Discussion
This is a large study using a national data set reflecting the adult hospital population (~75 000 participants) in England with six comorbidities. This provided sufficient data to closely match participants in the study group with those in the comparator group. While there are shortcomings using HES epidemiologic studies, using HES can be informative.17–19
Principal findings
We previously reported the variation in the likelihood of hospitalisation for CAP among adults with six comorbidities in England.20 The current analysis used the same data set to quantify the clinical and financial burden of hospitalised CAP in these patients over the same time period.
This study illustrates for the first time the increase in the rate of hospital admissions in patients with the six clinical comorbidities included following an episode of hospitalised CAP. This increase was statistically significant in all comorbidity categories apart from post-BMT and was maintained across all three study years. However, there is a lack of precision around point estimates for healthcare resource utilisation for post-BMT patients specifically due to the small number of these participants (n=271). As expected, there was variation between the different comorbidities included, which is consistent with previous findings on the impact of underlying comorbidities and the risk of invasive pneumococcal disease (IPD).19
Our study also shows that patients with these comorbidities who are diagnosed with hospitalised CAP subsequently cost more money to treat over the 3-year period following the initial episode of CAP compared with matched controls who did not have an episode of pneumonia. The costs varied substantially by comorbidity, with the mean difference ranging between £6000 over 3 years for CHD patients to over £11 000 for CRD patients. However, for CKD patients, this trend was inconsistent in 2014/2015, when those with hospitalised CAP cost less than their matched controls. A similar but not significant observation was also made for diabetic patients in 2015/2016. One possible explanation is that hospitalisation with CAP provides an opportunity to review the treatment for an individual’s underlying condition. This review subsequently leads to an improvement in treatment of the underlying condition, thereby averting future related hospital costs and admissions related to their underlying condition. This might be specific to CKD and diabetes and the way in which they are managed. We chose to calculate the difference in cost using HRGs which provides insight to the cost that hospitals would have received for treating these patients, but it does not reflect the full picture of costs and notably is not able to capture costs in the community following discharge. A separate specific cost analysis of hospitalised pneumonia would be needed to more accurately determine a more accurate cost.
Our study found that patients with certain underlying comorbidities have a significantly higher likelihood of in-hospital mortality following an episode of hospitalised CAP. The ORs were >4 for all six comorbidities which underlines the importance of measures to prevent episodes of hospitalised pneumonia in patients with comorbidities.
The key findings of this study suggest that following an episode of CAP, adults with underlying comorbidities are subsequently associated with increased healthcare resource utilisation and are at increased risk of mortality for an extended period. An episode of hospitalised CAP is therefore likely to have a prolonged adverse effect on the subsequent health of adults with underlying comorbidities, which supports considering pneumonia as a chronic rather than an acute condition.21 An episode of CAP in adults with underlying comorbidities appears likely to leave them particularly prone to long-term adverse health consequences.22 While this is the first time insights in this context have been obtained using a population of UK adults with underlying comorbidities specifically, similar research undertaken outside the UK has previously highlighted that adults may be left with a compromised health status following an episode of hospitalised CAP. A recent systematic review and meta-analysis reported an increased risk of myocardial infarction, heart failure, dysrhythmias and stroke after CAP, which is maximal in the acute phase but persists long-term after resolution of the pneumonia .23 The finding that there is an increased likelihood of mortality following an episode of hospitalised CAP is reflected by a study in Dutch adults which suggested long-term mortality was higher in those with an underlying comorbidity following an episode of IPD or pneumonia.24 A possible explanation for this is that an episode of hospitalised CAP can compromise the long-term health status of patients with underlying comorbidities.
This study suggests it is not appropriate to continue to consider an episode of hospitalised CAP as a discreet event for patients with comorbidities. Rather, the impact of hospitalised CAP should be considered over a longer period accounting for the impact on both the patient and the healthcare system. Furthermore, it is important to consider the personal impact on quality of life for these patients and their families along with some of the often unreported consequences of CAP, including wider societal implications such as time off work.25
The increasing numbers of patients with comorbidities and elderly patients hospitalised with CAP will consume a large percentage of health resources in the future.26 Our data suggest that those with underlying conditions continue to be at an increased risk of hospitalised CAP and its associated consequences. Since the risk varies by comorbidity, it should be possible to target these comorbidity groups with appropriate preventative measures including influenza and pneumococcal immunisations.
This study reflects the UK healthcare system, the NHS, and its findings are therefore mainly of relevance for the UK, but may nevertheless be of interest for other healthcare systems.
