How is Guillain-Barre syndrome diagnosed?

Guillain Barre Syndrome (GBS) is diagnosed on clinical grounds, examination of cerebrospinal fluid (CSF), EMG/NCS and by excluding other diseases which can cause similar symptoms.  There is no one diagnostic blood test for GBS. 

The most important diagnostic tool is clinical suspicion.  Your doctor will suspect GBS if you present with rapid onset of numbness, tingling, unsteadiness or muscle weakness which worsens over days.  Examination of the CSFhelps exclude other types of infections or inflammation which may cause similar symptoms; increased protein in the CSF is also supportive of GBS.  EMG/NCS (electromyography/nerve conduction study) is a test which evaluates the electrical activity of the peripheral nerves and can show findings which strongly support the diagnosis. 
As changes in the spinal fluid and EMG/NCS may not occur for up to 1-2 weeks after the onset of symptoms, your physician's clinical suspicion remains the most important diagnostic tool 

Guillain-Barre is called a syndrome rather than a disease because it is not clear that a specific disease-causing agent is involved. A syndrome is a medical condition characterized by a collection of symptoms (what the patient feels) and signs (what a doctor can observe or measure). The signs and symptoms of a syndrome can be quite varied, so doctors may, on rare occasions, find it difficult to diagnose Guillain-Barre in its earliest stages.

Several disorders have symptoms similar to those found in Guillain-Barre, so doctors examine and question patients carefully before making a diagnosis. Collectively, the signs and symptoms form a certain pattern that helps doctors differentiate Guillain-Barre from other disorders. For example, physicians will note whether the symptoms appear on both sides of the body (most common in Guillain-Barre) and the quickness with which the symptoms appear (in other disorders, muscle weakness may progress over months rather than days or weeks). In Guillain-Barre, reflexes such as knee jerks are usually lost. Because the signals traveling along the nerve are slower, a nerve conduction velocity (NCV) test can give some clues to aid the diagnosis. In Guillain-Barre patients, the cerebrospinal fluid that bathes the spinal cord and brain contains more protein than usual. Therefore, a physician may decide to perform a spinal tap, a procedure in which the doctor inserts a needle into the patient's lower back to draw cerebrospinal fluid from the spinal column.

This answer is based on source  information from the National Institute of Neurological Disorders and Stroke.

The symptoms of Guillain-Barre syndrome are similar to those of other disorders affecting the nerves, so diagnosis is sometimes difficult. Your doctor will want to get your medical history to clarify the extent of your symptoms. A nerve function test and a spinal tap can confirm the diagnosis. In the spinal tap, a small amount of fluid will be drawn to test for evidence of Guillain-Barre syndrome. Nerve tests determine how your muscles respond to electrical stimuli and if your muscle weakness is the result of nerve damage.

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