What defines Heparin Induced Thrombocytopenia (HIT)?

Heparin Induced Thrombocytopenia (HIT) is characterized primarily by a significant decrease in platelet count following heparin exposure. Specifically, a diagnosis of HIT is made when there is a platelet count that falls below 100,000 platelets per microliter or in cases where there is a reduction of 50% or more from the patient's baseline platelet count. This is crucial because it indicates an immune-mediated reaction to heparin where antibodies form against the heparin-platelet factor 4 complex, leading to platelet activation and consumption.

The other options do not accurately reflect the diagnostic criteria for HIT. An increase in platelet count after heparin would not indicate HIT, as this condition is associated with a decrease, nor would an increase in bleeding occur without thrombocytopenia as a defining feature of HIT. Improved partial thromboplastin time (PTT) readings post-heparin treatment does not pertain to HIT and reflects heparin’s efficacy in anticoagulation rather than its effect on platelet counts. Understanding these key definitions is essential for recognizing HIT in clinical practice.