Two-Step Testing for Heparin-Induced Thrombocytopenia - Results and Associations
Authors: Deloughery Emma P , Deloughery Thomas G 
Published: December 2024

Introduction: Heparin-induced thrombocytopenia (HIT) is a feared complication of heparin therapy, resulting in thrombocytopenia and, paradoxically, thrombosis. Testing at our institution involves the sensitive platelet-factor 4 (PF4) followed by, if positive, the specific serotonin release assay (SRA). Deciding who to test is imperative given the potential for false positives and resultant unnecessary and potentially harmful interventions. The 4T score is often used to determine the pre-test probability of HIT and need for testing, however this score requires knowledge of the platelet trend, heparin exposure timeline, and presence or absence of thrombosis, thus requiring significant testing and records that may not be available. Additionally, the 4T score includes a subjective component that relies on the scorer's judgement regarding alternative explanations for the thrombocytopenia. The extensive knowledge required for calculating the 4T score and its subjective nature make the development of new predictive models for HIT imperative. We utilized our institution's HIT data to identify risk factors for HIT and help guide testing.
Methods: Patients were identified by performance of PF4 or HIT antibody test followed by, if positive, SRA testing, and were categorized into three groups: negative (-) PF4, positive (+) PF4 and (-) SRA, and (+) PF4 and (+) SRA. Only those with both (+) PF4 and (+) SRA were considered to truly have HIT. Inclusion criteria included presence of additional laboratory data at or around the time of HIT testing as well as information on patient circumstances surrounding testing. All patients with a (+) PF4 were included as were 100 randomly selected patients with a (-) PF4. Change in platelet count was defined as the difference between the maximum platelet count in the past 30 days and platelet count at or around time of HIT testing. Data was analyzed using Welch's t-test, chi-square, and ANOVA.
Results: A total of 76 patients had a (+) PF4 and SRA from 2010-2024 while 100 had a (+) PF4 and (-) SRA (positive predictive value (PPV) of 43.2% for PF4). Only 31.5% of patients had a 4T score recorded around the time of HIT testing, including 50% of (+) PF4 and (+) SRA, 35% of (+) PF4 and (-) SRA, and 14% of (-) PF4 groups.
All three groups had similar platelet counts around time of HIT testing (72.7 (+) SRA vs 97.3 (-) SRA vs 83.3 (-) PF4, P = 0.2) but (+) SRA patients had a significantly greater decrease in platelet count than (-) PF4 patients (173 vs 126.1, P = 0.02).
Frequency of both surgery and cardiac/vascular surgery were higher in the (+) SRA group compared with the (-) PF4 (surgery 76.3% (+) SRA vs 55% (-) PF4, P = 0.006; cardiac surgery 60.5% vs 30%, P &lt; 0.001) as well as in the (-) SRA group compared with the (-) PF4 (surgery 71% (-) SRA vs 55% (-) PF4, P = 0.03; cardiac surgery 51% vs 30%, P = 0.004).
The (-) SRA group showed a tendency towards increased frequency of dialysis as compared with both the (+) SRA (21.1% (+) SRA vs 36% (-) SRA , P = 0.05) and (-) PF4 (36% (-) SRA vs 23% (-) PF4, P = 0.06) groups. There was no significant difference in presence of infection, cirrhosis, or cancer among the groups.
Positive PF4 (regardless of SRA) was associated with lower hemoglobin (9.3 g/dL (+) PF4 vs 9.9 (-) PF4, P = 0.03), higher white blood cell count (WBC) (13.0 vs 11.2, P = 0.04), greater change in platelet count (155.8 vs 126.1, P = 0.03), greater frequency of surgery (73.3% vs 55%, P = 0.003), and greater frequency of vascular surgery (55.1% vs 30%, P &lt; 0.001).
HIT was associated with a higher WBC (13.7 HIT vs 11.8 no HIT, P = 0.05), greater change in platelet count (173 vs 134.4, P = 0.03), greater frequency of surgery (76.3% vs 63%, P = 0.05), and greater frequency of cardiac surgery (60.5% vs 40.5%, P = 0.004).
Discussion: This data shows that HIT is associated with leukocytosis and large change in platelet count. This data also agrees with prior research that surgery, particularly vascular surgery, is a risk factor for HIT, and that dialysis may be a risk for false positive PF4. With the low PPV of the PF4 in our population as well as low utilization of the 4T score it is important to carefully select patients to undergo HIT testing to avoid false positive tests, and this data suggests possible HIT risk factors to consider before ordering testing.