RESEARCH ARTICLE Open Access Socioeconomic disparities in orthodontic treatment outcomes and expenditure on orthodontics in England’s state-funded National Health Service: a retrospective observational study Juliet Price ¹ , William Whittaker ² ⁰ , Stephen Birch ³ , Paul Brocklehurst ⁴ and Martin Tickle ⁵
Abstract
Background: This study aimed to assess whether there are potential areas for efficiency improvements in the National Health Service (NHS) orthodontic service in North West England and to assess the socioeconomic status (SES)-related equity of the outcomes achieved by the NHS. Methods: The study involved a retrospective analysis of 2008–2012 administrative data, and the study population comprised patients aged ≥10 who started NHS primary care orthodontic treatment in North West England in 2008. The proportions of treatments that were discontinued early and ended with residual need (based on post-treatment Index of Orthodontic Treatment Need [IOTN] scores that met or exceeded the NHS eligibility threshold of 3.6) and the associated NHS expenditure were calculated. In addition, the associations with SES were investigated using linear probability models. Results: We found that 7.6% of treatments resulted in discontinuation (which was associated with an NHS annual expenditure of £2.3 m), and a further 19.4% (£5.9 m) had a missing outcome record. Furthermore, 5.2% of treatments resulted in residual need (£1.6 m), and a further 38.3% (£11.6 m) had missing IOTN data (due to either a missing outcome record or an incomplete IOTN outcome field in the record), which led to an annual NHS expenditure of £13.2 m (44% of the total expenditure) on treatments that are a potential source of inefficiency. Compared to the patients in the highest SES group, those in the lower SES groups were more likely both to discontinue treatment and to have residual need on treatment completion. Conclusions: Substantial inefficiencies were evident in the NHS orthodontic service, with 7.6% of treatments ending in discontinuation (£2.3 m) and 5.2% ending with residual need (£1.6 m). Over a third of cases had unreported IOTN outcome scores, which highlights the need to improve the outcome monitoring systems. In addition, the SES gradients indicate inequity in the orthodontic outcomes, with children from disadvantaged communities having poorer outcomes compared to their more affluent peers. Keywords: Orthodontics, Malocclusion, Treatment outcomes, Socioeconomic status * Correspondence: william.whittaker@manchester.ac.uk ²Manchester Centre for Health Economics, School of Health Sciences, University of Manchester, Manchester, UK Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Background
In England, NHS expenditure on the primary care ortho- dontic service amounts to approximately £250 m per year [1, 2], which makes ensuring maximum value for money in the NHS orthodontic service critical. Moreover, the ex- penditure may continue to increase as a result of increases in social acceptance of fixed orthodontic appliances and expectations regarding dental appearance [3, 4]. However, the NHS is currently under a great deal of pressure to re- duce expenditure because NHS commissioners and pro- viders ended 2015/2016 with an aggregate deficit of £1.85 billion (a threefold increase on the previous year) [5]. A number of studies, which were largely hospital-based studies, have highlighted the potential for suboptimal out- comes in the NHS orthodontic service in England and Wales. These studies have focused on both discontinu- ation (i.e., failure to complete a course of active treatment) [6, 7] and suboptimal outcome scores based on occlusal indices such as the Index of Orthodontic Treatment Need (IOTN) [8], the Index of Complexity, Outcome, and Need (ICON) [8, 9], and the Peer Assessment Rating (PAR) index [8, 10] (the PAR index was developed as a simple tool for assessing the orthodontic outcomes of groups of patients, rather than individual patients and, as part of the NHS orthodontic outcome monitoring system, providers are required to report PAR scores for 20 patients plus 10% of the additional patients [11]). In addition, several studies have explored the associations between socioeconomic status (SES) and suboptimal ortho- dontic outcomes in the NHS. For example, a hospital-based study in England of 144 orthodontic patients aged between 9 and 19 reported that SES was not associated with dis- continuation [6]. In contrast, a study based on 1990–1991 Dental Practice Board data from England and Wales re- ported a positive association between deprivation and dis- continuation [7]. Moreover, a hospital-based study in England found a positive association between deprivation and 'low or no improvement in occlusion' (defined as a score lower than the sample median ICON improvement score of a modified version of the ICON), based on an ana- lysis of 135 12- to 16-year-olds treated for 1 year with fixed orthodontic appliances [9]. In this study, we aimed to identify the scale of poten- tial efficiency improvements in the NHS primary care orthodontic service. To do this, we quantified the pro- portion of, and NHS expenditure on, treatments that were discontinued early and those that ended with re- sidual post-treatment need. Secondly, we aimed to ana- lyse whether the proportions of treatments ending in discontinuation and residual need differed by SES in order to assess the equity of the outcomes achieved by the NHS and whether initiatives to improve the effi- ciency of the service should be targeted at specific populations.
