Variation in Outpatient Antibiotic Prescribing in the United States

Rates of outpatient antibiotic prescribing vary widely between US commercial health plans. High-utilizing health plans may improve quality and lower costs by reducing unnecessary antibiotic use.

Published Online: December 07, 2009
Michael A. Steinman, MD; Katherine Y. Yang, PharmD, MPH; Sepheen C. Byron, MHS; Judith H. Maselli, MSPH; and Ralph Gonzales, MD, MSPH

Objective: To evaluate variation in outpatient antibiotic utilization among US commercial health plans and the implications of this variation for cost and quality.

Study Design and Methods: We measured antibiotic utilization rates among 229 US commercial health plans that participated in the 2005 Healthcare Effectiveness Data and Information Set. Rates were adjusted to account for health plan age and sex distribution. To estimate antibiotic costs, we multiplied utilization data for each drug class by national estimates of intraclass distribution of drugs, duration of therapy, and median average wholesale price.

Results: Antibiotic utilization rates varied markedly among plans, ranging from 0.64 antibiotic fills per member per year (PMPY) at the 5th percentile of plans to 1.08 fills PMPY at the 95th percentile, with a mean of 0.88 (SD ± 0.15) antibiotic fills PMPY. US census region was the strongest predictor of antibiotic utilization. Antibiotic costs averaged $49 PMPY and ranged from $34 to $63 PMPY among plans at the 5th and 95th percentiles of cost, respectively. If a health plan with 250,000 members at the 90th percentile of antibiotic costs reduced its costs to the 25th percentile, annual drug cost savings would be approximately $4.1 million.

: Antibiotic utilization varies substantially among commercial health plans and is not accounted for by differences in the age and sex distribution of plan members. Because reducing rates of antibiotic utilization is likely to lower costs and improve quality, high-utilizing plans may reap considerable rewards from investing in
programs to reduce the overuse of antibiotics.

(Am J Manag Care. 2009;15(12):861-868)

Outpatient antibiotic utilization varies substantially among commercial health plans in ways not explained by patient case mix. As a result:

  • Antibiotic utilization is likely to be a valuable marker of prescribing quality for health plans.
  • Health plans with high rates of antibiotic utilization may benefit from targeted quality improvement programs to reduce unnecessary antibiotic use.
  • Cost savings from reducing unnecessary antibiotic use are substantial.
Antibiotics are widely overused in ambulatory practice, particularly in the management of acute respiratory tract infections. 1,2 Although bacterial infections cause a small minority of these illnesses, it is estimated that 40% to 50% of all patients in the United States who seek medical attention because of these conditions receive antibiotics.3 The consequences of this overuse are striking. Every year, millions of people are directly exposed to the side effects of antibiotics, ranging from common, bothersome symptoms to infrequent but devastating complications such as Clostridium difficile colitis and anaphylaxis.4 Moreover, research suggests that community levels of bacterial resistance occur in proportion to the volume of community antibiotic use.5-7 Thus,  perpetuation of antibiotic overuse promotes the continued evolution of ominous resistance profiles.

Since early in this decade, the European Surveillance of Antimicrobial Consumption project has been collecting country-level data on  antibiotic utilization and has documented large differences in per capita antibiotic utilization between countries in Europe.8,9 Because the European Surveillance of Antimicrobial Consumption measures are not based on a particular clinical condition, an optimal rate of overall antibiotic utilization is difficult to establish. Nonetheless, clinical evidence and expert opinion strongly suggest that prescribing is most appropriate in countries at the lower end of the spectrum and that the difference between low- and high-utilizing countries to a considerable extent represents potentially unnecessary and avoidable prescriptions.8,10,11

Recently, the United States adopted a similar (albeit more limited) approach to track and report antibiotic utilization. In 2005, the National Committee for Quality Assurance (NCQA) developed and implemented a new measure to compare overall rates of antibiotic utilization between US health plans. By evaluating variation in antibiotic use within a country, these data can improve understanding of factors that contribute to variation in overall antibiotic utilization without the confounding effects of different countries’ healthcare systems. In addition, implementation of this measure in an established program to compare and improve healthcare quality can directly facilitate efforts to improve quality of care by identifying high-prescribing plans and stimulating them to investigate the factors that contribute to potentially excessive antibiotic use.7,8,12-15

In this study, we analyze antibiotic utilization rates among nonelderly members of commercial health maintenance organization (HMO) and point-of-service (POS) health plans in the United States participating in the NCQA’s Healthcare Effectiveness Data and Information Set (HEDIS) program. We quantified the degree of variation in antibiotic utilization rates across health plans, controlling for member characteristics, and estimated the cost implications of this variation.


