Diagnostic Reassessment of a Historical Case of Atypical Heparin-Induced Thrombocytopenia: Between Spontaneous Heparin-Induced Thrombocytopenia and a Vaccine-Induced Immune Thrombotic Thrombocytopenia-Like Syndrome.

PF4-dependent disorders encompass a heterogeneous group of immune-mediated thrombotic syndromes, including heparin-induced thrombocytopenia (HIT), its autoimmune variants such as spontaneous HIT, and vaccine-induced immune thrombotic thrombocytopenia (VITT). The recent identification of VITT and VITT-like entities has significantly expanded the diagnostic spectrum, complicating the retrospective interpretation of cases that occurred before their formal recognition. We report the case of a young patient who initially presented with a clinical and biological presentation suggestive of atypical HIT, at a time when neither spontaneous HIT nor VITT were defined. The patient was re-evaluated during the COVID-19 vaccination campaign, prompting a reassessment of the initial diagnosis in light of current knowledge on PF4-related disorders, which continue to increase in both diversity and complexity. A critical review of clinical and laboratory findings now favors a diagnosis of VITT-like syndrome over spontaneous HIT, although confirmatory testing is no longer feasible given the time elapsed since the acute phase. This case highlights the importance of revisiting historical cases using updated diagnostic criteria to improve the identification and management of these emerging and underrecognized syndromes.