PF4 and Venous thrombosis: The Level 1 (highest level of certainty) Brighton TTS interim case definition is based on laboratory evidence of thrombocytopenia <150 x109/L with no known recent exposure to heparin, with radiologically-confirmed thrombosis/thromboembolism, or persistence headache with elevated D-dimer, or both a clinical presentation consistent with site-specific symptoms of thrombosis and raised D-dimer or positive anti-PF4 [5].