2.2. Study Design and Patient Population

From a sample population of almost 17 million health-assisted individuals, hospitalized adult (≥18 years) patients with a diagnosis of iTTP were identified by the ICD-9-CM code 446.6 from January 2018 to the end of the data availability (up to March 2023) inclusion period. Women with complications of pregnancy identified by the codes ICD-9-CM 634–639 (other pregnancy with abortive outcome) and ICD-9-CM 640–649 (complications mainly related to pregnancy) were excluded from the analysis. Moreover, patients treated with caplacizumab were identified by the presence of at least one prescription for caplacizumab (ATC B01AX07). The group untreated with caplacizumab consisted of patients with iTTP without a prescription for caplacizumab by considering all of the available observational period. The index date was the time of the first prescription of caplacizumab for the treated cohort and the time of first hospitalization with iTTP discharge diagnosis (throughout the whole inclusion period) for the caplacizumab-untreated cohort. The characterization period was all the time of data availability preceding the index date, and the follow-up was all of the available period after the index date.

For all patients with iTTP, demographic variables in terms of age, distribution of subjects below or over 50 years of age, and gender (proportion of males) were recorded at inclusion. The comorbidity profile was assessed during the characterization period using the Charlson Comorbidity Index, a score resulting from the sum of weight assigned to 19 concomitant conditions; thus, 0 indicates no comorbid conditions, while higher scores indicate a greater level of comorbidity [15]. Moreover, the mean number and the mean duration of iTTP-related hospitalization before the index date (if any) were collected.

The primary outcomes of this analysis were the number and length of ordinary and ICU hospitalizations and mortality rates among patients treated or untreated with caplacizumab. As secondary endpoints, the groups were described for their general demographic features, comorbidity profile, and previous history of TTP-related hospitalizations. Lastly, the trends in the annual incidence of iTTP in the 3-year period before and after caplacizumab introduction in Italy (2020) were estimated.