The Surgical Nosology In Primary-care Settings (SNIPS): a simple bridging classification for the interface between primary and specialist care

A panel of specialists at a large general hospital, Royal Darwin Hospital (RDH), in northern Australia, was convened to reach agreement on the major categories of surgical disorders. The panel included two general surgeons, an orthopaedic surgeon, an ophthalmologist, an obstetrician-gynaecologist, and an ENT registrar, all of whom were actively involved in both inpatient and outpatient care. Other specialists' advice was sought when necessary. The panel was asked to delineate the main categories of conditions and work in each of their specialties. For guidance, they were asked to consider the way conditions and procedures are grouped in subspecialty societies, and when presented at conferences or in journals and textbooks.

After an initial draft, the panel agreed that a four-level single axial hierarchy would be economical and still provide specificity for the most important groups of surgical disorders. The top three levels were to be delineated by the panel, and existing terms designed for primary care were to be used as the fourth level. The names of the major surgical specialties (e.g. general surgery, orthopaedic surgery) were allocated to Level 1. The first, second and third levels were given three, four and five-letter codes respectively, while the fourth level retained the original six-digit codes of the primary care terminology. An example of related Level 1, 2 and 3 categories is "Orthopaedics" (ORT), "Shoulder" (SHLD) and "Shoulder Injury/Instability" (SHINJ), to which terms for "Sprain;shoulder" (L79021), "Injury;rotator cuff" (L79041) and "Dislocation;shoulder" (L80003), among others, were classified. Another Level 3 category under "Shoulder" is "Shoulder Arthritis/Pain" (SHART), to which terms concerning osteoarthritis, rheumatoid arthritis, rotator cuff syndrome and frozen shoulder were classified.

We aimed for the hierarchy to meet the criteria for a robust classification, especially that the structure is appropriate for the majority of referrals from primary care; that it makes sense to specialists in all surgical disciplines; that categories are mutually exclusive; that each term has a place and that there is only one place for each term [11].

In development of the hierarchy some disciplinary areas were discrete and some overlapped. Where the overlap was such that several of the subcategories may be routinely managed by one or other of these specialties, the specialties were combined as one heading. Specialties whose work derives mostly from sources other than primary care, such as cardiothoracic surgery and transplant surgery, were grouped as "Other specialities". To maintain similar specificity at each level in each specialty, the panel decided that a three-level classification was not required in ENT surgery, urology, ophthalmology and obstetrics, in which cases Level 2 was omitted. Finally, a few Level 2 and Level 3 categories could have been allocated to more than one parent category. Examples included the allocation of hand surgery to 'Forearm, wrist and hand' under orthopaedics rather than plastic or general surgery, and thyroid surgery to breast/endocrine not ENT surgery. In such cases consensus about the most appropriate allocation was reached through panel discussion.

We used terms from the International Classification of Primary Care, Version 2-Plus (ICPC-2 Plus) [12, 13] for three main reasons. First, the validity and reliability of ICPC as a research tool in primary care have been established [13‐15]. Second, ICPC-2 Plus contained, at the time of this work, an extended vocabulary of 7640 'terms' derived from terminology used in over 500,000 general practice encounters. These terms greatly enhance the specificity of coding compared to what was possible with the 1388 higher-order 'rubrics' in ICPC-2. Third, ICPC-2 Plus has been used for the study of general practice activity in Australia since 1998 [16‐18] and a database was available to facilitate development of our nosology.

Furthermore, as part of ICPC-2 Plus, the Family Medicine Research Centre at the University of Sydney has developed an electronic interface providing easy keyword-prompted selection of appropriate ICPC-2 Plus terms and facilitating its integration into other databases [see: http://​www.​fmrc.​org.​au]. In addition, ICPC has widespread international use, and has been implemented in resource-poor settings where outreach, the subject of our research, is likely to be particularly relevant.

The codes for ICPC-2 Plus terms consist of a letter and five digits. For example, cataract can be coded as 'Cataract' (F92001), 'Cataract;senile' (F92002) and 'Lenticular opacities' (F92003). The letter indicates the ICPC-2 'chapter', organised mostly in body systems ('F' refers to the Eye). The letter plus two digits comprise the ICPC-2 'rubric', or major concept being represented (in this case, 'cataract'). The remaining three digits delineate terms that enable variations in and different representations of these rubrics to be coded more accurately. Due to their greater specificity, we classified the ICPC-2 Plus terms (rather than the broader ICPC-2 rubrics) to the Level 3 categories.

