Pressure relieving support surfaces (PRESSURE) trial: cost effectiveness analysis

Introduction

Older age, reduced mobility, impaired nutrition, vascular disease, faecal incontinence, and skin damage consistently emerge as risk factors for pressure ulcers.1 2 The annual cost to the UK NHS of implementing the current guidelines for the treatment of pressure ulcers was estimated in the region of £1.4b to £2.1b at 2000 prices.3 Experts differ in their views as to the extent to which pressure ulceration is preventable 4; however most would agree that pressure ulcers are often the consequence of poor preventive strategies. Between April 2002 and April 2004, the NHS Litigation Authority (www.nhsla.com/home.htm) received 56 claims for pressure damage (NHS Litigation Authority, personal communication, 2005), these included four instances where the patient died. The average cost of a claim was reported as £37 295 ($70 453; €54 713) but ranged up to £375 000. Inadequate nursing care, lack of assistance and care, and failure or delay in diagnosis were often cited as causal. Early identification of vulnerable people and timely provision of a cost effective programme for the prevention of pressure ulceration in hospitals could prevent this cost to the NHS and the adverse health consequences of those affected.

Current National Institute for Health and Clinical Excellence guidelines on the prevention of pressure ulcers recommend the use of high specification foam mattress as the standard in vulnerable patients and surfaces such as alternating pressure mattresses for high risk people.5 This latter recommendation was based on expert opinion in the absence of research. Alternating pressure surfaces are available as whole mattress replacement systems (typically costing about £4000) and as overlays (typically £1000), which are placed on top of a standard hospital mattress. The relative costs and effects of these different systems are uncertain. We carried out an economic evaluation alongside a large multicentre pragmatic randomised controlled trial of alternating pressure mattresses and alternating pressure overlays. The clinical results of the trial are published in the accompanying paper.6

Methods

Full details of the trial's methods are available elsewhere.7 Briefly, eligible people were aged at least 55 years who were admitted to vascular, orthopaedic, medical, or care of the elderly wards, either as acute or elective admissions, in the previous 24 hours. Inclusion criteria were an expected length of stay of at least seven days, and an expected (if elective) or actual limitation of activities and mobility or an existing pressure ulcer of grade 2, or both. Trial completion was defined as improved mobility and activity, resolution of pressure ulcers for three consecutive days, transfer to non-participating wards or consultants, discharge from hospital, 60 days from randomisation, or death.

Results

A total of 1972 participants were randomised from 10 of the 11 hospitals participating in the randomised controlled trial comparing alternating pressure mattresses with alternating pressure overlays for the prevention of pressure ulcers. One patient who had been inadvertently randomised twice was excluded, leaving an intention to treat population of 1971 participants.

Cost effectiveness analysis

The base case economic analysis indicated that the mattresses are a dominant strategy when compared with the overlays; they are associated with a delay in the development of pressure ulcers and lower hospital costs. The sampling uncertainty associated with this result was investigated in an incremental cost effectiveness plane. 1 shows the results of 1000 replicates of a non-parametric bootstrap of the estimated mean difference in cost and benefits. The majority of point estimates fall in the north west and south west quadrants of the cost effectiveness plane, suggesting that the overlays are associated with a more rapid development of pressure ulcers than are the mattresses.

Fig 1

Incremental cost effectiveness plane of using alternating pressure overlays or alternating pressure mattresses for the prevention of pressure ulcers in patients admitted to hospital

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2 is a cost effectiveness acceptability curve that represents the uncertainty associated with the decision to consider the mattresses a dominant alternative when compared with the overlays. The mattresses are associated with an 80% probability of being cost saving (the probability associated with a willingness to pay of zero) compared with the overlay. Similarly, a probability of about 15% was associated with overlays being cost effective for a range of willingness to pay values from £25 to £30 000 for an extra pressure ulcer free day.

Fig 2

Cost effectiveness acceptability curve for alternating pressure mattresses

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Discussion

Alternating pressure mattresses to prevent pressure ulceration in patients admitted to hospital are associated with lower costs and greater benefits and are more likely to be cost saving than alternating pressure overlays.

In this study we defined costs as mattress costs and hospital costs. During the trial we observed that grade 2 pressure ulcers did not usually receive dressings and other nursing care therefore we did not estimate for further treatment costs.7 The health benefits associated with the interventions were captured as pressure ulcer free days. We chose this approach rather than the more usual method of trying to capture a patient's utility through changes in quality of life because concurrent illness would dominate any quality of life measurement. Furthermore, the emphasis of our study was on prevention and participants who developed new ulcers were not followed-up until the ulcer healed. Although we did follow the healing of ulcers present at baseline, we did not have quality of life data that related to the time of ulceration (before admission) and therefore could not examine how quality of life changes with the status of pressure ulcers. The effect of pressure ulcer healing on people's health related quality of life may be the subject of future studies.

