Thrombotic thrombocytopenic purpura (TTP) is a rare disease (5–10 cases per million persons per year) characterised by the massive formation of platelet rich-thrombi in the microcirculation of multiple organs1,2. It affects both sexes, although the incidence is two to three times higher among females3. TTP was first described by Moschowitz in 1924 in a 16-year old girl who presented with fever, anaemia, thrombocytopenia and focal changes in the central nervous system and kidneys4. Several reports of TTP followed this initial description leading to a definition based upon the following pentad of symptoms: fever, mechanical haemolytic anaemia, thrombocytopenia, central nervous system abnormalities and renal impairment5. However, fever, neurological abnormalities and renal impairment are not constant symptoms, especially during the early stage of the disease, thus leading to the currently accepted definition of TTP consisting of the association of mechanical haemolytic anaemia with fragmented erythrocytes and thrombocytopenia (platelet count <100 × 109/L) without alternative causes6. In parallel to the evolution of clinical criteria, the pathophysiological mechanisms of TTP were elucidated when Joel Moake and his colleagues observed that the plasma of a patient with recurrent TTP contained very high molecular weight [so-called ultralarge (UL)] multimers of von Willebrand factor (VWF), a multimeric adhesion glycoprotein contained in endothelial cells, platelets and plasma7. Once released from the abnormally stimulated endothelial cells, these ultralarge forms of VWF, present in the endothelium but not in plasma in physiological conditions, promote intravascular aggregation of platelets and the consequent microvascular thrombosis and haemolytic anaemia caused by mechanical damage, particularly in blood flow conditions characterised by high fluid shear stress such as in the microcirculation3. Moake himself hypothesised the deficiency of a cleaving protease as being responsible for the presence of UL VWF7, but it was Furlan et al. and Tsai and Lian in 1996 who managed to isolate from human plasma a metal-dependent protease able to cleave the peptide bond between the tyrosine at position 1605 and the methionine at position 1606 in the central A2 domain of VWF. The same investigators subsequently and independently found that the VWF-cleaving protease was deficient in a retrospective cohort of patients clinically diagnosed as having TTP8,9. The protease, which is responsible for regulating the multimeric structure of VWF, was identified in 2001 by Zheng et al.10 as a new (the thirteenth) member of the ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin 1 repeats) family of metalloproteases and was called ADAMTS132. While the discovery of ADAMTS13 has renewed interest in TTP, as documented by the exponential increase in the number of publications on this topic in the last few years, it has raised the issue of the diagnostic and prognostic value of ADAMTS13 testing in this condition.