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*Pain Management Centers, Departments of Anesthesiology, Johns Hopkins University School of Medicine, Baltimore, MD and Walter Reed Army Medical Center, Washington, DC;
Department of Clinical Investigation, Walter Reed Army Medical Center;
Departments of Anesthesiology, Walter Reed Army Medical Center and Landstuhl Regional Army Medical Center, Landstuhl, Germany; and
Department of Anaesthesia and Critical Care, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
Address correspondence and reprint requests to Steven P. Cohen, MD, Pain Management Center, 550 North Broadway, Suite 301, Baltimore, MD 21205. Address e-mail to scohen{at}jhmi.edu
IV infusion tests performed to predict subsequent response to oral analgesics are an increasingly popular method used to enhance medical care and conserve resources. Because no infusion test is completely accurate, the potential benefits of these tests must be weighed against the frustration and waste in resources encountered with false-positive results, and the failure to use a potentially beneficial treatment with false-negative results. In recent years, drugs that act antagonistically at N-methyl-D-aspartate receptors have been shown to be valuable adjuncts in the treatment of pain. To determine the predictive value of small-dose (0.1 mg/kg) IV ketamine on an oral dextromethorphan (DX) treatment regimen, we analyzed the analgesic response to these drugs in 25 patients at 2 tertiary care military treatment facilities, institutions at which DX is not readily accessible. When 
50% response for both drugs was used as the outcome measure for success, the positive predictive value of the ketamine test was 64%, the negative predictive value 73%, and the observed agreement 68%. However, when 67% relief with ketamine was used as an outcome measure (as determined by a receiver operating characteristic curve), the positive predictive value was 90%, the negative predictive value 80%, and the observed agreement increased to 84%. Based on these results, we conclude that an IV ketamine test may be useful in predicting response to oral DX. More research is needed to determine the ideal candidates for such a test, and the optimal dose and cutoff value for the response to ketamine.
IMPLICATIONS: In this study, an IV infusion of ketamine, a parenteral N-methyl-D-aspartate receptor antagonist, was found to predict the analgesic response to an oral treatment regimen of dextromethorphan, an oral N-methyl-D-aspartate antagonist, in 84% of patients with neuropathic pain. A fine-tuned IV ketamine test may enhance patient care by saving time and conserving valuable resources.
This article has been cited by other articles:
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  E. Eisenberg, E. D. McNicol, and D. B. Carr Efficacy and Safety of Opioid Agonists in the Treatment of Neuropathic Pain of Nonmalignant Origin: Systematic Review and Meta-analysis of Randomized Controlled Trials JAMA, June 22, 2005; 293(24): 3043 - 3052. [Abstract] [Full Text] [PDF]
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