Abnormalities of serum glucose, liver enzymes, BUN, bile acids, ammonia, electrolytes, or blood gases can occur with metabolic dysfunctions. Serum cholinesterase is decreased in acute organophosphate toxicity, and serum lead determinations are increased in lead toxicity. Serum thyroid and cortisol determinations and stimulation tests are useful to detect endocrinopathies. Serum titers or PCRs for viral, fungal, protozoal, and rickettsial organisms can be evaluated. Serum muscle enzymes, especially CK, may be increased in myopathies. Serum acetylcholine receptor antibodies can be detected in dogs and cats with myasthenia gravis, and type II M muscle antibody titers are used to confirm masticatory muscle myositis. Muscle and nerve biopsies are essential for diagnosis and characterization of many neuromuscular disorders. In some cases, brain biopsy is necessary to confirm and characterize an inflammatory or neoplastic process so that proper antimicrobial therapy, chemotherapy, or radiation therapy can be administered.