Reference was made to the Bettering Evaluation And Care of Health (BEACH) study, the largest analysis of general practice activity in Australia, and which used ICPC-2 Plus for coding problems and activities. In the ongoing BEACH study approximately 1000 GPs randomly selected from all practising GPs in Australia are enrolled each year, and each enrolled GP prospectively records information regarding 100 consecutive encounters with patients, including the problems managed and new referrals made [17]. The BEACH data is cross-sectional and encounter-based.

In the development of SNIPS, we used the first three years of the BEACH data (1998–2001), which included 303,000 patient encounters and the 'terms' describing 449,277 problems managed. We allocated each term that had been recorded at least once in the dataset to the most appropriate Level 3 category. Conditions that could only be related indirectly to surgical specialties were coded to the group most likely to be relevant (e.g. 'Diabetes non-insulin dependent' to 'diabetes/systemic/retinal' under ophthalmology). Any that were obviously unrelated to surgery (eg Psoriasis) were categorised as 'Non-surgical', and those that were too non-specific to determine (eg 'Pain:arm') were categorised as 'Other.'

At this stage a preliminary mapping of any potential surgical terms had been achieved. The final stage of development of the classification aimed to define a restricted set of 'surgical problems' by examining which problems had been referred to surgeons in the BEACH study. Referrals were counted if they were coded in ICPC-2 Plus as having been made to any of a broad range of specialties in which surgical procedures form a large part of practice, including endoscopy and clinics usually attended by surgeons or obstetricians, such as breast, antenatal and in-vitro fertilisation clinics. In principle, surgical problems could be defined as those problems that a large, nationally-representative sample of GPs refers to surgical specialists via private clinics, hospital outpatients and emergency departments.

In practice, while this generally held true, there was also a considerable amount of 'noise' in the data, particularly from the very common presentations to general practice, such as hypertension and depression, which were recorded as being referred to a surgical specialist in a very small number of cases. Explanations for these aberrations undoubtedly lie in the fact that problems in primary care are not always discrete. Consider an elderly patient with multiple problems who has had trouble with her vision and who is depressed. Her poor vision may be contributing to her depression, and as part of its treatment, a referral to an ophthalmologist may be arranged. It is easy to see how, in this case, the problem of 'depression' can result in a specialist ophthalmological referral. However the rate of these referrals, relative to the rate of management of depression in primary care, for example, is trivial. The implication of having depression in the classification is that while it is not a surgical condition in any real sense, it could lead to many more encounters being included in studies of surgical problems. Analyses would therefore use larger denominators, and have lower estimates of referral rates and potentially less precise estimates. It was clear that inclusion of problems such as depression would reduce the usefulness of the nosology.

To improve the precision for 'real' surgical problems we defined a threshold percentage of total encounters with a problem in which a new surgical referral was initiated. Terms for which the percentage referred was above the threshold were automatically included. Terms not referred at all were excluded. Terms for which the percentage referred was between zero and the threshold value were then submitted to the most appropriate member of the panel and included only if he or she indicated that patients with that particular problem would be "a usual part of practice in that specialty." Prior to defining the terms to send to the panel we performed a sensitivity analysis using threshold percentages of 0, 0.01%, 1%, 5%, 10%, 15% and 20%. A threshold percentage was chosen to maximise both the number of surgical referrals in the data and the percentage of terms in the data that were referred.

SNIPS has been used in three ways to date. The first was to interrogate the BEACH data, already coded in ICPC-2 Plus terms, to describe in detail referrals to surgical specialists in a national sample [9]. The aim was to inform policy regarding surgical workforce distribution and training requirements, and the data were presented in the SNIPS Level 1, 2 and 3 categories. The second was to identify potential surgical problems in the same dataset and then determine whether proximity of a specialist influenced general practitioners' referral rates [10]. SNIPS was used to define and limit referrals that were to be included in the analysis, and to facilitate analysis of individual specialties separately. The third way SNIPS has been used, and the original impetus for its development, was to classify data collected in a primary care setting for a population-based study of the impact of specialist outreach visits on referrals and access to specialist care. A simple 'look-up' engine involving the ICPC-2 Plus keywords was developed, data were stored as ICPC-2 Plus terms in a Microsoft Access database, and results were reported for Levels 1, 2 and 3 as required.