Heparin-induced thrombocytopenia (HIT) is a rare complication of heparin or enoxaparin exposure where an autoantibody against platelet factor 4 (PF4) binds to heparin and creates a complex that leads to activation of platelets, causing a hypercoagulable state, which can result in both arterial and venous thrombosis. The serotonin release assay (SRA) is the gold standard for diagnosis; however, a small subset of patients presents with clinical features of HIT yet have persistently negative SRA results, creating diagnostic challenges. A 47-year-old male presented to multiple hospitals for heart failure exacerbations, was started on heparin, and then developed thrombocytopenia and a thrombus. The heparin-induced platelet antibody (HIPA) test was positive, but the SRA testing was negative on two occasions at two different institutions. Despite negative SRA results, the patient’s clinical course and laboratory results strongly suggest SRA-negative HIT. This case highlights the growing recognition needed with SRA-negative HIT, particularly in complex patients requiring mechanical circulatory support. Novel testing mechanisms have been suggested to improve sensitivity and improve the detection of true pathogenic HIT, but further research is needed. Additional clinical awareness is essential, as delayed recognition can lead to catastrophic thrombotic events. In such scenarios, clinicians should maintain a high index of suspicion and initiate alternative anticoagulation when warranted. This case underscores the importance of integrating clinical judgment with laboratory data, especially when conventional testing yields discordant results.

Heparin-induced thrombocytopenia, Heparin-induced platelet antibody, Serotonin release assay, Left ventricular assist device, Anticoagulation, Complications.

HIT is a prothrombotic adverse drug reaction caused by the formation of anti-platelet factor 4 4-heparin (PF4/H) antibodies. In vivo, PF4/H antibodies lead to increased platelet activation, causing immune-mediated platelet clearance and thrombosis. Thrombocytopenia in conjunction with new thrombosis is a key factor prompting further clinical investigation.[1,2] No single test provides 100% sensitivity or specificity; thus, a combination of immunoassay followed by functional tests is commonly used. Enzyme-linked immunosorbent assay (ELISA) is the immunoassay that detects presence of anti-PF4/H antibodies with high sensitivity (>95%) but lower specificity (50-89%). ELISA tests are rapid, cost-effective, and offer a high negative-predictive value (0.998; 95% confidence interval, 0.970-1.000), resulting in high utilization for initial testing following clinical suspicion.[3] Due to the low specificity, however, functional tests are necessary to confirm the diagnosis. The SRA measures the ability of anti-PF4/H antibodies to cause platelet activation via the release of serotonin with exceedingly high sensitivity and specificity (>95%). The SRA is the gold standard test but is limited by higher cost, lower widespread availability, and prolonged turnaround time.[4]

Rapid diagnosis and initiation of therapeutic anticoagulation are key to improving outcomes in HIT. Thrombosis, in a 3-4:1 venous: arterial ratio, can occur in up to 50% of patients if left untreated.[5] HIT produces a positive feedback reaction with PF4 released from α-granules contributing to further platelet activation, warranting prompt treatment to prevent life-threatening thrombotic complications such as limb or organ ischemia and skin necrosis. Despite the high sensitivity and specificity SRA tests provide, rare incidences of false-negative results have been reported.[6] This report describes a case of SRA-negative HIT in a patient undergoing left ventricular assist device (LVAD) implantation.

A 47-year-old male with a history of chronic systolic heart failure (EF < 20%), group 2 pulmonary hypertension (HTN), pulmonary embolism, and atrial fibrillation presented for evaluation for advanced heart failure therapies. He has multiple previous admissions for acute chronic heart failure exacerbation. In 2021, he was presented at an outside hospital with shortness of breath (SOB). He was found to have an LV thrombus and treated with intravenous heparin; however, he subsequently developed profound thrombocytopenia (platelets from 160 to a nadir of 92) with clinical concern for HIT. HIPA testing was positive with an optical density (OD) of 2.533; however, confirmatory SRA was negative. Heparin was discontinued, and the patient was transitioned to a fondaparinux bridging to a warfarin regimen. He presented to the same outside hospital approximately 1 month later with SOB and was found to have a new pulmonary embolus. Repeat HIT testing was obtained, demonstrating a positive HIPA test with an OD 1.535 and a negative SRA. Hematology was consulted and recommended a transition from warfarin to rivaroxaban therapy.

