Only one of the practice prevalence/comorbidity interaction terms was significantly different from 0: commonly occurring conditions presenting among patients with high levels of comorbidity. This finding implies that the comorbidity has a stronger influence on the chances of referral for patients presenting with common problems than those presenting with less common problems.
Table 4 shows the estimated probabilities of referral based on differences in practice prevalence and comorbidity. These probabilities were obtained from the b coefficients in Table 3. The reference group for the probability estimates is women aged 18 to 44 years with health problems categorized as medical conditions. The chances of referral varied as much as 8-fold based on only the practice prevalence of the principal diagnosis and level of comorbidity.
Discussion
Our results support the hypothesis that the frequency with which patients’ health problems present to primary care physicians (practice prevalence) has a strong inverse relationship with the chances of referral to specialty care. Primary care physicians were more likely to send patients with uncommon problems to specialists and retain those with the most common conditions. This finding highlights the responsible judgment primary care physicians employ in recognizing the boundaries of their scope of practice. Practice prevalence is a defining feature of the primary care–specialty care interface.
Referring patients with uncommon problems to specialists is a rational way to organize medical care. Outcomes are related to the volume of patients managed with a specific condition.18 Specialists need to care for an adequate number of patients with uncommon problems to maintain clinical competence. Patient self-referral, however, which dilutes the prevalence of health problems presenting to specialists, may result in potentially invasive and expensive diagnostic approaches to patients more appropriately evaluated by primary care physicians.19
In addition to a condition’s practice prevalence, the number and severity of comorbidities managed during the visit influenced primary care physicians’ decisions to make specialty referrals. Also, we found an interaction effect between high practice prevalence and high levels of comorbidity. In other words, patients with uncommon conditions were commonly referred, regardless of the complexity of other conditions. The chances of referral markedly increased for patients with common conditions when they also presented with co-existing medically complex health problems. Thus, the rare presentations for which specialist assistance is sought may be a result of either the practice prevalence of the presenting problem or the overall complexity of a patient.
Men were more commonly referred than were women, after accounting for differences in the nature of their problems. A possible explanation for this finding is that because women make more office visits over a year than men,20 their probability of referral during any given visit will be lower given roughly equal chances of referral between the 2 groups during the course of a year.
Further Research
We demonstrated that the potential need for surgical interventions was an important predictor of referral. Even after other clinical factors were controlled, medical conditions were 39% less likely to be referred than surgical ones. This is not surprising given that primary care physicians generally perform only minor office-based surgical procedures. But which surgical procedures should be in the scope of practice of primary care physicians? This question deserves further research and could be addressed in part by an analysis that is similar to the one presented here. Common outpatient procedures are candidates for inclusion as primary care services. Secondary considerations include the requirements and expense of necessary equipment, technical personnel, and training. Research that builds epidemiologic profiles of office-based procedures would be helpful in determining how responsibilities should be divided between generalists and specialists for these technical services.
Limitations
Several limitations in our study’s data source warrant consideration. First, the data set of visits provided information on primary care physicians’ referral decisions and did not elucidate whether patients actually received specialty care. Second, the sample was restricted to visits made to generalist physicians, excluding both obstetrician-gynecologists and medical subspecialists who may act as primary care physicians. Third, the NAMCS data set did not include hospital-based physicians, who are known to have higher referral rates than their office-based counterparts.13 Fourth, the unit of analysis was the visit rather than the patient. Patients with certain chronic conditions may have higher referral rates than suggested by our data if the measure used is the percentage of persons obtaining specialty care over a year. The advantage of focusing on the visit is that physician referral decisions can be examined rather than specialist use. Fifth, some conditions had lower than expected referral rates (eg, appendicitis had a referral rate of 46%), because the denominator for the referral rates was all visits made to generalists for the condition, which included both new presentations and follow-up visits. Finally, because of data limitations we did not assess the extent to which condition prevalence within an individual physician’s own practice affects his or her referral behavior.