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Objectives To define a fit-for-purpose clinical outcome assessment for thrombocytopaenia, as part of a novel multi-domain outcome measure for systemic lupus erythematosus (SLE) clinical trials, the Treatment Response Measure for Systemic Lupus Erythematosus (TRM-SLE).Methods An international working group comprising 14 expert clinicians (adult and paediatric rheumatologists and a clinical and laboratory haematologist), and 4 patients with lived experience of haematologic SLE participated in a multi-step consensus process. First, via structured discussion, the scope of thrombocytopaenia was defined for the SLE clinical trial context. Working group members then nominated and shortlisted potential response measures which were evaluated in a systematic literature review (SLR) focusing on relevant measurement properties and clinical trial utility. Informed by the SLR, nominal group technique (NGT) was used to reach consensus on the response measure, and thresholds for entry, minimum clinically meaningful response and complete response.Results The thrombocytopaenia domain was defined with 100% consensus as ‘immune-mediated thrombocytopaenia attributed to active lupus that impacts the patient and is modifiable by therapy to reduce or control disease activity’, irrespective of antiphospholipid syndrome/antibody status, and excluding other causes of thrombocytopenia (e.g. gestational, drugs, cancer, infection). 31 candidate measures (12 unique) were nominated, and after structured discussion 2 were shortlisted for evaluation via SLR: platelet count and blood film schistocytes. 4,755 studies were screened with 33 studies included in the SLR. The NGT process selected platelet count (Plt) as the consensus treatment response measure with the following thresholds: entry (Plt <50 x109/L), minimum clinically meaningful response (Plt ≥50 x109/L and at least 2x the baseline value), and complete response (Plt ≥100 x109/L). ( Figure 1)Abstract PO:05:138 Figure 1Conclusions Consensus was reached to use platelet count, with thresholds defined for entry and improvement, to classify thrombocytopaenia treatment response as part of the TRM-SLE multi-domain instrument for use in SLE clinical trials.