Trends in Well-Child Visits to Family Physicians by Children Younger Than 2 Years of Age

National Ambulatory Medical Care Survey

The NAMCS is administered by the National Center for Health Statistics (NCHS) for the Centers for Disease Control and Prevention (CDC).⁸ The NAMCS collects information on patient visits to office-based physician practices in the United States, including federally qualified health centers and nonfederal government clinics. The survey incorporates a multistage probability design, accounting for practice location, physician specialty, and individual visits within the practice.

Physicians are randomly selected from the master files of the American Medical Association and the American Osteopathic Association, and then randomly assigned to a 1-week reporting period. During this period, data for a systematic random sample of visits are recorded by the physician or office staff on a standardized encounter form and checked for completeness by NCHS field staff.

The survey collected 348,881 patient visits between 1995 and 2007.⁸ The participation rate of contacted physician practices in the NAMCS ranged from 59% in 2005 to 73% in 1995. Quality control was performed using a 2-way independent verification procedure for 10% of the sample visits. In 2007, coding errors for various items ranged from 0% to 1.6%.

The NCHS weights each visit to allow extrapolation to national estimates for all aspects of the survey. The weighting calculations account for practices that were invited to participate but declined to do so. National estimates are considered reliable with a standard error of 30% or less, which generally corresponds to a sample of at least 30 patient visits.⁸

Study Variables

Deidentified clinical and demographic data, including age, insurance status, and race, were collected for each visit.⁹ The NCHS institutional review board approved the protocol for the NAMCS, including a waiver of the requirement for informed consent. Health professional variables included self-reported specialty. Clinical variables included the primary diagnosis and up to 2 secondary diagnoses coded according to the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM).¹⁰ The NCHS divides the country into 4 geographic regions—Northeast, South, Midwest, and West—in NAMCS. Rural locations were identified based on the designation of non–metropolitan statistical area by the US Office of Management and Budget.

We examined all office visits with the primary diagnoses of routine infant or child health check (ICD-9-CM codes V20, V20.1, V20.2), routine general medical examination (V70.0), and health examination of defined subpopulations (V70.5). To most accurately capture the direct impact that the location, distribution, and timing of prenatal services by family physicians may have on well-child care, we limited well-child office visits to children younger than 2 years of age. We further categorized visits by children into 3 age categories: (1) younger than 6 months, (2) 6 to 12 months, and (3) 1 to 2 years. Additionally, we eliminated visits to physicians whose specialty was not family medicine/general practice or general pediatrics.

Statistical Analysis

We calculated standard errors (SEs) for all results as recommended by the NCHS using Stata software, version 10.0 (StataCorp, College Station, Texas).⁹ Stata was programmed with the masked survey design variables to take into account the complex multistage design of the survey that allows for making population estimates and generating variance estimates that result in conservative tests of significance.¹¹ The NCHS weights each visit to permit extrapolation to national estimates for all aspects of the survey. We used these visit weights for all analyses.

We performed linear regression analysis with biennial period as the predictor variable to assess for a trend in the proportion of all US well-child visits by children younger than 2 years of age provided by family physicians. To decrease annual variation in visit estimates, we combined 2 successive years of data (1 year for the last period), as recommended by NCHS, for a total of 7 periods (1995–1996, 1997–1998, 1999–2000, 2001–2002, 2003–2004, 2005–2006, 2007) for the trend analysis.

Categorical variables were evaluated with the χ² test. All statistical tests take into account data from all 13 years, from 1995 through 2007. All P values are 2-tailed; P <.05 was considered significant.