Data for this study were collected by commercial HMO and POS health plans in the United States that participate in NCQA’s HEDIS program.16 HEDIS is a voluntary program that collects data from health plans on various domains of effectiveness and utilization. These data are used to benchmark and compare the quality of care across health plans. Participating health plans account for more than 85% of individuals enrolled in US commercial plans. We report data from 2005, the first year plans reported data on overall antibiotic utilization for a new HEDIS measure.

We received a core set of data from 248 commercial health plans, representing 83% of commercial plans in established parts of the HEDIS program. The antibiotic utilization measure was a first-year measure. All first-year measures in HEDIS are not publicly reported in order to evaluate the measure, and, as is common, some plans choose not to report during this first year of data collection. We were able to obtain information on approximately 80% of the plans not contributing core data. Plans that did not report data had fewer members than those which did report (55% of nonreporting plans had fewer than 10,000 members vs 7% of reporting plans; P <.001).

Among 248 plans contributing core data, we excluded 4 health plans that did not provide basic data necessary for our analyses, 14 health plans whose rate of overall antibiotic utilization was extreme enough to suggest discrepancies in the way these data were collected or reported, and 1 plan that did not use an HMO or POS model of care. Our final analytic dataset thus comprised 229 health plans.

Our main outcome variable was each plan’s rate of antibiotic utilization per member per year (PMPY), as assessed by pharmacy claims billed. In calculating this rate, we counted each antibiotic prescription fill equally, regardless of drug dose or duration. Our analyses focused on people age 0 to 64 years, as most people age 65 years and older are enrolled in government-sponsored Medicare plans and those remaining in commercial plans may not be representative of the larger population of elders.

Each plan reported antibiotic utilization data for enrollees stratified into age-sex groups. To calculate standardized rates of antibiotic utilization, for each plan we multiplied the PMPY utilization rates for each of these age-sex strata by the proportion of the overall study cohort within each of these strata. Next, we summed each of these weighted rates to create an overall antibiotic utilization rate for all plan members age 0 to 64 years. This approach yielded a PMPY antibiotic utilization rate that adjusted for the age and sex distribution of each plan’s members. This standardized rate yielded figures similar to those of a crude rate that did not adjust for differences in the age and sex distribution of plans (see the eAppendix, available at

Next, we evaluated variation in use of specific types of antibiotics. Health plans provided utilization information for each of 15 classes of antibiotics, following a categorization scheme defined by a  multistakeholder expert panel that worked with NCQA to arrive at a consensus definition that was both clinically relevant and feasible to implement. This panel further grouped each of the antibiotic classes into 2 categories: “antibiotics of concern,” agents generally considered to have broad-spectrum activity; and all other agents (“other antibiotics”), generally considered to have a narrow spectrum of antimicrobial activity (see Table 2). For purposes of clarity, we will use the terms “broad spectrum” and “narrow spectrum” to refer to these categories. We identified potential discrepancies in class-level utilization data for 20 plans. These discrepancies were most likely due to rounding errors, as plan-level data on utilization rates were provided to only 2 decimal places (eg, if the reported utilization rate for an antibiotic class was 0.04 antibiotic fills PMPY, the actual

rate could be anywhere from 0.0351 to 0.0449, an error rate of up to ±12%). We excluded these 20 plans from our analyses of antibiotic classes, plus an additional 3 plans that did not report data necessary to complete these analyses, leaving 206 plans with usable data. Because these discrepancies could affect estimates of drug expenditures, we also excluded these plans from our cost analyses, described below.

Next, we evaluated plan characteristics associated with the overall rate of antibiotic utilization. First, we conducted bivariate analyses using linear regression, where the outcome of interest was the plan’s age-sex adjusted rate of antibiotic utilization and the predictor variables were plan characteristics available from HEDIS that we hypothesized might be associated with antibiotic utilization. Next, we entered all variables into a multivariable linear regression model, on which further diagnostic testing confirmed the adequacy of model fit.

To estimate the cost of antibiotic utilization for each plan, we used several steps (see the eAppendix for details). Health plans provided data on utilization of antibiotic classes but not specific drugs within those classes. To compensate, we used data from the 2004 and 2005 National Ambulatory and National Hospital Ambulatory Medical Care Surveys to estimate the frequency of use of specific antibiotics within each antibiotic class. Next, we estimated the typical course of therapy for each antibiotic, including the most commonly used formulation, dose, dosing frequency, and duration of therapy, repeating this process separately for persons age 0 to 3 years, 4 to 9 years, and 10 to 64 years. For each antibiotic, we used data from the 2005 Red Book to assess the median price per pill (or other formulation), averaging across all manufacturers of a given drug and all bottle sizes.17 For multisource (generically available) drugs, we evaluated only the price for the generic versions. Finally, we calculated the estimated cost of a typical course of therapy for each antibiotic and then integrated this cost at the level of the antibiotic class and ultimately at the level of the health plan.

This research was approved by the Committee on Human Research at the University of California, San Francisco and the Research and Development Committee at the San Francisco VA Medical Center.


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