Closing analysis of the patient and disease characteristics of the UK primary paediatric ITP registry

Abstract Introduction: Immune thrombocytopenia (ITP) is a rare autoimmune disorder characterised antibody-mediated platelet and megakaryocyte destruction. The UK Childhood ITP Registry (UKCITP) opened in January 2007 and closed to recruitment in January 2024 and final data entry in March 2024. It includes 2271 cases from 139 UK centres (Research ethics reference: 06/MRE03/09). This analysis aimed to describe the patient (pt), and disease characteristics of UK children diagnosed with primary ITP. Methods: For this analysis, pt were excluded if they had platelet count >100 x 109/L (plt x 109/L), had secondary ITP as defined by the international working party consensus, or an alternative cause of thrombocytopaenia identified. Data was analysed in different age groups: <2 years (y), 2-5y, 6-11y and 12 to <18y. Results: 499 pts were excluded from analysis: 22 for secondary ITP, 2 for alternative causes of low plt, and 475 due to insufficient data or plt>100 at diagnosis. Data from 1772 children (52.9% male, 84.4% white) were analysed. Age at presentation varied, peaking ay 2-5y (<2y = 368, 2-5y = 784, 6-11y = 393 and 12-17y = 227). The male-to-female (M:F) ratios at diagnosis shifted from male predominance in the younger cohorts to female predominance in the older cohorts (age <2 1.6:1, age 2 to 5 1.22:1, age 6 to 11 1:1.1 and age 12 to 17 1:1.5). The majority of this registry is of Caucasian origin with a gradual reduction of non-Caucasian presentation occurring with age (<2y = 82.1%, 2-5y = 83.9%, 6-11y = 84.7% and 12-17y = 89.0% white). Mean time of follow up (FU) for documented plt counts was 23.2 months (mo), however, pt were discharged from local follow up and the registry once counts remained >150, per routine practice, but could re-enter the study at a later date. The rate of persistent ITP (plt <150) falls from 39.8% at 6mo to 23.4% at 12mo and 16.6% at 24mo FU. The rate of persistent disease is lowest in the <2y cohort at 25.0% at 6mo, 13.0% at 12mo and 6.8% 24mo; this contrast to the higher rate in the 12-17y cohort at 63.9%,45.8% and 31.7% at 6,12 and 24mo respectively. At presentation 81.7% had a plt<20 and 95.8% <50. The median plt was lowest in the <2y cohort (6.5%) and highest in the older cohort (9.0%). The proportions with persisting plt <20 drops from 14.5% at 6mo to 7.5% at 12mo and 4.4% at 24mo across all age groups. The rates are lowest in the <2 cohort at 7.3%, 2.7% and 1.6% at 6, 12 and 24mo; and highest in the older cohort at 14.5%, 7.5% and 4.4% at 6, 12 and 24mo. The proportions with plt<50 follows the above trend reducing from 21.8% at 6mo to 11.6% at 12mo and 7.2% at 24mo across all age groups. Again, rates are lowest in the <2y cohort at 7.1%, 4.1% and 1.9% at 6,12 and 24mo and highest rates in the 12-17y cohort at 18.5%, 12.8% and 6.2% at 6, 12 and 24mo. The most common factor associated with the onset of ITP was a preceding viral illness, reported in 58.2% of <2y, 58.0% of 2-5y, 45.3% of 6-11y and 41.0% of 12-17y. Vaccination preceded ITP significantly more often in the <2y cohort (57.6%), reflecting the UK paediatric vaccination schedule compared to 2-5, 6-11 and 12-17y (7.4, 2.5 and 9.7% respectively). Of the 284 bone marrow aspirations, the procedure was more common in the 6-11y and 12-17y cohorts (20.9% and 23.8% respectively). 744 pts were admitted to hospital at presentation for a median of 24 hours. 45.43% were between 2-5y, followed by 25.1% at <2y, 18.8% at 6-11y and 10.6% at 12-17y. Conclusion: This analysis offers unique insight into the paediatric primary ITP population; gathering data from both specialist and local centres across the UK. As such, this cohort reflects real-world data, including spontaneous remitters, though recruitment bias may reduce alignment with current UK census ethnicity statistics. These data indicate that, the shift to predominant female disease occurs in the 6-11y cohort, not in the adolescent group, which has been previously suggested. The rate of persistent disease is shown to increase with age reinforcing the need for plt supporting therapies across all ages but particularly in the older age group. Further analysis will assess the proportion of pts with persistent severe thrombocytopaenia receiving rescue and second line therapies and compare the UKCITP against adult and international paediatric counterparts.