Normal pressure hydrocephalus Guide for patients and families
The spinal tap test for NPH: what happens and what the result means
The short answer
A spinal tap test, also called a large-volume lumbar puncture, removes 30 to 50 milliliters of spinal fluid from the lower back to see whether walking improves over the next day or so. When it does, a shunt is very likely to help. When it does not, NPH has not been ruled out, and a longer drainage trial is often the next step.
Written and medically reviewed by Carlos Esmond Sanchez, MD, neurosurgeon in Washington, DC. Last updated November 2026.
What the test is for
A rehearsal for what a shunt would do.
A brain scan can show that the ventricles are enlarged. It cannot show whether draining fluid will help you walk better. The tap test answers that question directly, by removing some fluid for a short time and measuring what changes.
Before the tapthe usual walk, timed
After the tapa positive test
What happens, step by step
Before the day
Tell my office about every medication, especially blood thinners, aspirin and other drugs that affect clotting. Some have to be paused beforehand, and you will be told which ones and for how long. Do not stop any of them on your own.
Bring the shoes and the cane or walker you normally use, and a family member who knows how you usually walk.
Your walking is measured
You are timed rising from a chair, walking a set distance, turning and sitting down again, and the steps are counted. Short tests of attention and memory are sometimes added.
The tap
You lie on your side or sit leaning forward. The skin of the lower back is numbed, and a thin needle is passed between two bones of the lower spine, below the level where the spinal cord ends. The fluid pressure is measured, and 30 to 50 milliliters of fluid, two to three tablespoons, is allowed to drain. Most people feel pressure rather than pain.
Your walking is measured again
The same timed walk is repeated later that day. Improvement can take hours to appear and sometimes shows best the following day, so what the family sees at home afterward is part of the result.
Going home
You go home the same day. Plan for someone else to drive, and to be with you for the rest of the day.
What the result means
A positive test is strong evidence. A negative test is not the end of the question.
9 in 10when walking improves
In a review of eight studies, 92% of people whose walking improved after a tap went on to improve after a shunt. Across the individual studies the figure ranged from 73% to 100%.1
Not finalwhen nothing changes
A tap that shows no improvement is a poor guide. Among people who had a shunt despite a negative tap, the test’s prediction of failure was right only 37% of the time.1 Published guidelines say the tap test should not be used to rule NPH out, because it identifies only 26% to 61% of the people who would benefit.2
2 to 3 daysthe longer trial
When a single tap leaves doubt, a soft drain is placed in the lower back for two or three days in the hospital. Draining more than 300 milliliters this way identifies 50% to 100% of the people who will respond, and a positive result predicts improvement after a shunt 80% to 100% of the time.2
0.23 m/sfaster after surgery
The 2025 placebo-controlled trial in the New England Journal of Medicine enrolled only people whose walking speed improved after a drainage trial. Three months after surgery, those with a working shunt walked 0.23 meters per second faster. Those whose valve was not draining gained 0.03.3
If neither the tap nor the longer trial changes anything, NPH becomes unlikely, and the search for the cause continues with a neurologist. What happens if it is not NPH.
After the test
A sore lower back for a day or two is common. Some people develop a headache that is worse when sitting or standing and better lying flat; rest, fluids and time usually settle it. Bleeding and infection are rare.
The improvement is temporary. The brain replaces the fluid at about a pint a day, so the walk usually returns to where it was within days. That fading is expected and does not mean the test failed.
When to call
Call my office the same day for a headache that is severe or does not ease when you lie down, a fever, or fluid leaking from the needle site.
Call 911 for new weakness or numbness in the legs, loss of bladder or bowel control, or sudden confusion or drowsiness.
How the family can help
- Record a short video of the walk before the tap: rising from a chair, walking ten steps, turning and coming back.
- Record the same walk that evening and again the next day.
- Note anything else that changes, including bladder urgency and alertness.
- Bring the videos and the notes to the follow-up visit.
Questions families ask
Is the spinal tap test for NPH painful?
The skin is numbed first. Most people feel pressure in the lower back rather than pain, and a sore back for a day or two afterward is common.
How long does the improvement last after a tap test?
Hours to a few days. The brain replaces spinal fluid at about a pint a day, so the effect fades. A temporary improvement is what the test looks for; a shunt makes the drainage permanent.
What if the tap test shows no improvement?
NPH is not ruled out. A single tap identifies only about 26% to 61% of the people who would benefit from a shunt. The usual next step is a longer drainage trial in the hospital over two or three days, which is more reliable.
How accurate is the spinal tap test for NPH?
When walking clearly improves, about 92% of people go on to improve after a shunt, according to a review of eight studies. When there is no improvement the test is much less reliable, which is why a negative result leads to further testing and is not treated as final.