That the mattress is likely to be economically dominant may initially seem counterintuitive. The mattress has a higher purchase cost and no statistically significant effect on the proportion of patients developing an ulcer, the time to ulceration, the severity of ulcers, or length of stay. However, the purchase cost of these pressure relieving surfaces is low when viewed over their life span, and the difference in costs between overlays and mattresses is small over this time period. For a two year life span, the average cost per day would be £1.38 for an overlay and £5.71 for a mattress (table 1). This cost difference is small in the context of the daily inpatient treatment costs of £165-£385 (depending on specialty) and the reduced length of stay (on average, 1.22 of a day less) for mattress recipients in this trial and translates into an average reduction in costs of £283.6 per patient. The health benefit associated with the interventions was measured as the difference in mean time to develop a pressure ulcer, and also favoured the mattress (on average by 10.64 days). The accompanying clinical paper for this trial reports the median (rather than mean) time to ulceration, as is common practice owing to the skewed distribution of time to event data.6 By contrast, and as recommended for economic analyses, we use the mean time to pressure ulceration since the median would greatly underestimate the costs.8 9

The delay in ulceration associated with alternating pressure mattresses is crucial because the longer a patient avoids ulceration the less likely they are to go on to develop a pressure ulcer; a delay in ulceration allows the patient time to recover sufficiently from their acute episode so that their risk of ulceration recedes. We argue that although time to ulceration was a secondary end point in this trial, it should be considered as a primary end point in future studies since it is more informative economically and clinically. Furthermore, many of the risk factors for pressure ulceration in the participants do not completely disappear on discharge from an acute hospital (for example, impaired mobility), and simple proportions of pressure ulcers developed ignore the censored nature of these data.

Total cost is a function of length of hospital stay which itself is mainly a function of overall health status. Patients who are more ill are both more at risk of pressure ulcers15 and likely to have longer lengths of stay. It is probable that good nursing care, such as the provision of a support surface, will completely prevent pressure ulcers in a proportion of patients and will merely delay the appearance of ulcers in others. If the appearance of an ulcer is a proxy for acuity, and more acutely ill patients consume more healthcare resources, then delays in pressure ulceration suggest a potentially lower consumption of healthcare resources. The longer length of stay in patients with pressure ulcers is probably a consequence of comorbidities rather than the pressure ulcer itself. We found no effect when we tested to see whether the surfaces had a differential effect on length of stay depending on the presence of a pressure ulcer; overall lengths of stay for patients who developed a pressure ulcer on either surface were similar.

Since there are no robust estimates of the cost of a pressure ulcer we were not able to estimate the savings from pressure ulcers prevented. Such a costing study would need to be large, with a considerable length of follow-up, carried out in several centres to ensure generalisability, and would require careful observation of clinical practice in nursing care. It was not possible to undertake such a costing study within this trial.

Our results oppose those from the model based cost effectiveness analysis by Fleurence because data in that trial were based on expert opinion whereas we collected data within the trial.16 Furthermore, the previous model disregarded the fact that pressure ulcers are rarely the main reason for people being admitted to hospital. Thus the marginal cost of treating a pressure ulcer in hospital may be small compared with the overall costs of hospital treatment. In this sense our analysis reflects actual practice. Our assumption that participants remained on the allocated pressure relieving surface over their entire hospital stay is a conservative one; in reality patients are moved on to standard mattresses or higher specification surfaces after major changes in their risk of pressure ulcers. Given participants in the overlay group developed pressure ulcers earlier than those on the mattresses, this assumption will have over-estimated the cost of the mattresses, thus strengthening our conclusions.

Although neither the difference in mean overall hospital costs nor the difference in mean time to development of pressure ulcers between the two surfaces were statistically significant, decision makers need to make a decision. Failing to do so until statistically significant evidence becomes available would deny patients the possibility of experiencing the expected health benefits at an earlier stage and prevent the realisation of expected savings to the NHS.9 Although there was considerable uncertainty around the point estimates of mean health benefits and costs shown in table 3, this should not result in large decision uncertainty for decision makers since even for large willingness to pay values (fig 2) the probability of the overlays being cost effective is only between 10% and 20%.17

We thank the clinical research nurses Helen Barrow, Caroline Cooper, Fiona Corcoran, Patricia Hutchinson, Yvonne Meades, Dawn Parkes, Fiona Smith, Ann Fotheringham, Sarah Gowland, Ann Warriner, Caroline Smith, Richard Buckland, Monica Clark, Morag Doherty, Andrew Young, and Helen Marson; the participants and their relatives; ward nursing and medical staff of the six participating NHS trusts; and the independent trial steering committee: Jenny Hewison (chair), David Machin, and Gerben ter Riet.