walking clearly gets better, a shunt (a thin tube that drains extra fluid) is more likely to help. The improvement usually lasts only hours. But a person who does not improve may still benefit from a shunt. Some centers use a lumbar drain over several days in the hospital instead.⁹ See normal pressure hydrocephalus.

Cost and insurance

Coverage for dementia biomarker tests varies. Medicare and other plans may cover only some tests in some situations.² Before the test, call your plan or ask the clinic's billing office:

• Is the spinal tap covered, and is the lab test covered?
• Will I get separate bills from the doctor, the facility and the lab?
• What will I pay out of pocket?

Questions to ask your doctor

Why do you suggest a spinal tap instead of a blood test or PET scan?

Ask what the test will add, and whether a blood test could answer the same question.¹² Ask how the result will change your care or treatment choices.

Who will do the test, and where?

Ask whether it will be done by a neurologist, a radiologist or another specialist, and whether they do these often. Ask what kind of needle they use.⁵

What should I do about my medicines?

Bring a full list, including aspirin, blood thinners and supplements. Ask exactly what to do before the test. Do not change anything until your doctor tells you to.¹

When and how will I get results?

Ask how long the lab takes, and whether you will get the results at a visit. A visit gives you time to ask questions and bring a family member.

Is a spinal tap safe for someone in their 70s or 80s?

In the large memory clinic study, people over 65 had fewer spinal tap headaches than younger people.⁵ Your doctor will also look at your bleeding risk, back problems and ability to stay still.

Education only. This page is general information written from the sources listed. It is not medical, legal or financial advice and does not replace a doctor, therapist or lawyer who knows your situation. How we write and check pages.