This study is a multicenter, retrospective, observational study in patients with acquired thrombotic thrombocytopenic purpura (aTTP) treated with plasma exchange (PEX) in association with caplacizumab, and immunosuppression between Q4-2019 and 28 February2021. The retrospective study will measure: Age, sex, BMI, blood pressure at diagnosis, Platelet count at diagnosis and at the follow up visits, Hb level at diagnosis at the follow up visits, White blood cell count at diagnosis at the follow up visits, Creatinine at diagnosis at the follow up visits, schistocytes count at diagnosis at the follow up visits, LDH at diagnosis at the follow up visits, Coombs' assay at diagnosis, alanine-leucine-amino-transferase (ALT) at diagnosis at the follow up visits, total bilirubin at diagnosis at the follow up visits, Troponin above ULN at any point, ADAMTS13 activity (where measured) at diagnosis at the follow up visits, Anti-ADAMTS13 antibodies (where measured) at diagnosis at the follow up visits.

The primary objective in this study is the description and quantification of clinical response in terms of platelet count recovery in patients with aTTP treated t with caplacizumab , in addition to PEX and immunosuppression in the real-world setting. The secondary objectives include: number of exacerbations, defined as recurrent thrombocytopenia within 30 days after the end of therapy; rate of relapse, defined as a TTP event occurring more than 30 days after the end of daily plasma exchange; refractoriness; defined by the lack of a doubling of platelet count after 4 days of treatment and a lactate dehydrogenase level that remained above the upper limit of the normal range, TTP-related mortality and evaluation of adverse events.

This study is a multicenter, retrospective, observational study in patients with acquired thrombotic thrombocytopenic purpura (aTTP) treated with plasma exchange (PEX) in association with caplacizumab, and immunosuppression between Q4-2019 and 28 February2021.

Study design

The retrospective study will measure the following vital, clinical chemistry, hematological, and hemostatic parameters, according to the consensus guidelines of aTTP diagnosis and as a part of routine diagnostic work-up:

• Age, sex, BMI, blood pressure at diagnosis
• Platelet count at diagnosis and at the follow up visits
• Hb level at diagnosis at the follow up visits
• White blood cell count at diagnosis at the follow up visits
• Creatinine at diagnosis at the follow up visits
• schistocytes count at diagnosis at the follow up visits
• LDH at diagnosis at the follow up visits
• Coombs' assay at diagnosis
• alanine-leucine-amino-transferase (ALT) at diagnosis at the follow up visits
• total bilirubin at diagnosis at the follow up visits
• Troponin above ULN at any point
• ADAMTS13 activity (where measured) at diagnosis at the follow up visits
• Anti-ADAMTS13 antibodies (where measured) at diagnosis at the follow up visits The clinical signs will be parametrized by the PLASMIC score and the Glasgow score, where calculated in the various centers. The PLASMIC score and the Glasgow score will be evaluated according to the obtained values (0-4, 5-6, and 7; <13 and 13-15, respectively). All the clinical and laboratory parameters will be taken in each center and reported in the CRF.

OBJECTIVES The primary objective in this study is the description and quantification of clinical response in terms of platelet count recovery in patients with aTTP treated t with caplacizumab , in addition to PEX and immunosuppression in the real-world setting. The Secondary objectives include: Number of exacerbations, defined as recurrent thrombocytopenia within 30 days after the end of therapy; Rate of relapse, defined as a TTP event occurring more than 30 days after the end of daily plasma exchange;Refractoriness; defined by the lack of a doubling of platelet count after 4 days of treatment and a lactate dehydrogenase level that remained above the upper limit of the normal range, TTP-related mortality and evaluation of adverse events.

Data collection and management A customized eCRF (RedCap) will be created for the study and only the principal investigator and collaborating investigators of the center will have access to patient medical records during the study.

STATISTICAL CONSIDERATIONS Qualitative variables will be expressed by absolute and percentage frequency. Quantitative variables will be previously assessed for their distribution by the Shapiro-Wilk test. Normally distributed data will be described by mean and standard deviation (SD), whilst non-normally distributed variable by median and interquartile range (IQR).

INFORMED CONSENT Informed consent form will be collected for all except for all patients whom death is ascertained

Inclusion Criteria:

• The patients included in this study should have received caplacizumab for treatment in the period between Q4-2019 and the end of February 2021 while the end of follow up observation is scheduled for Q1-2021 (to observe at least one month of post treatment follow up). The diagnosis should be based on either clinical/laboratory parameters inclusive of measurement of ADAMTS13 level <10%) or the PLASMIC score (platelets, lysis, active cancer, stem cell or solid organ transplant, MCV, INR, and creatinine) with intermediate and high risk (sore>5) already computed or retrospectively calculated as previously detailed for centers that did not measure the ADAMTS13 level.

Exclusion Criteria:

• Patients treated with uncertain aTTP diagnosis according to the above inclusion criteria and manifesting clinical signs like aTTP but characterized by a different pathogenesis (e.g. cancer, sepsis)

The study population consists of patients treated for acute aTTP episodes in the hematology, internal medicine, and critical care centers between Q4-2019 and Q1-2021 in Italy.