The only definitive diagnostic test for CJD is to look at brain tissue. Brain tissue can be examined during life - a brain biopsy - or after death, during a post-mortem examination.

As a brain biopsy carries the risk of causing brain damage or seizures, it is only undertaken in a few cases, where the concern is that the patient does not have CJD but some other treatable condition.

Diagnosis of CJD is normally based on medical history, symptoms and diagnostic tests. A neurologist (a doctor who specializes in conditions of the nervous system) will carry out the series of tests to rule out other conditions with similar symptoms such as Alzheimer's, Parkinson's disease or a brain tumor. The tests can also check for some common signs of CJD. These tests include:

• MRI scan - an MRI scan of the brain can rule out other conditions, and find abnormalities in the brain that are distinctive to CJD.
• EEG - an EGG (electroencephalogram) scan can measure the electrical activity of your brain. Sporadic CJD is known to cause a distinctive pattern of electrical activity, which can be found by the EEG scan.
• Lumbar puncture - a lumbar puncture involves inserting a needle into the base of your spine and taking a sample of the fluid that surrounds the brain and spinal column - cerebro-spinal fluid (CSF). The fluid can be studied to rule out other conditions. It is also checked for a protein called 14-3-3. This protein is found in almost all cases of sporadic CJD and 50% of variant CJD.
• Tonsil biopsy - a small piece of tissue can be taken from the tonsils and checked for abnormal prions. These prions are present with sporadic CJD, but not variant CJD.