Sorted By Test Name - Mayo Medical Laboratories

GABA
80826
cerebrospinal fluid may be helpful when serum testing is negative.
Useful For: Evaluating new onset encephalopathy encompassing 1 or more of the following:
confusional states, psychosis, delirium, memory loss, hallucinations, seizures, dyssomnias, coma,
dysautonomias, or hypoventilation The following accompaniments should prompt suspicion for
autoimmune encephalopathy: -Headache -Autoimmune stigmata (personal or family history, or signs of
diabetes mellitus, thyroid disorder, vitiligo, poliosis [premature graying], myasthenia gravis, rheumatoid
arthritis, systemic lupus erythematosus) -History of cancer -Smoking history (20+ pack years) or other
cancer risk factors -Inflammatory cerebrospinal fluid or isolated protein elevation -Neuroimaging signs
suggesting inflammation Evaluating limbic encephalitis Directing a focused search for cancer
Investigating encephalopathy appearing in the course or wake of cancer therapy, and not explainable by
metastasis or drug effect
Interpretation: GABA-B-receptor IgG is a valuable serological marker of autoimmune
encephalopathy and of a patient's immune response to cancer (usually small-cell carcinoma). This
autoantibody is usually accompanied by subacute neurological symptoms and signs, and is not found in
healthy subjects.
Reference Values:
Negative
Clinical References: 1. Lancaster E, Lai M, Peng X, et al: Antibodies to the GABA(B) receptor in
limbic encephalitis with seizures: case series and characterisation of the antigen. Lancet Neurol
2010;9(1):67-76 2. Boronat A, Sabater L, Saiz A, et al: GABA(B) receptor antibodies in limbic
encephalitis and anti-GAD-associated neurologic disorders. Neurology 2011;76(9):795-800
Gabapentin, Serum
Clinical Information: Gabapentin is an antiepileptic drug that is effective in treating seizures,
neuropathies, and a variety of neurological and psychological maladies. Although designed as a gamma
amino butyric acid (GABA) analogue, gabapentin does not bind to GABA receptors, nor does it affect the
neuronal uptake or content of GABA; its precise mechanism of action is not known. Gabapentin circulates
essentially unbound to serum proteins. Unlike most antiepileptic drugs, gabapentin does not undergo
hepatic metabolism; it is eliminated almost entirely by renal excretion with a clearance that approximates
the glomerular filtration rate. The elimination half-life is 5 to 7 hours in patients with normal renal
function. Because gabapentin does not bind to serum proteins and is not metabolized by the liver, it does
not exhibit pharmacokinetic interactions with other antiepileptic drugs; its serum concentration is not
changed following the addition or discontinuation of other common anticonvulsants (eg, phenobarbital,
phenytoin, carbamazepine, valproic acid), nor is their serum concentration altered upon adding or
discontinuing gabapentin. Adverse effects are infrequent and usually resolve with continued treatment.
The most common side effects include somnolence, dizziness, ataxia, and fatigue. Experience to date
indicated that gabapentin is safe and relatively nontoxic.
Useful For: Monitoring serum concentration of gabapentin, particularly in patients with renal disease
Assessing compliance Assessing potential toxicity
Interpretation: Therapeutic ranges are based on specimens drawn at trough (ie, immediately before
the next dose). Most individuals display optimal response to gabapentin with serum levels of 2 to 20
mcg/mL. Some individuals may respond well outside of this range, or may display toxicity within the
therapeutic range; thus, interpretation should include clinical evaluation. Some patients require high doses
to achieve response, resulting in concentrations as high as 80 mcg/mL. Dosage reduction should be based
on signs of toxicity, not the serum concentration.
Reference Values:
2.0-20.0 mcg/mL
Clinical References: 1. Patsalos PN, Berry DJ, Bourgeois BF, et al: Antiepileptic drugs-best practice
guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug
monitoring, ILAE Commission on Therapeutic Strategies. Epilepsia 2008;49(7):1239-1276 2.
Current as of January 4, 2013 7:15 pm CST 800-533-1710 or 507-266-5700 or MayoMedicalLaboratories.com Page 782