External Multicenter Validation of the APTTO Model for Prolonged APTT Using Clot Waveform Analysis

Prolonged activated partial thromboplastin time (APTT) is a frequent laboratory finding that may reflect a broad spectrum of underlying conditions, ranging from benign laboratory abnormalities to clinically relevant hemostatic disorders. Clot waveform analysis (CWA), automatically generated during routine APTT testing by optical coagulation analyzers, provides additional quantitative and qualitative information on clot formation dynamics. The APTTO model is a previously developed two-step predictive algorithm based on CWA features designed to estimate the probability of a pathological cause of prolonged APTT and to differentiate lupus anticoagulant from intrinsic pathway factor deficiency or von Willebrand disease. Internal validation has demonstrated good discrimination and calibration. This multicenter observational study aims to perform an external validation of the APTTO model in independent patient cohorts, assessing its discrimination, calibration, and decision-analytic performance without model updating.

This multicenter observational cohort study is designed to externally validate the APTTO predictive model in patients with prolonged APTT and normal prothrombin time evaluated in routine clinical practice across multiple hospitals. All laboratory data, including CWA parameters and waveform morphology, are generated as part of standard diagnostic workflows. No additional blood sampling, laboratory testing, or modifications to clinical management are introduced for research purposes. The study focuses on evaluating model performance in independent cohorts by applying the original APTTO model coefficients and predefined cut-offs without recalibration or re-estimation. Model discrimination, calibration, and decision-analytic measures will be assessed. Secondary analyses will explore model performance across predefined subgroups and analytical robustness. This study adheres to the TRIPOD statement for validation of prediction models.

Inclusion Criteria: 1. Patients of any age (pediatric and adult populations) undergoing coagulation testing with: \- Prolonged activated partial thromboplastin time (APTT), defined as an APTT ratio ≥ 1.25. \- Normal prothrombin time (PT), according to local laboratory reference ranges. 2. Availability of clot waveform analysis (CWA) data obtained during routine APTT testing using: * Optical coagulation analyzers (ACL TOP platform). * Silica-based APTT reagent (SynthASil®). 3. Completion of the standard laboratory evaluation for prolonged APTT as part of routine clinical care, when clinically indicated. 4. Samples collected and processed in accordance with the standardized preanalytical protocol defined in the study SOP. 5. Patients evaluated in either: * Preoperative assessment, or * Routine clinical practice (non-preoperative setting). Exclusion Criteria: 1. Prolonged prothrombin time (PT) or combined prolongation of PT and APTT. 2- Inadequate preanalytical conditions, defined as non-compliance with the study SOP, including but not limited to: * Incorrect blood-to-anticoagulant ratio. * Delayed plasma processing beyond protocol-defined time limits. * Inadequate centrifugation or plasma quality. 3\. Absence of required CWA data or unavailable clot waveform images. 4\. Samples in which APTT values are outside the measurable range of the analyzer, preventing extraction of CWA-derived parameters. 5- Patients with missing essential clinical or laboratory data required for application of the APTTO models.