With the increasing availability of multimodal CT and IA procedures, we examined the cost-effectiveness of multimodal CT compared to NCCT with the option for conventional angiography. Multimodal CT for subjects presenting with symptoms of an acute stroke severe enough to warrant consideration of IV tPA had lower costs and greater QALYs than NCCT, making multimodal CT cost-saving over the 3-month poststroke phase and over the lifetime of the cohort.
An alternative model evaluated a cohort with more severe symptoms and a higher prevalence of thrombus. In this alternative model, multimodal CT was cost-effective at 3 months, but NCCT became the dominant strategy over the lifetime of the cohort. With the increased prevalence of thrombus, the detection of thrombus by multimodal CT became an extra screening test in those who ultimately needed conventional angiography combined with IA procedures. Outcomes after IA procedures were based on recent studies which did not incorporate CTP-guided treatment, thus our base case analysis did not include the sensitivity and specificity of perfusion maps to identify viable parenchyma. If CTP maps can improve patient selection for IA procedures and subsequent outcomes, multimodal imaging would be more effective.
We tested the ability of perfusion-guided IA procedures to improve outcomes using a relative risk term in a 1-way sensitivity analysis. Assuming that an improvement in the percentage of mRS of 0-2 reflects greater discriminating power of CTP, then only a small change (4%) in favorable outcomes would make multimodal CT dominant for a cohort with a high prevalence of thrombus. Two small cohort studies reported that 16 of 34 patients (47%) and 12 of 27 patients (44%) had favorable outcomes following CTP-guided procedures.26,27
The probabilities of 44-47% for mRS of 0-2 following CTP-guided procedures compared to 41% from the sample weighted data suggest that a relative improvement of 7%, let alone 4%, is possible.
Our model assumed immediate access to multimodal imaging, that physical capabilities for multimodal CT were already in place, and that no cost was incurred for setup. We excluded sequential NCCT, with or without IV tPA, followed by multimodal CT prior to IA procedures as an alternative imaging strategy. First, it is convenient to proceed with the entire multimodal CT protocol once the person is in the CT scanner rather than arranging a return trip to the CT suite. Second, the angiographic suite can be prepared based on the multimodal CT data while IV tPA is administered. Finally, sequential NCCT with multimodal CT prior to angiography would only reduce costs compared to the multimodal CT branch. QALYs would be equivalent between sequential NCCT and multimodal CT.
Price per QALY boundaries in the NE and SW quadrants may not be the same. That is, the 2008 USD saved for a QALY loss in the SW quadrant may not have the same relative value as 2008 USD spent for QALY gains in the NE quadrant.25,28
Though we presented a threshold of $100,000/QALY as a point of reference, we also presented data over a wide range of ICERs because there is no accepted US economic value for a QALY.24
This presentation style also allows for evaluation of other prices for a QALY (e.g., $40,000/QALY), which vary by payer or by country.
A previous study examined the cost-effectiveness of mechanical thrombectomy.29
Mechanical thrombectomy itself was cost-effective compared to no other IA therapy. Imaging modalities to qualify someone for IA treatment were not clearly outlined nor was a probabilistic sensitivity analysis conducted. Our study differs by also addressing imaging modalities, a necessary component prior to thrombectomy, and by simultaneously sampling multiple variables.
Our model has several limitations. Only 1 acute event was allowed, such that after the initial acute ischemic stroke, no future events, other than death, were allowed. This assumption allowed us to address the cost-effectiveness of multimodal CT as a diagnostic tool for IA procedures in a group of subjects with an immediate life-threatening condition and immediate payoffs. Risks of exacerbating renal disease or a contrast reaction were not included in our model (although both multimodal CT and catheter angiography involve ionizing radiation and contrast administration). The prevalence of clot was a weighted estimate (8.6%) from recent trials (range 1.5%-16.1%).7,8,10,11
This expected value was reasonable because it did not exceed the clot burden reported in Multi-MERCI and was near the midpoint of the prevalence range. Randomized data were not available for outcomes after IA procedures or for multimodal imaging. Our sample weighted outcome estimates after IA procedures may be subject to unknown confounding factors.
Multimodal CT is a cost-effective screening tool for individuals presenting with an acute stroke who would be considered for IV tPA or IA procedures. This assessment of imaging screening strategies with the potential for acute thrombectomy or thrombolysis suggested that multimodal CT would be the imaging modality of choice 90.1% of the time for a WTP of $100,000/QALY over a lifetime. For a cohort with a high prevalence of clot, improving outcomes following multimodal CT guided IA procedures by 4% (e.g., by incorporating CTP data in patient selection) would enhance the cost-effectiveness of multimodal CT. While the hypothesis of improvements with CTP-guided IA procedures cannot be directly tested yet, increasing evidence points to the utility of CTP in determining salvageable penumbra in stroke patients, and such patients may fare better following revascularization.2,3,26,27
Future models should incorporate new data with longer follow-up from such cohorts.