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Development of an economic model to assess the cost effectiveness of treatment interventions for chronic obstructive pulmonary disease

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Abstract

Objective: To develop a Markov model that allows the cost effectiveness of interventions in patients with chronic obstructive pulmonary disease (COPD) to be estimated, and to apply the model to investigate the cost effectiveness of an inhaled corticosteroid/long-acting β2-adrenoceptor agonist (β2-agonist) combination (salmeterol/fluticasone propionate) versus usual care.

Methods: A Markov model consisting of four mutually exclusive disease states was constructed (mild, moderate and severe disease, and death). The transition probabilities of disease progression (for smokers and ex-smokers) and death were derived from the published medical literature. The model outputs were costs, exacerbations, survival, QALYs and cost effectiveness. The model was made fully probabilistic to reflect the joint uncertainty in the model parameters. Efficacy data for the combination of inhaled salmeterol/fluticasone propionate 50/500µg twice daily in poorly reversible COPD patients with a history of exacerbations were obtained from the 1-year TRISTAN (TRial of Inhaled STeroids ANd long-acting β-agonists) study and applied to the model, based on patient profiles representative of COPD clinical trials.

Results: According to the model, the mean life expectancy with usual care alone (placebo group) was 8.95 years, which decreased to 4.08 QALYs once adjusted for quality and discounted, at a lifetime discounted cost of $Can16 415 per patient (year 2002 values). Assuming that salmeterol/fluticasone propionate reduced exacerbation frequency only (base case analysis), the estimated mean survival time remained unchanged but there was an increase in the number of QALYs (4.21) for an estimated lifetime cost of $Can25 780, resulting in a cost-effectiveness ratio of $Can74 887 per QALY (95% CI 21 985, 128 671) versus usual care. If a survival benefit was assumed for salmeterol/fluticasone propionate, the incremental cost per QALY was $Can11 125 (95% CI 8710, dominated) versus usual care. If the combination achieved around a 10% improvement in forced expiratory volume in 1 second, leading to delayed progression to more severe disease states, the benefits translated into an incremental cost per QALY of $Can49 928 (95% CI 37 269, 66 006) versus usual care.

Conclusions: This Markov model allows, for the first time, a means of estimating the long-term cost effectiveness and cost utility of interventions for COPD. Initial evidence suggests that for patients with poorly reversible COPD and a documented history of frequent COPD exacerbations, the addition of salmeterol (a longacting β2-agonist) to fluticasone propionate (an inhaled corticosteroid) is potentially cost effective from the Canadian healthcare payer’s perspective. However, the precision of this estimate will be improved when additional data are available from clinical trials such as the ongoing TORCH (TOwards a Revolution in COPD Health) study.

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Acknowledgements

Michael Spencer and Andrew Briggs were responsible for the technical development and programming of the model. Ron Grossman provided clinical input and validation of the model assumptions and costing estimates. Laureen Rance was responsible for developing Canadian-specific cost estimates. All authors contributed to the writing of the manuscript.

Funding for the study was provided by GlaxoSmithKline.

Michael Spencer and Laureen Rance are employees of GlaxoSmithKline in the UK and Canada, respectively. Andrew Briggs and Ron Grossman have both worked as paid consultants to GlaxoSmithKline.

The authors wish to thank Dr Roger Goldstein, West Park Hospital, Toronto, Canada, for consultation regarding costings, and Dr Peter Bach, Memorial Sloan-Kettering Cancer Center, New York, USA, for consultation regarding standardised mortality ratios (SMRs).

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Correspondence to Michael Spencer.

Electronic supplementary material

Appendices

Appendices

Appendix I. Disease Progression

Disease progression of COPD is dependent on the annual rate of decline in FEV1 and the initial severity of the patient’s disease (FEV1% predicted). Given these parameters, the average time t spent in a given severity state can be calculated, and hence a transition probability can be estimated as 1/t.

Figure A1 illustrates the estimation task for progression of COPD for an example patient. The predicted FEV1 line shows the predicted FEV1 estimated from the reference equation with its characteristic age- (and time-)related decline. Threshold 1 occurs at 50% of predicted FEV1 and represents the threshold for transition to moderate disease. Similarly, threshold 2 at 35% of predicted FEV1 represents the threshold for transition to severe disease. The actual FEV1 decline is faster than that predicted by the reference equation, and the points at which this crosses the severity thresholds, can be read on the x-axis as time to progression. It is notable that even a decline parallel to that of the predicted FEV1 would result in progression between stages, highlighting the low probability of transitions to less severe severity states.

Fig. A1
figure 6

Diagrammatic representation of the estimation task for disease progression of chronic obstructive pulmonary disease (COPD) for an example patient. The predicted FEV1 line shows the predicted FEV1 estimated from a reference equation (see text). Threshold 1 (50% of predicted FEV1) represents the threshold for transition to moderate disease, and threshold 2 (35% of predicted FEV1) represents the threshold for transition to severe COPD. FEV 1 = forced expiratory volume in 1 second.

