(1) Case data. Methicillin-resistant
S. aureus (MRSA) bacteraemia case data by hospital for the financial year 2007-08 (6 April 2007 – 5 April 2008) were received from Health Protection Scotland (HPS). This financial year was chosen because at the time it was the only year for which data were available for all risk factors (hospital size, type and connectivity). (2) Risk factors
. Data of potential risk factors were obtained from Information Services Division (ISD) for each hospital for the financial year 2007-08 (a full list of variables in Additional file
1). As the aim was to test specific hypotheses regarding the effect of hospital size, type and connectivity; the majority of the data that were obtained represent different measures of those characteristics. Although various other measures including average occupancy rate, total number of whole time equivalent (medical and dentistry, nursing and midwifery, domestic, and support services) staff, patient-staff ratios (number of patients to medical and dentistry, nursing and midwifery, domestic, and support services staff), and cleaning by hospital size were considered (Additional file
1). (2a) Hospital size. Measures of size included occupied bed days (OBD), surface area (m
2), average staffed beds, total inpatients discharged, total staff members and others (Additional file
1). (2b) Hospital type. ISD group Scottish into the following categories: Category A (General (mainly acute) including Teaching (A1), large General (A2), General (A3), and Sick children’s (A4)); Category B (Long stay), Category C (Mental); Category D (psychiatry of learning difficulties); Category E (Maternity); and Category J (Community). Information on the presence and absence of 46 specialties was also acquired. For a complete list of these specialties see Additional file
2. (2c) Connectivity. To quantify movements of patients between hospitals, patient admission data were obtained from ISD. The patient admission data covered all admissions to healthcare facilities in Scotland for the calendar year 2007 (1 January – 31 December 2007). From this dataset the movements of all patients between hospitals were extracted either as direct transfers, i.e. from one hospital directly to another hospital, or as indirect transfers, i.e. when a patient was discharged from one hospital into the community and subsequently (within the period covered by the data) admitted to another hospital. From these data, a movement matrix was derived for all connected hospitals in Scotland [
11] and then various summary measures of hospital connectivity were generated (Table
1).
Table 1
Summary of connectivity measures
Patients in | Total number of patients moving to this hospital from other hospitals adjusted by number of staffed beds | 3.61 | 3.26–4.09 | <0.001 |
Indegree | Number of hospitals that transferred patients to this hospital [ 11] | 11 | 6–25 | <0.001 |
Outdegree | Number of hospitals that receive patients from this hospital [ 11] | 8 | 8–12 | <0.001 |
Closeness centrality | Normalized measure of the centrality of a node in a network based on the mean length of all shortest paths from that node to every other reachable node in the network. [ 24] | 0.3898 | 0.3898–0.3915 | <0.001 |