Methods
The study involved a retrospective analysis of data from 1 January 2008 to 31 December 2012 from all orthodontic activity records (i.e., FP17O records) submitted by primary care dentists who were working under state-funded NHS contracts in North West England [12]. The pseudo- anonymised data set was provided by the National Health Service Business Services Authority (NHSBSA). The study population comprised patients ≥10 years old who started orthodontic treatment in 2008. The analysis was restricted to patients aged ≥10 because those aged ≤9 receive only interceptive treatment (which amounted to 1.6% of the treatments in the data set), while those aged ≥10 receive full courses of treatment. The NHS provides the vast majority of orthodontic treatments for children in the UK, with only 2.4% (95% CI: 0.7–4.0%) of children having private treatment by the age of 15 [13]. As the data set used in this study represented the entire population of patients aged ≥10 who received orthodontic treatment under state-funded NHS contracts in North West England, inferential statistics were unnecessary. The data set included clinician-reported information on whether each treatment was completed or discon- tinued, along with IOTN outcome scores that were used to calculate residual need (these are assessed at the final appointment before treatment completion or discontinuation). Discontinuation was defined as failure to complete a course of active treatment before the end of the treatment plan (as recorded by clinicians in the FP17O records). Dis- continuation can be initiated by patients (who can request to stop treatment early or fail to return for treatment; once it has been established that a patient does not wish to return, the dentist is required to submit an outcome record) or by dentists (e.g., when a patient fails to comply with oral hygiene advice, repeatedly misses appointments, or breaks their orthodontic appliance) [12]. Residual need was measured based on whether each patient's IOTN score at the end of active treatment met or exceeded the NHS IOTN eligibility threshold of 3.6 (i.e., a Dental Health Component [DHC] score of 3 with an Aesthetic Component [AC] score of 6) [14]. Discon- tinuation and residual need are not mutually exclusive in that, for example, a patient can discontinue treatment (e.g., because the patient's parents can no longer take their child to orthodontic appointments) and yet have no residual need by the time they discontinue (despite not completing the full planned course of treatment). As self-assessment of treatment outcomes can be subject to bias, providers are encouraged to utilise the services of an independent third party calibrated in the use of the IOTN and the PAR index [15]. In addition, the NHSBSA carries out independent monitoring of five patients of 450 orthodontic clinicians per year, which involves requesting full patient records, including photographs, radio- graphs, and pre- and post-treatment models of the pa- tients' occlusions [16]. While this monitoring process often involves flagging up issues for discussion with the orthodontic clinicians (which are then resolved without further scrutiny), it very rarely leads to further investi- gation being conducted [16]. Firstly, we calculated the proportions of patients who a) discontinued treatment, b) completed treatment, and c) had a missing outcome record (i.e., no record submit- ted within the study period indicating that treatment had been completed or discontinued). The analysis was restricted to treatments that started in 2008, treatment usually takes 18–24 months to complete [17], and the data set provided outcomes for the subsequent 4 years, so missing outcome records were considered to be re- flective of reporting issues. Secondly, we calculated the proportions of patients with residual need, no residual need, and incomplete IOTN outcome fields among a) patients who discontinued treat- ment and b) those who completed treatment. Thirdly, we calculated the overall proportions of patients who had re- sidual need, no residual need, incomplete IOTN outcome fields, and missing outcome records. NHS expenditure was determined by calculating the numbers of Units of Orthodontic Activity (UOAs) carried out in 2008 by the NHS orthodontic providers. Under NHS payment arrangements, the quantity of services that an orthodontic provider is contracted to deliver is expressed on the basis of an annual target number of UOAs (as agreed by the NHS and the provider), and the annual payments are issued as monthly instalments, 1 month in arrears [1]. Courses of treatment (including an initial assessment) for patients aged 10–17 attract 21 UOAs and those for patients aged >17 attract 23 UOAs; one UOA (the value of which differs between different contracts) has a mean value of approximately £59 [1]. To assess whether the proportions of treatments ending in discontinuation and residual need differed by SES, or- dinary least squares (OLS) linear regressions were used (whilst