The estimation task for progression of COPD is described algebraically as follows (equation 1):

$$\matrix{{{\rm{\gamma}} \bullet {\rm{predFE}}{{\rm{V}}_{0}} - {\rm{\delta}} \bullet {\rm{T}} = {\rm{\tau}} \bullet ({\rm{predFE}}{{\rm{V}}_{0}} + {{\rm{\beta}}_{2}} \bullet {\rm{T}})} \cr {{\rm{\gamma}} \bullet {\rm{predFE}}{{\rm{V}}_{0}} - {\rm{\tau}} \bullet {\rm{predFE}}{{\rm{V}}_{0}} = {\rm{\tau}} \bullet {{\rm{\beta}}_{2}} \bullet {\rm{T}} + ({\rm{\delta}} \bullet {\rm{T}})} \cr {{\rm{T}}({\rm{\tau}} \bullet {{\rm{\beta}}_{2}} + {\rm{\delta}}) = {\rm{predFE}}{{\rm{V}}_{0}}({\rm{\gamma}} - {\rm{\tau}})} \cr {{\rm{T}} = {{{\rm{predFE}}{{\rm{V}}_{0}}({\rm{\gamma}} - {\rm{\tau}})} \over {({\rm{\tau}} \bullet {{\rm{\beta}}_{2}} + {\rm{\delta}})}}} \cr}$$
((Eq. 1))

(Eq. 1) where:

  • T is the time from baseline

  • β2 is the normal rate of decline amongst patients without COPD, taken from the reference equation

  • age is defined as age at baseline such that (age + T) gives current age

  • predFEV0 is the baseline predicted FEV1 score at T = 0

  • baseline FEV1 score is a proportion γ of the predicted FEV1 at baseline

  • annual decline in lung function is given by δ

  • the threshold level of FEV1 is a proportion τ of predicted FEV1.

When a stochastic estimation is performed, estimates for the rate of decline in lung function are drawn from the second-order distribution around the decline estimates, while (in keeping with its definition as a reference equation) no uncertainty is assumed around the parameters of the prediction equation.

Appendix II. Mortality

A Gompertz function was used to estimate mortality risks according to age; this approach allows calculation of the mortality risk for each patient profile run through the model dependent on age and sex. This baseline risk may then be adjusted by a standardised mortality ratio (SMR) to provide an estimate for an age/sex profile by disease severity.

Mortality risk in the model is therefore related to time, i.e. there is an increasing mortality risk experienced by individuals in the model as they age, independent of their progression through COPD disease stages. It has been observed that mortality risk increases exponentially with age, and this relationship can be described by the Gompertz function[12] (more detail on the Gompertz function is available as electronic supplementary information at http://www.adisonline.info.PEC/extras.asp).

The age-related mortality risk was estimated by integration of the Gompertz curve over the 3-month period represented by each model cycle. This mortality risk was then adjusted for severity of disease by application of the SMR (table AI), as discussed by Haybittle.[51]

Table AI
figure Tab5

Standardised mortality ratio by disease stage for chronic obstructive pulmonary disorder (COPD)

Appendix III. Costing Assumptions

General Assumptions

  1. 1.

    All hospital and emergency room visits were exacerbation related, and thus were not part of maintenance therapy.

  2. 2.

    Antibacterials were used only during exacerbations and not during maintenance therapy.

  3. 3.

    Ontario Drug Benefit[52] drug costs were used for all therapies.

  4. 4.

    Drug therapies for moderate or severe disease were taken daily at recommended doses.

Specific Assumptions

Clinic Visits

Canadian estimates were available for patients with severe COPD,[53] but not patients with mild or moderate COPD. Since US estimates for mild, moderate and severe COPD patients were available,[54] a conversion factor of ≈0.70 was calculated by dividing the Canadian estimate for frequency of clinic visits for patients with severe COPD (4.1 visits per year) by the US estimate (5.96 visits per year). This factor was then used to convert the US estimates for frequency of clinic visits for patients with mild and moderate COPD to approximate Canadian values. Standard error estimates were included as upper and lower range values. The calculated estimates were as follows:

Mild COPD: 2.0 ± 3.7 physician visits per year. All clinic visits for mild COPD were assumed to be with a family physician.

Moderate COPD: 3.2 ± 3.7 physician visits per year. Of all clinic visits, 50% were assumed to be with a specialist and 50% with a family physician.

Severe COPD: 4.1 ± 2.4 physician visits per year. The patients considered in this estimate were all hospitalised for their COPD;[1] 75% of clinic visits were assumed to be with a specialist and 25% with a family physician.

For all family physician visits, 50% were assumed to be general assessments and 50% reassessments. For specialist visits, all visits were assumed to be medical-specific assessments, as judged by expert opinion.

Rehabilitation Visits

It was assumed that resource utilisation was the same for both moderate and severe COPD patients (i.e. 0.14% of the COPD population required rehabilitation). Costing was based on clinical opinion, and confirmed in the literature (inpatient programme cost).[55] Three-month costs were determined by dividing the total programme cost per year by four. A description of the outpatient and inpatient rehabilitation programmes and costs is provided in table AII.

