ObjectiveTo characterise the microscopic structure of thrombus induced by direct effect of sclerosants (sclerothrombus) compared with that of post-sclerotherapy retained coagulum (trapped blood; sclerocoagulum) and thrombus in acute superficial venous thrombosis (SVT).MethodsScanning electron microscopy (SEM) and fluorescence microscopy were used to assess the structural architecture of thrombus samples. Sclerothrombus samples were generated in vitro following incubation of whole blood or platelet-rich plasma with detergent sclerosants sodium tetradecyl sulphate (STS) or polidocanol (POL). Sclerocoagulum samples were collected from patients treated for varicose veins with STS 1.5% foam. Samples of naturally occurring thrombus were collected from patients with SVT.ResultsAcute sclerothrombus contained a mesh-like distribution of branched fibrin fibers dispersed with red cells and microparticles. STS-induced sclerothrombus showed thicker fibrin strands with less branching and contained more microparticles when compared to POL-induced clots. Sclerothrombus at 1 week exhibited a fibrin sponge structure. Sclerocoagulum showed a disorganised distribution of thin, tortuous and branching fibers containing large gaps dispersed with haemolysed red cells and cell debris. SVT contained a smooth fibrin mesh of relatively thick fibrin strands with scarce branching and fewer microparticles.ConclusionThe three entities of sclerothrombus, sclerocoagulum and SVT are distinct and distinguishable entities. Whilst sclerothrombus and SVT demonstrate features of a fibrin clot, sclerocoagulum is haemolysed blood containing disorganised fibrin strands.