not used to generate statistical tests, this approach provided details on the proportions of children with the treatment outcomes and enabled our analyses to be ad- justed for potential confounding). The data set included area-level SES data in the form of Index of Multiple Deprivation (IMD) scores [18] that had been matched by the NHSBSA to the patients' postcodes at the level of Lower-layer Super Output Areas (LSOAs). Cut-points for IMD quintiles (with respect to the population of England) were used to calculate each patient's IMD quintile. The first SES analysis assessed whether discontinuation dif- fered by SES, and the second and third analyses assessed whether residual need differed by SES for patients who discontinued and completed treatment, respectively. Initially, unadjusted SES analyses were carried out (i.e., only the SES dummy variables were included in the models). Subsequently, the analyses adjusted for poten- tial confounding (to separate the effect of confounders from the effect of SES) by adding the following potential confounders to the models: IOTN AC score at the start of treatment (1–10; which was used to reflect the com- plexity of treatment), gender, and age. This helped to en- sure that any SES-related differences would not reflect, for example, differences in treatment complexity or late uptake of treatment. In each model, the SES coefficients represent the mean percentage-point difference (compared to being in the ref- erence category, which was the least deprived group) in the proportion of children with the treatment outcome variable (discontinuation or residual need) when all other variables besides SES (i.e., the potential confounders) are held constant. To assess potential biases to the SES analyses due to incomplete IOTN outcome fields and missing outcome records, we explored whether the number of incomplete IOTN outcome fields differed by SES (for patients who discontinued and completed treatment) and whether the number of missing outcome records differed by SES. The statistical analyses were conducted using Stata version 13 [19].
Results
There were 24,501 treatment starts in 2008. Of these, 4746 (19.4%) were missing an outcome record, while the remaining 19,755 (80.6%) had an associated outcome record. Of the 19,755 outcome records, 4636 (23.5%) had incomplete IOTN outcome fields (3456 for patients who completed treatment and 1180 for those who dis- continued treatment). As shown in Table 1, 7.6% of treatments were discon- tinued and 73.1% were completed, while the remaining 19.4% had missing outcome records. Among the treat- ments associated with outcome records ( n = 19,755), 9.4% were discontinued and 90.6% were completed. The proportion of patients with residual need among those who discontinued and completed treatment was 16.3% and 5.4%, respectively. As shown in Table 2, overall, 5.2% of patients had re- sidual need and 56.5% had no residual need, while the remaining 38.3% had either incomplete IOTN outcome fields (18.9%) or missing outcome records (19.4%). Among the treatments with IOTN outcome scores (n = 15,119), 8.4% had residual need and 91.6% had no residual need. Compared to female patients, male pa- tients were more likely, overall, to discontinue treatment and to have residual need. In 2008, £2.3 m was expended by the NHS on treat- ments in North West England for patients aged ≥10 that
GBP Great Britain Pound, IOTN Index of Orthodontic Treatment Need, NHS National Health Service ªAn outcome could be specified as either treatment discontinuation or treatment completion ended in discontinuations (Table 1). The corresponding value for treatments that were discontinued or completed with residual need was £1.6 m (Table 2). A further £11.6 m was expended on treatments that had missing IOTN out- come data (Table 2). Thus, residual need and missing IOTN outcome data was associated with an expenditure of £13.2 m (i.e., 44% of the total NHS expenditure). Table 3 shows the treatment outcomes by SES. Discontinuation was more likely for those in the lower SES groups compared to those in the highest SES group, and this association persisted after adjusting for poten- tial confounders (Model 1); the distributions of the po- tential confounders by IMD quintile are provided in Additional file 1. Furthermore, among the patients who completed treatment, those in the lower SES groups were also more likely to have residual need than those in the highest SES group, and this association persisted in the adjusted analysis (Model 2). However, among the pa- tients who discontinued treatment, there was no associ- ation between SES and residual need (Model 3). As shown in Table 4, higher SES was associated with incomplete IOTN outcome fields among patients who completed treatment (Model 4), but there was no associ- ation with SES among those who discontinued treatment (Model 5). In contrast, lower SES was associated with missing outcome records (Model 6). The SES-related as- sociations persisted in the adjusted analyses.
