134 Chapter 8 CI: 48.2-71.7) and specificity was 81.4% (95% CI: 68.7-89.9). DSA as standard of reference was regarded to have a sensitivity and specificity for aneurysm detection and determination of treatment of 100%. Evaluation of costs and benefits: A decision tree A decision tree (using Excel Software) was developed to determine differences in health benefits and costs in patients with ruptured aneurysms for each diagnostic pathways.21 For patients with acute non- traumatic SAH, the decision tree delivers different health states due to characteristics of diagnostic imaging and choice of therapy. Health states were based on whether patients were alive and, when they were alive, whether they were well or disabled. Figure 1 shows the decision tree for MRA. A hypothetical cohort of patients passes through the model based on a one year period. This allowed us to evaluate health costs of diagnostic test and treatment as well as related quality of life and associated costs determined by diagnostic decision. A health care perspective was used, which included only direct health care costs. Basic model and scenario model In the basic model, patients underwent either DSA, MRA or CTA. Following our standard clinical practice, if no aneurysm was detected on CTA or MRA, an additional DSA study was performed. Patients without an intracranial aneurysm were not treated. In patients in whom an aneurysm was detected, feasibility of coiling of the aneurysm was determined. Depending on the result of each test, either a coiling or clipping procedure was initiated. In cases where false positive feasibility of coiling was determined, angiography during the coiling procedure would show no feasibility, and transfer to surgical clipping would have been performed. In case of false negative determination of coiling, surgical clipping would have been performed although coiling would have been feasible. In a scenario analysis in each patient in whom an aneurysm was detected which was deemed not suitable for coiling by MRA or CTA, an additional DSA study was considered to be performed. Model parameters Sensitivity and specificity of the diagnostic test for detection of aneurysms and determination of treatment possibility from our prior study, as described in Chapter 2 and 3, were taken as input parameters to the decision model. For the costs of DSA, MRA and CTA, standard prices from the Dutch manual for cost research were used.22 Total one year costs of surgical clipping and endovascular coiling were derived from a literature search for western countries.13 Health outcome after one year of treatment was derived from the ISAT trial.7,8 Input parameters for related utilities23 and costs19,22,24 as well as health risk of DSA are based on available literature.15,16 All costs were updated to 2010 by means of national price index figures25 and converted to Euro (1$ = 1,37€). Standard discount rates of 1.5% for effects and 4% for costs according to Dutch guidelines were used.22 Table 1 shows the model input parameters and their sources. Model assumptions We assumed that DSA as standard of reference has a sensitivity and specificity of 100% in detecting aneurysms and determining feasibility of coiling. Furthermore we assumed no significant gender or age
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