CAP is a significant contributor to winter pressures that the NHS faces each year. Strategies to prevent pneumonia, including smoking cessation, pneumococcal vaccination and seasonal influenza vaccination, are important. Clinicians are advised to provide patients with written advice on pneumonia recovery when they are discharged from hospital. It is therefore disappointing that the most recent British Thoracic Society audit of adult CAP found that only 5.8% of participating hospitals routinely provided written advice to pneumonia patients when discharged from hospital.27
Strengths and weaknesses of the study
This study used HES data which has acknowledged limitations, particularly regarding quality and consistency of coding for pneumonia28 29 and potential errors of omission and commission of underlying comorbidities. The most recent national audit of hospitalised cases of CAP, conducted on behalf of the British Thoracic Society,27 compared prospectively identified pneumonia cases with HES data. The accuracy of a diagnosis of CAP at the national level varied widely between 124 participating hospitals. The median accuracy across all participating institutions was 65.6% (IQR 52.8% to 79.3%.27 The most common reason for exclusion of the diagnosis of CAP was the absence of new radiographic changes on chest X-ray. It is therefore possible that some admissions for ‘pneumonia’ may in fact have been as a result of other conditions, including heart failure or decompensated underlying comorbidity. However, coding accuracy in HES has improved ever since the roll-out of financial incentives that are based on diagnosis and procedure codes.30 Additionally, because of further reporting requirements in the NHS, coding completeness has increased substantially.17 20 HES continues to be used in multiple studies for studying disease epidemiology and healthcare resource use in the NHS.17 20 We chose to interrogate HES from financial year 2012/2013 when data reliability improved, following the introduction of payment by results.30
Due to the nature of coding in HES, it was not possible to fully differentiate between hospitalised CAP and HAP, although we did exclude all cases of pneumonia with onset occurring over 48 hours after admission. Patients admitted with a comorbidity diagnosis might be at an increased risk of developing HAP compared with those who have not been admitted with an underlying illness. Therefore, it is possible that presence of HAP within the data set may have resulted in overascertainment for all the outcomes measured.
Since we included patients whose comorbidity was coded in either the primary or secondary position, it is probable that a large proportion of patients will have multiple comorbidities.26 The risk of developing CAP increases when patients have several risk factors, a phenomenon known as ‘risk stacking’.11 31 In order to correct for these confounders, we used PSM to compare the outcome variables between the two categories.32 The propensity scoring method has been used in other HES and CAP studies.8 33 We have not assessed the effect of multiple comorbidities in this study, but it is likely that the healthcare costs incurred, and in-hospital mortality would be elevated in patients with multiple risk factors. In a study of the impact of risk stacking on mortality from pneumococcal infections in adults, each additional risk factor increased the risk of mortality by 55%.34 HES data does not include data on other known risk factors for CAP, for example, smoking, alcohol abuse and use of proton pump inhibitors. It is therefore unclear whether these additional risk factors or a worsening clinical condition rather than an episode of hospitalised CAP are predictors of a worse outcome. A prospective study would be needed to establish the relative importance of these factors.
Frailty increases the likelihood of hospitalisation with CAP.35 For example, in one study using the FRAIL index36 (FRAIL is a five-item scale of fatigue, resistance, ambulation, illnesses and weight loss) a score >3 was associated with an increase in duration of stay in hospital and an increase in in-hospital mortality.37 Since prevalidated frailty scores such as the FRAIL index are not recorded in the HES database, we were unable to adjust for frailty. Another potential confounder is chronic disease severity. Based on the data extracted as part of this study, it was not possible to determine the contribution of the chronic disease severity or the degree of frailty to the clinical and financial burden consequent on an episode of hospitalised CAP.38 It is unclear whether an episode of CAP requiring hospitalisation per se alters the course of a chronic disease, or is in fact a marker of worsening disease severity or increasing frailty resulting in a worse outcome. Millett et al18 investigated the factors associated with hospitalisation for CAP among adults aged ≥65 years in England, using linked primary and secondary care data sets; the Clinical Practice Research Datalink39 and HES. After adjusting for age, sex and year, they found frailty factors (inability to self-care, mobility problems, tiredness and a history of falling) did not increase the risk of hospitalisation for CAP. The authors did note that frailty factors and smoking were suboptimally recorded by general practitioners, preventing a full assessment of the role of these factors and highlighting the need for better data on these parameters.
Finally, it was not possible to account for loss of patients from the study due to mortality outside of the hospital setting. The HES data warehouse only includes records of patients’ contacts with hospitals in England. The available data would therefore only reflect death in hospital during an admission, rather than longer-term mortality. The increased likelihood of dying for patients who have had an episode of hospitalised CAP presented in this analysis may therefore be an underestimate. By linking the HES database to Office for National Statistics central mortality data, it would be possible to estimate mortality without restricting the analysis to those patients who died in hospital.
Meaning of the study: implications for clinicians and policymakers
This study suggests the adverse effect of an episode of hospitalised CAP for those with underlying comorbidities, both for the individual patient and for the NHS. Quantification of these effects in patients with underlying comorbidities could be useful for policy makers when deciding about preventative measures.
Unanswered questions and future research
This study examined the impact of an episode of hospitalised CAP on patients with at least one of six selected underlying comorbidities over a period of 3 years. The longer-term duration of the impact of an episode of hospitalised CAP on healthcare utilisation and mortality for patients with the six comorbidities studied has not been determined. The study did not include patients with other comorbidities, including immunosuppression and functional asplenia. Future research could address these unanswered questions.