Table AII
figure Tab6

Details of rehabilitation programmes

Concomitant Medications

Based on dosage recommendations obtained from Canadian guidelines,[56] and Ontario Drug Benefit (ODB)[52] drug costs, an overall average daily cost was obtained for recommended drugs for COPD within each drug class. A 10% mark-up was added (representing the maximum mark-up for a drug that is reimbursed by the ODB) and an average cost for 3 months (90 days) calculated. A dispensing fee of $Can6.47 was added to overall 3-month costs. Resource utilisation estimates were taken from a combination of sources as described below.

Mild COPD

The lower range of dosage recommendations from Canadian guidelines[56] was used. It was assumed that 50% of patients used long-acting bronchodilators, as recommended by the Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines,[1] and that all patients were taking short-acting bronchodilators as needed. The frequency of short-acting bronchodilator medication use was estimated by expert opinion to be once to twice daily. A once-daily estimate was included in base case costs for a more conservative estimate.

Moderate COPD

The upper dosage recommendations from the Canadian guidelines[56] were used. Given that the estimates for resource use from Hilleman et al.[54] were very similar between patients with moderate and severe COPD, it was assumed that drug therapy use would also be very similar between those with moderate and severe COPD in Canada. Therefore, a large portion of resource utilisation estimates were based on published Canadian data.[53] It was also assumed that inhaled long- and short-acting β2-agonist use was identical between patients with moderate and severe COPD.[54] Anticholinergic and theophylline use was also assumed to be identical between moderate and severe disease. It was assumed that no oral corticosteroids were used. Inhaled corticosteroid use was assumed to be 25% of that for patients with severe COPD.[54]

Severe COPD

The upper dosage recommendations from the Canadian guidelines[56] were used. Resource utilisation estimates were taken from Canadian data,[53] because this population most likely represents severe COPD patients. In the Canadian data,[53] medication use was reported for 90 days after hospital discharge, which may slightly overestimate actual medication usage. It was assumed that oral antimicrobials were not continued beyond 14 days after hospital discharge, thus oral antimicrobials were not included in general maintenance costs. It was further assumed that all inhaled β2-agonists were long-acting β2-agonists. The following resource utilisation estimates according to drug class were used:

  • long-acting inhaled β2-agonists: 69.5%

  • inhaled anticholinergics: 52%

  • oral corticosteroids: 30%

  • oral theophyllines: 18.4%

  • inhaled corticosteroids: 51%.

In addition, it was further assumed that all severe COPD patients were on a short-acting bronchodilator (upper dosage recommendation used).

Laboratory Testing and Devices

The costs included under this category relate only to those occurring in the maintenance phase of COPD. Those occurring during emergency department or inpatient care were included as exacerbation costs.

Mild COPD

It was assumed that the only outpatient tests performed were spirometry and complete blood count (CBC) on an annual basis.

Moderate COPD

Only certain Canadian resource utilisation estimates were available.[57] It was assumed that these estimates were representative of patients with moderate COPD; the data indicated that approximately 80% of patients had moderate chronic bronchitis, but they were not formally diagnosed according to FEV1 levels. Laboratory tests that were generally completed in hospital or the emergency room setting were not included. A number of very infrequent tests were also excluded (e.g. tympanogram, erythrocyte sedimentation rate, Mantoux test). Overall, the number of chest x-rays per year and medical devices used were obtained from Torrance et al.[57] On the basis of expert opinion, it was assumed that one CBC test was performed per year and that spirometry was performed at each specialist visit.

Severe COPD

Resource use involving laboratory testing and devices for patients with severe COPD was based on expert opinion. It was assumed that spirometry was performed at each specialist visit, and CBC performed annually. Further, full pulmonary function tests were performed at the initial consultation visit and may have been repeated annually thereafter. Thus, it was assumed that approximately 50% of patients had annual full pulmonary function tests. Outpatient chest x-rays were assumed to be performed once per year; other chest x-rays for patients with severe COPD are mainly performed in the emergency department or during inpatient hospitalisation so were not included in the maintenance costs. All other laboratory tests are completed in hospital and are captured in the inpatient exacerbation costs.

Oxygen Therapy

Oxygen therapy resource use estimates according to disease severity were provided by expert opinion (Goldstein R, personal communication). Standard government costs were used for continuous oxygen therapy in the home ($Can383/month) as quoted by two companies (Medi-Gas-Toronto, Vital Air Canada). It was assumed that those on oxygen therapy required oxygen every day. Nocturnal estimates were not provided separately because costing would be roughly the same price and most patients use continuous oxygen therapy. Prices ranged from $Can12.35 to $Can13.68 per day.

Mild COPD: It was assumed that no patients required oxygen therapy.

Moderate COPD: It was estimated that 3% of patients with moderate COPD use oxygen therapy.

Severe COPD: It was assumed that 9.4%. of patients with severe COPD would require oxygen therapy. This was based on the estimation that of approximately 160 000 patients with severe COPD in Ontario, approximately 15 000 use oxygen therapy.

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Spencer, M., Briggs, A.H., Grossman, R.F. et al. Development of an economic model to assess the cost effectiveness of treatment interventions for chronic obstructive pulmonary disease. Pharmacoeconomics 23, 619–637 (2005). https://doi.org/10.2165/00019053-200523060-00008

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