Diagnosing causes of seizures through blood, cerebrospinal fluid and urine tests

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To diagnose a child's seizures, the treatment team may need to do tests on samples of blood, cerebrospinal fluid or urine. Read about collecting procedures.

Key points

  • Blood work is usually required to test for diabetes, other metabolic problems and abnormal levels of certain minerals and other substances in the blood, all of which can cause seizures.
  • A lumbar puncture is done to get a sample of cerebrospinal fluid from your child's lower back. It is used to rule out meningitis or other infections in children aged under 18 months with febrile seizures and to help with diagnosis in older children.
  • A urine test may be done to look for infections, metabolic disorders or abnormal levels of drugs, which may be responsible for seizures.

To help diagnose your child’s seizures, the treatment team may need to do tests on samples of your child’s blood, cerebrospinal fluid, or urine. The procedures for collecting these samples are described on this page.

Blood work

For uncomplicated seizures, it is often not necessary to do blood work. However, the doctor may order blood work to test for diabetes or other metabolic problems that may be causing seizures. Your doctor may also decide to test your child’s blood for levels of blood cells, sodium, potassium, calcium, magnesium, glucose and other substances. Abnormal levels of any of these may cause seizures.

If a child is taking anti-epileptic drugs, blood work is usually needed for monitoring.

Is there any preparation for blood work?

There is usually no preparation for blood work. If your child needs to stop eating or drinking before the blood sample is collected, the nurse or technician will let you know ahead of time.

How is the blood sample collected?

Your child will need to have blood taken with a needle at a blood work clinic. The technician who is trained to take blood (called a phlebotomist) will roll up your child's sleeve and tie an elastic band around the arm, above the area where the blood will be drawn. Then alcohol or antiseptic is rubbed over the vein into which the needle will be inserted.

Blood is drawn into special, small, air-tight tubes called vials which are labelled and sent off for analysis. It doesn't take a long time. The elastic is removed when enough blood is taken to let the blood flow normally. A cotton swab and pressure are applied to the needle site to stop the bleeding, followed by a bandage. For some blood tests, only a finger prick is needed.

For babies, a blood sample may be taken from the heel using a small device called a lancet. Babies have a small volume of blood, so the technician will take the smallest possible amount of blood.

Will the needle hurt my child?

A local anaesthetic (numbing) cream may be given to help reduce the pain involved when blood is taken. Your child may experience some minor bruising or throbbing where the needle was inserted. This will disappear in a day or two. Some children feel faint when they have blood drawn.

How can you help your child through blood work?

Getting a needle is not a fun experience for children (or adults, for that matter). And it can be upsetting for many parents to see their children have needles. Experienced nurses can give needles almost painlessly and it doesn’t take long at all. As a parent, you can help your child by reassuring them while they get a needle. Explain that it won't be more painful than a pinch and that it will be over quickly. Tell them you will be with them the whole time.

Encourage your child to be strong and reinforce that, while needles may look frightening, they’re not dangerous. Talking calmly to your child will help. Also, giving them a favourite toy to hold onto may make it easier. Taking deep, slow breaths and having them look away from the needle can also help.

Many children worry about not having enough blood after some has been taken from their arm. You can reassure your child that the nurse is only taking a very small amount of blood, just enough to test it. There is lots of blood left in their body. Our bodies make new blood all the time, so we never run out of blood.

Lumbar puncture (LP)

A lumbar puncture is used to get a sample of cerebrospinal fluid (CSF). A fine needle is inserted between the bones of the spine, and a small sample of CSF is withdrawn. Lumbar punctures are typically performed in children under 18 months old with febrile seizures to rule out meningitis, other infections or metabolic disorders. They may also be performed in older children to help with diagnosis.

Cross section of lower spine with needle taking sample of cerebrospinal fluid
A small needle is inserted between two vertebrae in the lower (lumbar) spine. This is below the point where the spinal cord ends. A sample of cerebrospinal fluid is taken, and is sent to a lab for testing.

