the diagnosis of guillain–barré syndrome depends on findings such as rapid development of muscle paralysis, absent reflexes, absence of fever, and a likely cause. cerebrospinal fluid analysis (through a lumbar spinal puncture) and nerve conduction studies are supportive investigations commonly performed in the diagnosis of gbs. testing for antiganglioside antibodies is often performed, but their contribution to diagnosis is usually limited. blood tests are generally performed to exclude the possibility of another cause for weakness, such as a low level of potassium in the blood. an abnormally low level of sodium in the blood is often encountered in guillain–barré syndrome. this has been attributed to the inappropriate secretion of antidiuretic hormone, leading to relative retention of water.