Discussion
In North West England, 7.6% of orthodontic treatments that were started in 2008 for patients aged ≥10 resulted in discontinuation and 5.2% resulted in residual need.
AC Aesthetic Component, IMD Index of Multiple Deprivation, IOTN Index of Orthodontic Treatment Need ªOnly patients with outcome records could be included (4746 had missing outcome records) bOnly patients with completed IOTN outcome fields could be included (out of those who completed and discontinued, 3456 and 1180 had incomplete IOTN outcome fields, respectively). The same sample sizes were used for the unadjusted and adjusted analyses (as there were no missing data on gender and age, and a dummy variable was used for the missing IOTN AC data) The proportion of patients with residual need among those who discontinued and completed treatment was 16.3% and 5.4%, respectively. It is likely that early dis- continuation has a much larger effect on residual need than discontinuation near the end of a course of treat- ment. However, our results show that residual need is evident for some patients even when a course of treat- ment is completed. NHS expenditure on treatments that resulted in dis- continuation amounted to £2.3 m, and £1.6 m was expended on treatments that ended with residual need (for treatments that were either discontinued or com- pleted). These figures highlight the need to increase the cost-effectiveness of NHS orthodontic care. Moreover, previous studies of NHS orthodontic treat- ment outcomes [6–10] have tended to report higher rates of poor treatment outcomes compared to those identified in our study. For example, a study in England of 144 patients who were treated at several hospitals and a primary care practice found that 43% failed to complete their treatment (the most common reasons being poor oral hygiene, multiple missed appointments, and orthodontic appliance breakages) [6]. However, most of the patients were hospital patients, and treat- ment provision may also have been affected by the three orthodontic clinicians involved being aware that the discontinuation rates would be published. Another study compared the differences in residual need rates (as measured using the IOTN, PAR index, and ICON) for 130 patients who were treated in hospitals in the North of England [8]. The study found that different occlusal indices indicated differing levels of residual need, for example, 20.1% of patients had residual need according to their IOTN DHC scores and 17.2% ac- cording to their ICON scores [8]. In contrast to our study, the study involved hospital patients (as in the abovementioned study), so the sample may represent a more complex case mix than those treated in primary
*Only patients with outcome records could be included (4746 had missing outcome records) care practices, and the skills and training of the clini- cians involved would differ from those of primary care orthodontic clinicians. We also found that lower SES was associated with dis- continuation and residual need after completing treatment, indicating SES-related inequality in outcomes. However, there was no association between SES and residual need among the patients who discontinued treatment, which suggests that ceasing treatment early does not contribute to SES-related inequality in residual need. Previous UK studies have also indicated that there are associations between low SES and poor treatment out- comes. First, a study in England and Wales of 1431 pa- tients (based on a 1990–1991 Dental Practice Board data set that covered all patients who discontinued treatment and 2% of those who had completed treatment) found a larger percentage of those from the more deprived groups (using multiple area-level SES measures) discon- tinued treatment [7]. A study in England of 135 12- to 16-year-olds treated for one year with fixed orthodontic appliances in the hospital dental service found that deprivation (based on characteristics of parental employ- ment) was associated with high improvement in occlu- sion (defined as a score equal to or higher than the sample median ICON improvement score of a modified version of the ICON) [9]. However, other elements of SES, namely parental education and employment status, were not associated with treatment outcomes [9]. More- over, a study in North West England of 144 9- to 19- year-olds reported that SES (measured using Townsend scores) was not associated with discontinuation [6]. However, this study largely involved patients treated in hospitals, who (like the patients in the abovementioned study) may have had a different treatment experience compared to those treated in primary care. SES may be linked with discontinuation because pa- tients in the more deprived groups have been reported to be more likely to miss orthodontic appointments [20] and discontinuation can be increased by orthodontic practices having a strict policy on discontinuing treatment for pa- tients who miss appointments [21]. It is likely that the more deprived groups are more affected by prohibitive transport costs and the potential impact of lost pay for the accompanying parents [22]. In addition, the SES associa- tions with treatment outcomes may be due to the effect of SES on the patient's development of self-efficacy [23] (i.e., the strength of one's belief in one's ability to complete tasks and reach goals [24]). Self-efficacy influences health behaviours [25], and health behaviours (e.g., patient