The ADD-RS and D-dimer, alone or in combination, provide useful diagnostic information that can assist clinical decision-making, specifically the selection of patients for CTA. ADD-RS > 0, D-dimer > 500 ng/ml and combinations of ADD-RS and D-dimer have high sensitivity (> 90%) with variable specificity (22% to 67%). The combination of ADD-RS of zero and D-dimer < 500 ng/ml effectively rules out AAS (sensitivity 99.8%). However, unselective use of ADD-RS or D-dimer in patients with very low prevalence of AAS will result in a high rate of FP cases requiring CTA.
Clinical scores other than the ADD-RS and biomarkers other than D-dimer currently lack sufficient evidence of accuracy to be recommended for clinical use in diagnosing AAS. Further research using an appropriate design and sample size is required to determine the accuracy of alternative biomarkers for AAS.
Decision-analytic modelling showed that a strategy based on the Canadian guideline (CTA if ADD-RS > 1 or if ADD-RS = 1 with D-dimer > 500 ng/ml) is cost-effective but, if applied unselectively to any patient whose symptoms were compatible with AAS, would require CTA capacity to be tripled or quadrupled.
Secondary analysis suggested that if the diagnostic strategies were only applied to patients that clinicians considered to have a non-zero probability of AAS, then the strategy based on the Canadian guidelines would be cost-effective and deliverable with only a 60% increase in CTA capacity. A strategy of CTA if ADD-RS > 1 or D-dimer > 500 ng/ml could also be cost-effective and deliverable if clinical judgement were used to select a higher prevalence cohort. Research is required to determine how these strategies would be delivered in practice and how an appropriate population would be selected for investigation.
For patients currently receiving CTA, the strategy of ADD-RS > 1 or D-dimer > 500 ng/ml could halve the number of CTA undertaken and would be cost-effective according to NICE thresholds for willingness to pay but would result in an estimated 143 cases of missed AAS per year across the NHS in England and Wales.