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Interpretive Handbook

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Test 80826:
Gabapentin, Serum

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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 Suggests clinical disorders or settings where the test may be helpful

Monitoring serum concentration of gabapentin, particularly in patients with renal disease

 

Assessing compliance

 

Assessing potential toxicity

Interpretation Provides information to assist in interpretation of the test results

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.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

This test cannot be performed on whole blood. Serum must be separated from cells within 2 hours of draw. Specimens that are obtained from gel tubes are not acceptable.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

2.0-20.0 mcg/mL

Clinical References Provides recommendations for further in-depth reading of a clinical nature

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. Johannessen SI, Tomson T: Pharmacokinetic variability of newer antiepileptic drugs: when is monitoring needed? Clin Pharmacokinetics 2006;45(11):1061-1075