Introduction Research into occlusion factors has substantially increased in recent years, including imaging, flow patterns, clot composition, histology, immunohistochemistry, and biomechanical properties. However, integrating them into clinical practice to identify a challenging occlusion prior to clot retrieval is not well established.

Methods A modified DELPHI technique was used before and during CLOT SUMMIT 7.0, which included experts in thrombectomy and clot research from different specialties. Panelists answered three iterative question rounds, in which they indicated their certainty level on the association of 30 specific clot features as indicators for difficult-to-recanalize target occlusions. The features were grouped into 5 domains: histological, imaging, biomechanical, procedural, and clinical factors. The first round included open-ended questions and formed the basis for subsequent rounds, in which closed ended questions were used. Consensus was defined as ≥ 50% agreement among the panelists. Certainty was rated from 1 (‘very uncertain’) to 4 (‘very certain’) in the final round, and mean levels of 3.0 or greater were regarded as high certainty.

Results A total of 3 DELPHI rounds were performed, the last two in a live setting. Consensus was reached on 16 out of 30 questions, of which 8 were of high certainty (23%). Except for the clinical factors domain, all others had at last at least one clot feature with consensus and high certainty. Of those, the biomechanical domain produced the most clot features with consensus and high certainty (75%) while the imaging domain produced the least (8.3%). The 8 clot features were combined to produce a holistic definition of a challenging clot: A white coloured or calcified clot that’s stiff, hard, sticky or adherent, that could be calcified on imaging, and during thrombectomy is difficult to pass and resistant to pulling. There was also consensus but with less certainty (2.6/4) that the endovascular (EVT) technique should be switched after the third unsuccessful attempt.

Conclusions A live DELPHI consensus from experts in thrombectomy and clot research suggest the features of a challenging clot, which most aptly describe a tough clot: A white coloured or calcified clot that’s stiff, hard, sticky or adherent, that could be calcified on imaging, and during thrombectomy is difficult to pass and resistant to pulling. This may help clinicians and researchers focus on using and developing specialized tools for a priori identification of tough clots for swift recanalization.

Disclosures M. Mirza: 5; C; Cerenovus. J. Ospel: None. P. Brouwer: 5; C; Cerenovus.