According to a paper in the Journal of Thrombosis and Haemostasis the timing of the thrombocytopenia is highly predictive:
The high frequency of thrombocytopenia in post-cardiac surgery patients makes it challenging to diagnose heparin-induced thrombocytopenia (HIT). Two platelet count profiles are reported as indicating possible HIT in these patients: profile-1 describes a platelet count fall that begins between postoperative days 5 to 10, whereas profile-2 denotes early-onset thrombocytopenia that persists beyond day 5.
...Thrombocytopenia that begins between 5 to 10 days post-cardiac surgery is highly predictive for HIT. In contrast, early-onset and persisting thrombocytopenia is usually caused by non-HIT factors with coinciding heparin-dependent antibody seroconversion.
Many patients with antibodies coincidentally have non-HIT related thrombocytopenia.