In total 4536 patients were seen in PC+; 69.0% of them were referred back to the GP after PC+ consultation, whereas 29.1% were referred to secondary care. The outcome was unknown in 2.9% of patients. There was wide variation in the rate of referral to secondary care after PC+ consultation between specialties (8.6% (gynaecology) to 43.8% (orthopaedic surgery)), specialists (14.5 to 65.2%) and diagnosis groups (11.1 to 93.4%). These differences underline that, when participating in substitution initiatives like PC+, some specialties, specialists and diagnosis groups seem more appropriate than others. Differences between specialties might be explained by a variation in the need for additional in-hospital facilities and the possibility to equip PC+ consultation rooms with diagnostic devices. Differences between specialists from the same specialty may be caused by the individual way a specialist performs his or her occupation. A subspecialist who narrowed his scope down to a limited amount of diseases seems less equipped to work in a primary care setting than a specialist with a broader scope and interest in his field of specialization. Selection of specialists according to their profile might therefore reduce referrals to secondary care. Experience within PC+ may be another factor that influences referrals to secondary care; the more patients a specialist has seen in PC+, the better he is adapted to using fewer diagnostic tools, and as a consequence the fewer referrals to secondary care he is expected to make. Differences in referral patterns between diagnosis groups might be explained by the varying need for diagnosis specific in-hospital equipment. Whether referrals back to the GP reflect true substitution cannot be concluded based on this data, but will be subject to future reports about the PC+ intervention. Nonetheless, this information should be used to select appropriate specialties, specialists and patient groups for PC+. Combining those efforts might result in further declines of referral from PC+ to secondary care.
Few studies that explore the effectiveness of substitution initiatives have been published [7,8,9,10,11,12,13,14]. Most of them examined small-scale substitution initiatives with shared GP-specialist consultations in GP practices. Gruen et al.  reviewed a wide variety of studies evaluating the effectiveness of outreach clinic initiatives across the UK. This study showed ambiguous results regarding the effectiveness of outreach clinics in reducing referrals to secondary care. No clarity on specialty-specific referral patterns was provided. Vierhout et al.  explored the effects of shared consultations by GPs and orthopaedic surgeons. Within this study the orthopaedic surgeons held consultation hours in GP practices across the region of Maastricht in the Netherlands. The GP was able to attend the consultations in order to learn, share knowledge and deliberate with the specialist. Vierhout et al. found a 33.0% decline in referrals to secondary care within the intervention group compared to care as usual. Schulpen et al. [12, 13] found comparable results when exploring the effectiveness of joint consultations in GP practices for rheumatology patients, and described a positive learning effect for GPs after shared consultations. Van Hoof et al. [7, 8] presented comparable outcomes during a feasibility study of PC+ consultations in GP practices in 2013. Based on interviews with GPs and specialists it was expected that 32.0% of patients would remain in primary care after PC+ consultation. By contrast, exploring the current arrangement of PC+ in this study (specialist consultations in two out-hospital facilities) shows that 69.0% of all patients are sent back to the GP after consultation. This arrangement therefore seems most promising in achieving substitution of secondary care with primary care.
A report from NIVEL  indicated that care as usual referrals (i.e. without PC + -like initiatives) from the GP to secondary care vary between patient groups and are partly attributed to specialty-, GP- and patient group-specific characteristics. According to this report, in 2012 more female patients (303 per 1000) than male patients (220 per 1000) received a referral to secondary care, and older patients were likelier to end up in secondary care than younger patients. For both females (36%) and males (48%), most referrals were made for musculoskeletal complaints. Reflecting on the referral numbers in this study, it seems reasonable that the orthopaedic surgery and rheumatology departments refer most patients to secondary care. Also, gynaecology patients in this study were considerably younger than patients from other departments, which might partly explain the low number of referrals to secondary care. Further identification of specialty-, specialist- and patient group-specific factors that influence the chances of a referral to secondary care might be helpful in deciding which parts of secondary care are appropriate for substitution with primary care.
This study found differences in referrals after PC+ between several specialties, specialist and diagnosis groups. The data however, does not provide explanations for these differences that could be beneficial when improving substitution initiatives like PC+. Also, a small discrepancy between referral numbers based on the PC+ databases and true referral information from the Maastricht University Medical Centre was found, which could indicate that referral numbers from the PC+ databases do not correctly reflect actual patient flows. If considerable amounts of patients end up in secondary care shortly after a PC+ consultation, PC+ might, as a consequence, become an additional step in between primary and secondary care, generating double care loops instead of substitution. As was underlined by the head of the department of otorhinolaryngology, the fact that the department still treats roughly 8500 patients per year inside the hospital, as it did prior to the PC+ project, and an additional 1000 patients in PC+, could indicate that PC+ leads to a supply-driven demand. However, this could have also resulted from a priming effect of PC+ on patients from other regions, since waiting times to access specialist care have decreased after the foundation of PC+. This effect is not necessarily unbeneficial because GPs potentially refer their patients in an earlier stage to PC+ than they would to secondary care, which could prevent delays in diagnosis and treatment and, as a consequence result in higher quality of care. Analysis of patient flows on a national level, as well as comparative analyses of health outcomes will be conducted to address these questions. Also, since several specialties make use of a second consultation for a considerable number of their patients, and as equipment in PC+ consultation rooms is expanding with costly medical devices that do not reflect a primary care setting, a situation in which PC+ is hitting the target of fewer referrals to secondary care but missing the point of substitution of low complex hospital care might develop. Improving referral numbers from PC+ by transforming the PC+ facilities into small-scale hospitals is undesirable and might eventually lead to higher care costs. Finally, when generalizing this data, two main region specific factors should be considered that impact the feasibility to achieve substitution. First, in regions like Maastricht-Heuvelland, specialists receive a fixed salary regardless of the number of visits to the outpatient clinic of the hospital, which increases the willingness to adopt substitution initiatives like PC+ among specialists. This could translate into improved substitution compared to regions where a declining number of patients seen in secondary care does directly affect the specialists’ income. Therefore, varying specialist reimbursement agreements are important region specific factors that should be considered when generalizing this data. Second, specialists with a more generalist approach seem better equiped to work in initiatives like PC+, as opposed to super-specialists. Since the Maastricht University Medical Centre fulfils both the role of a highly specialized tertiary care provider and a regional hospital where non-super-specialized routine care is being delivered, improved substitution is expected since more generalist doctors could have taken part in the PC+ initiative. Substitution could be hampered when hospitals that merely deliver highly specialized care take part in these initiatives.