Some parents worry that putting a needle so close to the spinal cord may cause the child to become paralyzed. However, there is almost no risk of paralysis because the needle is put into the space below the end of the spinal cord.

Other names for a lumbar puncture include spinal tap, spinal puncture, thecal puncture and rachiocentesis.

Is there any special preparation for a lumbar puncture?

There is usually no special preparation for a lumbar puncture. However, many children need medicine to help them sleep so they will lie still for it. The type of sedation your child needs depends on their age and medical condition. Some children have a sedative by mouth and others will receive a general anaesthetic. In such cases, an anaesthetist will explain the procedure to you.

If sedation is required, your child will not be allowed to eat or drink for several hours (usually eight hours) before the lumbar puncture.

Your child’s treatment team will let you know if any preparations are needed.

What happens during a lumbar puncture?

It takes five to 10 minutes to do a lumbar puncture. Some children may have a short general anaesthetic or an oral sedative to have a lumbar puncture. If they are sedated they will have the following monitors attached:

  • blood pressure cuff: to keep track of blood pressure
  • electrocardiogram (EKG or ECG) monitors: to show how the heart is working
  • oxygen saturation clip: placed on the finger to keep track of how much oxygen the red blood cells are carrying

A nurse will also hold and watch your child during the test.

Your child will have a cream put on their lower back to make the area numb to prepare for the local anaesthetic. EMLA (Eutectic Mixture of Local Anaesthetic) is one brand of this type of cream. The local anaesthetic is a type of medicine injected in the back so your child won’t feel anything. Some older children choose not to have any sedation, but will have the local anaesthetic.

Your child will lie curled on their side for the test. Most babies do not like to be held in this position and will cry during the test.

Their back will be cleaned and then a fine needle will be inserted between the bones (vertebrae) of the spine. A sample of cerebrospinal fluid will be withdrawn. Drugs can also be injected through the needle if they are needed. The fluid will then be sent to the laboratory for testing.

What happens after a lumbar puncture?

Your child will need to lie down for an hour or so after the lumbar puncture and will be monitored by a nurse.

Urine tests

The doctor may ask for a urine sample from your child to test for infections, metabolic problems, or drugs.

Is there any preparation for a urine test?

There is no special preparation for a urine test. Your child can eat and drink as usual before the test.

How is the urine sample collected?

Older children will be asked to urinate into a sterile container, after first cleaning the genital area. You may need to help your child with the sample.

If you are told to collect a urine sample at home, try to collect the first one in the morning. Use a sterile jar. You may be given a sterile container at the doctor’s office or clinic.

Wash off the genital area several times with cotton balls and warm water. Have your child sit on the toilet seat. If your child is a girl, ask her to sit with her legs spread widely so that the skin folds of the vagina don't touch. Your child should start to urinate into the toilet. Then, place the clean container directly in line with the urine stream. Remove it after you have collected a few ounces but before your child stops urinating. The first or last ounce that comes out of the bladder may be contaminated. This is called a midstream urine sample.

Keep the urine in the refrigerator until you take it to the office. Keep it cool when you bring it to the office by placing it in a cooler or a plastic bag with some ice. The container will be labelled with your child’s name and the date, and sent to the laboratory for testing.

What if your child is too young to give a midstream urine sample?

Younger children and babies may not be able to give a urine sample as described above. If this is the case, it may be necessary to insert a urinary catheter to get a urine sample. A urinary catheter is a small, soft plastic tube. It is inserted through the urethra, the tube that carries urine out of the bladder. This may or may not feel uncomfortable.

As a parent, you can help your child by reassuring them and helping them to relax while the catheter is inserted. Explain that it will be over quickly and that you will be with them. Talking calmly to your child and giving them a favourite toy to hold will help. Help them to take deep, slow breaths in and out while the tube is being inserted and removed.

What happens after the test?

After a catheter is removed, some children feel a stinging or burning sensation while trying to urinate. Encourage your child to drink a lot after the test to help them urinate. Reassure them that it will feel better soon. If the discomfort lasts for more than 48 hours or your child develops a fever, contact your child’s doctor.

Last updated: February 4th 2010