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Many outpatients taking drugs with a narrow therapeutic range do not receive drug concentration monitoring to prevent toxicity

Therapeutic drug monitoring is useful for avoiding drug toxicity for patients taking drugs that have a narrow therapeutic range (NTR). Yet a substantial number of outpatients using these drugs go 12 months or longer without receiving blood tests to monitor drug concentration, according to a new study. NTR drugs are those in which there is a narrow range between the dose necessary to achieve beneficial effects and the dose that causes serious adverse effects when there is a direct concentration-effect relationship. Drug monitoring that evaluates the drug's concentration in the blood can help individualize the drug dosage and the dosing interval. This, in turn, can minimize the toxicity and maximize the therapeutic benefit of NTR drugs, explain researchers at the HMO Research Network Center for Education and Research in Therapeutics.

The researchers retrospectively examined the monitoring of NTR drugs among 17,748 ambulatory patients at 10 health maintenance organizations (HMOs), who were receiving ongoing continuous therapy with specific NTR drugs for at least 12 months between January 1, 1999 and June 30, 2001. They assessed serum drug concentration monitoring from administrative data and from medical record data. Fifty percent or more of patients receiving digoxin, theophylline, procainamide, quinidine, or primidone were not monitored, and 25 to 50 percent of patients receiving divalproex, carbamazepine, phenobarbital, phenytoin, or tacrolimus were not monitored. In contrast, lithium and cyclosporine drug serum concentrations were monitored in most patients.

Younger patients were 1.5 to nearly 2 times more likely than older patients to not be monitored for digoxin and theophylline, respectively. Older patients were half as likely to be monitored for carbamazepine and divalproex. Patients with fewer outpatient visits were less likely to be monitored in general. The study was supported by the Agency for Healthcare Research and Quality (HS11843).

More details are in "Monitoring of drugs with a narrow therapeutic range in ambulatory care," by Marsha A. Raebel, Pharm.D., Nikki M. Carroll, M.S., Susan E. Andrade, Sc.D., and others, in the May 2006 American Journal of Managed Care 12, pp. 268-274.

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