com- pliance with treatment instructions such as to regularly re- place intraoral elastics) are associated with orthodontic outcomes [26–28]. Studies have shown that low SES is as- sociated with some of the elements of low patient compli- ance, such as poor oral health practices [29–31]. The data set provided comprehensive individual-level data on all NHS primary care orthodontic treatments provided under NHS contracts in North West England, and the IMD score for each patient. While the results reflect orthodontic outcomes in North West England, they may not be generalizable to other populations if there are regional variations in practitioner processes and patient preferences. However, practitioner processes should largely be uniform across England given that the same standards and procedures exist across NHS England. Also, patient preferences are likely to be generalizable, and approximately 13% of individuals aged ≥ 10 in England re- sided in North West England in 2011 [32], so the analyses were conducted on a significant percentage of the popula- tion of England. Additionally, North West England has a diversity of individuals from different SES backgrounds, with all five IMD quintiles being represented (though the lower SES groups are overrepresented, with approximately a third of North West LSOAs being in the most deprived quintile in England [33]). We used the IMD (which is an area-level measure that takes into income, employment, health, education, crime, housing and services, and living environment) be- cause there were no individual- or household-level data on income, occupation, or other indicators of SES. The IMD is the most commonly used measure of SES in the UK; however, one limitation of the use of the IMD is that not everyone living in a deprived area is deprived and not all deprived people live in deprived areas. This implies that there can be misclassification error, which could bias the SES-related associations. IMD scores are typically reported at the LSOA level, which represents the smallest area for reporting UK census data, with population sizes of 1000–3000 individuals. As the popu- lation size of small-area deprivation measures decreases, the risk of misclassification error decreases. Although in- dividual- or household-level SES indicators would help to avoid misclassification error, collecting individual-level data on, for example, self-reported income, increases the likelihood of non-response. In addition, despite the risk of misclassification error, small-area SES measures can help to identify areas with higher proportions of deprived households, so they are useful for planning and targeting healthcare services [34]. Another major limitation of the study is the missing out- come data, i.e., 18.9% of treatments (associated with an NHS expenditure of £5.9 m) ended without an outcome record being submitted and 19.4% were associated with incomplete IOTN outcome fields (£5.7 m). This inevitably led to under- estimation of the proportions and NHS expenditure associ- ated with discontinuations and residual need. In addition, higher SES was associated with incomplete IOTN outcome fields among patients who completed treatment (but not among those who discontinued treatment), for reasons that are unclear. In contrast, lower SES was associated with missing outcome records. If missing outcome records were partly reflective of dis- continuations (e.g., if dentists did not submit records be- cause patients discontinued treatment and the dentists were initially unsure whether the patients would return), the association between lower SES and discontinuations would be attenuated. Another limitation of the study relates to criticism re- garding the ability of the IOTN to measure outcomes. The developers of the ICON argue against the use of the IOTN to investigate outcomes on the basis that it was 'de- veloped and validated to assess treatment entry and exits as separate phenomena, when they are clearly part of the same clinical process. This requires additional training and duplicates the effort of measuring what are often simi- lar occlusal traits' [35]. Nonetheless, the IOTN was used on the basis that it indicates the degree of residual norma- tive need at the end of active treatment. Moreover, the IOTN remains the principle index used to assess individ- uals' need for NHS orthodontic treatment need and the prevalence of malocclusion in the population [36, 37], and using the same index to establish both a baseline assess- ment of need and the treatment outcome is practical when assessing the effects of treatment. However, the IOTN outcome scores were measured at the end of active treatment, but there can be relapse after active treatment has finished [38], particularly if there is poor compliance with retention instructions. From this perspective, both the proportion of treatments that end with residual need and the expenditure on these treatments are underesti- mated. Lastly, the IOTN outcome scores were not inde- pendently validated, which may also have led to bias in the data, and thus an underestimation of residual need. Regarding the implications of the findings, the quality of the NHS activity data could be increased by ensuring that outcomes are reported (and validated) for all treat- ments (except in cases where the patients fail to return, when only discontinuations, rather than IOTN scores, can be reported), and monitored by the service