The effects of group size on test ordering for hypertensive patients.

There has been a tremendous increase in the number of patients receiving care from physicians in group practice rather than in solo practice. To determine whether group size influences the number of tests ordered for ambulatory patients, we studied the records of 351 hypertensive patients cared for by 30 internists in group or solo private practice. Patients of physicians who were members of large groups received twice as many tests (6.4 per patient per year) as patients of physicians in small groups (3.5) or in solo practice (3.3) (P less than 0.001). The charges for testing were also nearly double for patients of physicians in large groups ($123 vs. $58 and $71, P less than 0.01). A multiple regression analysis of individual tests showed that large group practices were associated with increased use of electrocardiography (P less than 0.001), blood counts (P less than 0.01), urinalysis (P less than 0.05), and chest radiography (P less than 0.05). These effects were independent of the patient's age and sex, the year the physician graduated from medical school, and the availability of testing machinery at the practice site. These findings suggest that membership in a large group practice is associated with increased test ordering for hypertensive patients. This may be due to self-selection of particular types of physicians or patients, or to a property of the organizational structure itself.