Primary Care Physician Supply and Colorectal Cancer

Data on physician supply were obtained from the 1994 American Medical Association (AMA) physician masterfile, which includes allopathic and osteopathic physicians regardless of AMA membership. County-level population estimates were obtained from the 1990 United States Census. Physician supply variables were created for total physician supply, and for primary care physician supply and non–primary care physician supply. Physicians were classified as primary care if their self-designated specialty was family practice, general practice, obstetrics/gynecology, or general internal medicine.

Physicians who indicated they were engaged in full-time direct patient care were counted as one full-time equivalent (FTE); those who indicated in the masterfile that they were either “semi-retired,” in residency training, or engaged in teaching or research were counted as 0.5 FTE. Physicians who indicated they were no longer involved in direct patient care were excluded. On the basis of this information, we calculated for each county the proportion of all physicians engaged in primary care and used this as our measure of primary care supply.

Counties were the unit of analysis for our study. We explored associations between primary care physician supply, and colorectal cancer incidence and mortality rates in 2 ways. First we constructed scatterplots to explore possible linear relationships, and to exclude nonlinear associations, and also calculated Pearson correlation coefficients. Second, we used multiple linear regression to explore the multivariable relationship between primary care physician supply and outcomes, controlling for other county-level characteristics.

Parameter estimates were determined using the method of ordinary least squares. Potential confounding variables included in each initial model were median household income, percentage of county residents with less than a high school education, percentage residing in urban census areas, percentage who were white, percentage who were married, and total physician supply. Final regression models included all variables that remained statistically significant (P < .05), using a backward variable selection algorithm. We also confirmed that all outcomes were normally distributed using graphical methods.

Results

The average physician supply for Florida’s 67 counties (physicians per 100,000 population) was 134.9, with primary care supply at 49.7 and specialist supply at 85.2. The average supply of primary care specialties was as follows: family physicians, 17.5; general practitioners, 10.7; general internists, 13.9; and obstetrician-gynecologists, 7.2. There was substantial variation in physician supply, with some counties having as few as 15 physicians per 100,000 population and other counties having more than 500 physicians per 100,000 population. The average proportion of physicians who were in a primary care specialty was 0.36 across Florida’s 67 counties (standard deviation = 0.19; range = 0.17-1.00).

There was also substantial variation in both incidence and mortality rates across Florida’s 67 counties. Some counties had incidence rates as low as 9.6 cases per 100,000 and others as high as 72 cases per 100,000. Mortality rates varied from a low of 3.8 cases per 100,000 to a high of 26.4 cases per 100,000. Incidence and mortality rates were both higher in men than in women.

Associations between primary care physician supply and colorectal cancer incidence and mortality rates were assessed both graphically and using the Pearson correlation coefficient Table 1.* ( Figure 1, Figure 2, Figure 3) Primary care physician supply was negatively correlated with colorectal cancer incidence and mortality rates in the 67 counties studied. For colorectal cancer incidence rates, negative correlations were observed for both proximal and distal cancers, and among both men and women. For mortality rates, correlations were stronger for men and did not reach statistical significance among women. Scatter diagrams did not suggest the presence of nonlinear relationships.

Table 2 presents the results of linear regression analyses. Primary care physician supply was a statistically significant predictor of all outcomes examined. Each 1% increase in primary care physician supply was associated with a reduction in overall colorectal cancer incidence of 0.25 cases per 100,000. Each 1% increase in primary care physician supply was similarly associated with a reduction in overall colorectal cancer mortality of 0.08 cases per 100,000. In stratified analysis, primary care physician supply had similar effects for both proximal and distal cancers, with slightly greater effects among men than women. Overall physician supply was not a significant predictor of any of the outcomes examined.

Discussion

We found that an increasing supply of primary care physicians was associated with lower incidence and lower mortality rates of colorectal cancer in Florida counties. Each 1% increase in primary care physician supply was associated with a reduction in colorectal cancer incidence of 0.25 cases per 100,000 and a reduction in mortality of 0.08 cases per 100,000. In contrast, overall physician supply was unrelated to any of the outcomes examined. Findings were similar in men and women and for proximal and distal cancers.