com- missioners. This is in line with 2015 guidance on the delivery of NHS orthodontic care in Wales, which highlighted the importance of the development of local health board policies to ensure that treatment outcomes in terms of completions and discontinuations are re- ported for each patient [39]. Monitoring outcomes more closely could help service commissioners to determine which providers provide the best value for money. In addition to recording information on discontinua- tions and IOTN outcome scores, there is a contractual requirement for dentists to monitor the outcomes of 20 patients plus 10% of the remainder of their patients using the PAR index [11], though these data are not col- lected using the NHS activity forms [12]. Implementing a contractual requirement to monitor the PAR scores of consecutive patients and to utilise independent third par- ties who are calibrated in the use of the index may help to reduce bias [15]. The British Orthodontic Society have noted that NHS commissioners may make partici- pation in a peer review process a contractual require- ment [15], and NHS guidance on commissioning has stated that 'PAR scoring will in future be undertaken within a managed orthodontic clinical network…under a peer review mechanism' [1]. Payment by Results (PbR) remuneration is used in many areas of the NHS [40], and could potentially help to im¬ prove orthodontic treatment outcomes [11]. However, a difficulty with implementing this policy is that there is a variation in the case mix (e.g., different percentages of pa- tients from more deprived groups) between orthodontic clinicians, which would influence the percentages of pa- tients who discontinue treatment and have residual need. Further, a PbR approach may generate perverse incentives for orthodontists to reject referrals of patients from disad- vantaged backgrounds, widening inequalities further. In addition, although contract penalties for discontinuations may help to reduce the SES-related inequity in discontinu- ation, care would need to be taken to ensure that treat- ment was not continued in cases where there could be risk of harm to the patient (i.e., if compliance with oral hy- giene advice was poor). Key areas to be explored include how factors related to patient compliance underlie the associations between SES and orthodontic outcomes, and why higher SES was associated with incomplete IOTN outcome fields among patients who completed treatment.
Conclusions
We found evidence of inefficiencies in the NHS ortho- dontic service, with 7.6% and 5.2% of NHS primary care orthodontic treatments in North West England ending in discontinuation and residual need, respectively. These outcomes were positively associated with lower SES, which raises concerns about inequity in the service and indicates that policies aimed at improving the levels of discontinuations and residual need may be more effect- ive if they were targeted at patients from lower SES groups. In addition, 38.3% of treatments had missing outcome data, highlighting the need to improve the out- come monitoring systems.
Additional file
Additional file 1: Percentage distribution/means of variables used in the adjusted treatment outcomes analyses, by socioeconomic status (SES). Additional file 1 contains information on the distribution of variables used to adjust for potential confounding: IOTN AC scores, gender, and age. This information is provided by IMD quintile. (DOCX 16 kb)
Abbreviations
AC: Aesthetic Component; Cl: Confidence interval; DHC: Dental Health Component; ICON: Index of Complexity, Outcome, and Need; IMD: Index of Multiple Deprivation; IOTN: Index of Orthodontic Treatment Need; LSOA: Lower-layer Super Output Area; NHS: National Health Service; NHSBSA: National Health Service Business Services Authority; OLS: Ordinary least squares; PAR: Peer Assessment Rating; PbR: Payment by Results; SES: Socioeconomic status; UK: United Kingdom; UOA: Unit of Orthodontic Activity
Acknowledgements
Not applicable.
Funding This study received no specific grant from any public, commercial, or not-for- profit funding agency.
Availability of data and materials
The data that support the findings of this study are available from NHSBSA (nhsbsa.dentalservices@nhsbsa.nhs.uk) but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of NHSBSA.
Authors' contributions
Authors JP, WW, SB, PB and MT contributed to the conception and design of the study, data interpretation, and manuscript revisions. MT acquired the data. JP and WW drew up the detailed analysis plan. JP carried out the analysis and drafted the manuscript. All authors read and approved the final manuscript.
Ethics approval and consent to participate
Ethics approval was not sought for this study. The study was a retrospective analysis of pseudo-anonymised administrative data that were provided by the NHSBSA (in line with their Information Governance procedures, i.e., the data can only be used for non-commercial research, and the copyright holder remains the NHSBSA), with the request for the data approved by the Caldicott Guardian. In the process of obtaining the data, the Caldicott Guard- ian did not consider a request for ethical approval to be sought. Consent for publication Not applicable. Competing interests The authors declare that they have no competing interests.
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Author details
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