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National Disease and Therapeutic Index


The National Disease and Therapeutic Index (NDTI) is a commercial data resource maintained by IMS Health, in Plymouth Meeting, Pennsylvania. This resource was developed in 1958 to provide representative data on the population for whom drugs are prescribed, as well as the prescribers, in the United States. Analogous data resources are maintained by this company in many other countries.

The basic data is gathered in an ongoing fashion from a panel of 3,700 physicians selected to represent a statistical sample of practicing physicians. Since a large proportion of practitioners are primary-care providers (e.g., general practitioners or internal medicine specialists), these groups are well sampled. Smaller specialty groups, such as ear, nose, and throat physicians, are sparsely represented.

Each panel physician records data every quarter for a two-day period, using a special duplicate prescription form for all drugs prescribed. When a prescription is written, not only is the usual information (drug name, amount, dosing instructions and duration) written, but also the indication for the drug, the patient's gender and age, the site of prescription (hospital, clinic, etc.), other drugs the patient is taking, other diagnoses, and some physical exam and laboratory data. This data, excluding patient identification, is provided to the company for inclusion in the database.

This NDTI database provides an ongoing national estimate of the pharmaceutical prescribing practices in the United States. This data is expressed as "mentions" of a pharmaceutical, since the information on pharmaceuticals is mentioned in both the actual prescription as well as concomitant therapy. Accordingly, this information does not provide a precise estimate of the frequency of prescribing any product due to the nature of the methodology. Frequency data (e.g., number of prescriptions dispensed per quarter) is available in other databases that sample very large numbers of dispensed prescriptions at retail pharmacies. Nonetheless, because of the careful sampling of practitioners, the actual numbers of mentions are extrapolated to provide approximate national estimates of the characteristics of patients exposed to specific drug products and other products they are using.

This data includes information primarily on prescription pharmaceutical products, although there is some information on over-the-counter concomitant therapy. There is little information on other therapies or herbal drugs used by patients. Due to the sample, this data resource's information is most reliable in describing therapies in relatively common use by primary care physicians. It is more limited for use in describing prescription or patient characteristics typical of specialty practices such as urology or plastic surgery.

NDTI data is most often used by pharmaceutical manufacturer's marketing departments. Post-marketing surveillance and drug safety groups both within and outside the pharmaceutical industry also use this data to estimate the characteristics of populations for both epidemiological studies and for developing exposure denominators for pharmaceutical benefit and risk assessment.

Judith K. Jones

(see also: Personal Health Services; Pharmaceutical Industry )

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