The diagnosis of GBS is largely clinical, but the standard investigations are listed below.

The key diagnostic tests for the ED are the lumbar puncture, along with a screen of blood tests.

Lung function tests are vital for safe management of the patient.


Acute investigations include:

  • Blood tests: FBC, ESR, U+E, LFT, CPK, CRP, glucose, Mg, calcium and phosphate
  • Stool culture (for C jejuni and others)
  • ECG
  • Lumbar puncture
  • Lung function tests if any respiratory compromise, or unable to walk

Other diagnostic tests include:

  • MRI may have a place in diagnosis. Selective anterior nerve root enhancement appears to be strongly suggestive of GBS. The cauda equina nerve roots are enhanced in 83% of patients
  • Nerve conduction and other electrodiagnostic studies may confirm and classify the diagnosis and is also used to prognosticate.
  • Immunological tests: Antibodies to GQ1B may confirm a Miller Fisher variant.
  • Nerve or muscle biopsy
  • Specific disease related tests e.g. HIV test

Interpretation of results

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A raised CSF protein (>0.4 g/L) with normal white cell count is the typical CSF finding in GBS.

However, CSF protein may be normal in the first week of the disease, but will have increased in more than 90% of cases by the end of the second week.

A raised serum CK suggests an alternative diagnosis such as a myositis.

Hypokalaemia and hypoglycaemia may also mimic GBS symptoms, and should be excluded.

Liver function tests are often mildly elevated in GBS.

Serial lung function tests are important as the onset of respiratory compromise can be rapid. Patients with an FVC less than 20mL/kg are candidates for prophylactic intubation and mechanical ventilation.

There is much current work regarding immunological studies in GBS. These may confirm an immune pathogenesis, and may affect long-term treatment in some cases, but currently have little place in the acute diagnosis, or treatment, of GBS.

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