He had several subsequent hospital admissions over the next 3 years and was referred to the Advanced Heart Failure Clinic for further evaluation. He was approved for the implantation of an LVAD. On admission, he was initiated on fondaparinux therapy given his history of HIT, but was transitioned to enoxaparin for venous thromboembolism prophylaxis given multiple negative SRA tests. Platelet count remained stable, and a repeat SRA test was obtained at that time, which was negative. The decision was made to use intraoperative heparin during LVAD placement.

On post-op day 0, the platelet count fell from 160 to 128 and trended down to a nadir of 77 on post-op day 2. Platelets recovered to 141 on post-op day 4, and the patient started on subcutaneous heparin prophylaxis on post-op day 5. The platelet count dropped from 121 to a standard of 53 on post-op day 9. The patient was also found to have a new left upper extremity deep vein thrombosis on post-op day 9. At this time, the patient’s 4T score was 7 (high probability of HIT at ~64%), and heparin therapy was discontinued. Warfarin therapy started on hospital day 6, and therapeutic INR was achieved on post-op day 10. The patient was discharged on warfarin therapy.

This case describes an incidence of SRA-negative HIT in a medically complex patient undergoing LVAD implantation. While SRA testing remains the gold standard for HIT diagnosis, the true incidence of false-negative results remains unknown. Previous literature has reported incidence ranging from 50% to <10%.[6] One study found 27 of the 430 patients evaluated had SRA-negative HIT, and a significantly increased 30-day mortality was observed in SRA-negative patients compared to SRA-positive (29.6% vs 8.6%, p=0.045). This case adds to the growing body of evidence demonstrating the rare but serious subset of HIT.

Patients undergoing cardiac surgery are at a uniquely increased risk of developing HITHeparin was re-challenged given the ability for direct measurement intra-operative, availability of a rapid and effective reversal agent (protamine), and familiarity of use in intensive cardiopulmonary procedures. While the patient had high OD (>1) previously, he also had multiple negative SRA tests and stable platelets when re-challenged with enoxaparin. This case highlights the difficulty in detecting true pathogenic HIT in medically complex patients

PF4 ELISA tests are commonly used as the initial step in suspected HIT cases, but are limited by poor sensitivity and an inability to distinguish non-pathologic antibody formation, resulting in a high incidence of false positive results. Functional tests are necessary to confirm pathological platelet activation via the PF4/H antibodies. Following activation, platelets release their α-granules and dense bodies to elicit a serial reaction resulting in platelet aggregation. SRA measures serotonin released by platelet dense granules via liquid chromatography tandem mass spectrometry methods. A positive SRA test occurs when >20% release of radiolabeled serotonin. Some clinicians have suggested using a modified PF4-SRA to increase sensitivity. The modified PF4-SRA tests either remove heparin and supply exogenous PF4 or add supplemental PF4 to heparin-dependent SRA to enhance the detectability of pathogenic anti-PF4/H antibodies.[9-11] These enhanced assays may prove beneficial in cases of SRA-negative HIT, but have limited data at this time.

This case highlights the difficulty in detecting true pathogenic HIT in medically complex patients. It also supports previous reports and highlights the need for further investigation into modified tools to supplement currently available assays. Further research is necessary to fully elucidate the true incidence of SRA-negative HIT, particularly in cardiac surgery populations.

None

No financial disclosure

Corresponding Author:
Alivia Adkins
Department of Internal Medicine
The University of Tennessee Health Science Center, Memphis, USA
Email: alivia_adkins@teamhealth.com

Co-Authors:
Sarah Harlan, Cassidy Ruckel
Department of Pharmacy
Baptist Memorial Hospital, Memphis, USA

All authors were involved in conceptualizing, drafting, and editing the manuscript.

Verbal consent was given by the patient.

The authors declare no conflicts of interest.

None

https://doi.org/10.63096/medtigo30612311

Adkins A, Harlan S, Ruckel C. A Rare Presentation of SRA-Negative Heparin Induced Thrombocytopenia in LVAD Patient. medtigo J Pharmacol. 2025;2(3):e30612311. doi:10.63096/medtigo30612311 Crossref