A CSF smear is part of a series of laboratory tests performed on samples of a patient’s cerebral spinal fluid (CSF). A CSF smear is often combined with a CSF Gram stain to diagnose certain infectious diseases of the central nervous system. These diseases include meningitis and encephalitis. They are caused by fungi, bacteria or other pathogens and result in inflammation of the central nervous system.
CSF is a clear liquid that cushions, nutures, and surrounds the brain and spinal cord. It delivers nutrients and carries away waste. It is continually produced by the choroid plexus in the brain and absorbed into the blood stream. The body completely replaces the CSF every few hours.
Although obtaining a spinal fluid sample is difficult, testing CSF is one of the best ways to correctly diagnose specific types of infections of the central nervous system. This is because CSF is in direct contact with the brain and spine.
The most common method of collecting cerebral spinal fluid is lumbar puncture, sometimes called spinal tap.
Lumbar puncture usually takes less than 30 minutes and is performed by a physician specially trained to safely collect CSF. CSF is usually withdrawn from the lower back area. It is very important to remain completely still to avoid incorrect needle placement or trauma to the spine.
You will either be seated with your spine curled forward or lie on your side with your spine curved and your knees drawn up to your chest. Curving the spine allows the doctor to find ample space to insert a thin spinal needle between the vertebrae (bones in the lower back). Sometimes fluoroscopy (X-ray) is used to guide the needle safely between the vertebrae.
When you are in position, the doctor or a nurse will clean your back with a sterile solution such as iodine. A sterile area is maintained throughout the procedure to reduce the risk of infection. Numbing cream is applied to the skin, and then the site is injected with an anesthetic (pain-killing) solution. When the site is numb, the doctor inserts the spinal needle.
Once the needle is in, CSF pressure (opening pressure) is usually measured using a manometer (pressure gauge). High CSF pressure can be a sign of certain conditions and diseases, including:
- intracranial hemorrhage (bleeding in the brain)
Pressure may also be measured at the end of the procedure (closing pressure).
The doctor then takes fluid samples up through the needle and into an attached syringe. Several vials of fluid may be taken. When fluid collection is complete, the doctor takes the needle out of your back. The puncture site is cleaned again with sterile solution and a bandage is applied.
You must remain lying down for about one hour to avoid headache, a common side effect of the procedure.
Rarely, if you have a back deformity, infection, or possible brain herniation (when structures of the brain have shifted out of place), it is necessary to use more invasive CSF collection methods. These methods usually require hospitalization. In a ventricular puncture, a doctor drills a hole into the skull and inserts a needle directly into one of the ventricles of the brain. In a cisternal puncture, a doctor inserts a needle below the base of the skull.
A lumbar puncture requires a signed release stating that you understand the risks of the procedure.
Be sure to tell your doctor if you take any blood-thinning medications such as warfarin (Coumadin). You may need to stop taking them for a couple of days before the procedure.
You’ll be asked to empty your bowels and bladder before the procedure.
Primary risks associated with lumbar puncture include:
- bleeding from the puncture site into the spinal fluid (traumatic tap)
- discomfort during and after the procedure
- allergic reaction to the anesthetic
- infection at the puncture site
- headache after the test
- damage to spinal cord nerves (especially if you move during the procedure)
Your risk of bleeding is higher if you take blood thinners.
There are serious additional risks if you have a brain mass such as a tumor or abscess (puss surrounded by inflamed tissue) putting pressure on the brain stem. In these cases, lumbar puncture can cause brain herniation. Brain herniation results in high intracranial pressure, which can eventually cut off the supply of blood to the brain. This can result in brain damage or even death. Lumber puncture will not be performed if a brain mass is suspected.
Cisternal and ventricular puncture have additional risks. They include:
- damage to spinal cord or brain
- bleeding within the brain
- disturbance of the blood/brain barrier in the skull
Lumbar puncture is extremely dangerous for people who have thrombocytopenia (low platelet count) or other blood clotting problems.
A CSF smear analyzes the color of your spinal fluid as well as the number and shape of cells it contains. Your doctor will order this test to check for signs of infection. A CSF Gram stain looks for microorganisms. It is ordered when your doctor suspects you have meningitis or encephalitis caused by bacteria or fungi.
Meningitis progresses quickly, and you should seek medical attention if you experience:
- sudden fever
- severe headache
- stiff neck
Bacterial encephalitis causes inflammation of the brain. Symptoms can range from mild and flu-like to more severe symptoms, including:
- severe headache
- sudden fever
Cases of bacterial and fungal meningitis are less common than viral meningitis, but people who have trouble fighting infections are more likely to get them.
When doing a CSF smear, a laboratory technician spreads (smears) a small amount of your CSF fluid on a glass slide and looks at it under the microscope. Infection can make it cloudy instead of clear. The appearance of the fluid and the presence of any abnormalities are diagnostic tools.
With a CSF Gram stain, a lab technician adds special dyes (stains) and other solutions to the CSF smear to help identify microorganisms. Certain types of bacteria and fungi are known to absorb the special stains.
If there are no microorganisms found, it is possible you do not have an infection caused by bacteria or fungi.
However, if you are on antibiotics, bacteria may not be present in the CSF. Further tests will be needed to rule out bacterial infection.
If microorganisms are found, it likely means that you have one of the following diseases of the central nervous system:
- tuberculosis, a bacterial infection that usually starts in the lungs but may spread to the spinal cord
Other types of bacterial and fungal infections are also possible.
Bacterial meningitis is a medical emergency. Early treatment can help prevent serious illness and death in patients with meningitis. However, this infection is hard to diagnose because its symptoms are similar to viral meningitis, a less life-threatening illness.
Prompt treatment is essential. Your doctor may begin broad-spectrum antibiotics (medication that treats a variety of bacteria types) while conducting additional tests to find the exact cause of the infection. Bacterial meningitis vaccines can prevent some types of the infection. Students living in college dorms or common housing should consider getting the vaccine.
In mild cases of encephalitis, you may get better with bed rest, forced fluids (drinking more than usual to ensure hydration), and an over-the-counter pain and fever reliever. For severe cases, hospitalization is usually needed.
Cases of encephalitis are uncommon in the United States.