Progressive thrombocytopenia after cardiac surgery in a 67-year-old man

Autoimmune Heparin-Induced Thrombocytopenia

Autoimmune thrombocytopenia (aHIT) is a severe subtype of heparin-induced thrombocytopenia (HIT) with atypical clinical features caused by highly pathological IgG antibodies ("aHIT antibodies") that activate platelets even in the absence of heparin. The clinical features of aHIT include: the onset or worsening of thrombocytopenia despite stopping heparin ("delayed-onset HIT"), thrombocytopenia persistence despite stopping heparin ("persisting" or "refractory HIT"), or triggered by small amounts of heparin (heparin "flush" HIT), most cases of fondaparinux-induced HIT, and patients with unusually severe HIT (e.g., multi-site or microvascular thrombosis, overt disseminated intravascular coagulation [DIC]). Special treatment approaches are required. For example, unlike classic HIT, heparin cessation does not result in de-escalation of antibody-induced hemostasis activation, and thus high-dose intravenous immunoglobulin (IVIG) may be indicated to interrupt aHIT-induced platelet activation; therapeutic plasma exchange may be required if high-dose IVIG is ineffective. Also, aHIT patients are at risk for treatment failure with (activated partial thromboplastin time [APTT]-adjusted) direct thrombin inhibitor (DTI) therapy (argatroban, bivalirudin), either because of APTT confounding (where aHIT-associated DIC and resulting APTT prolongation lead to systematic underdosing/interruption of DTI therapy) or because DTI inhibits thrombin-induced protein C activation. Most HIT laboratories do not test for aHIT antibodies, contributing to aHIT under-recognition.

Rivaroxaban Treatment for Heparin-Induced Thrombocytopenia: A Case Report and a Review of the Current Experience

Heparin-induced thrombocytopenia is a life-threatening complication of exposure to heparin. Heparin-induced thrombocytopenia results from an autoantibody directed against platelet factor 4 in complex with heparin. Heparin-induced thrombocytopenia is traditionally treated with bivalirudin, argatroban, danaparoid, or fondaparinux. Recently, direct oral anticoagulants administration to treat heparin-induced thrombocytopenia has been reported. Direct oral anticoagulants do not cause platelet activation in the presence of heparin-platelet factor 4 antibodies, nor do they provoke autoantibody production. Direct oral anticoagulants offer advantages such as consistent and predictable anticoagulation, oral administration with good patient compliance, and a good safety profile. We report a case of heparin-induced thrombocytopenia with deep venous thrombosis successfully treated with rivaroxaban and review the current experience with rivaroxaban for the treatment of heparin-induced thrombocytopenia.

Potential Role of Direct Oral Anticoagulants in the Management of Heparin-induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a rare, potentially life-threatening condition secondary to unfractionated heparin or low molecular weight heparin exposure. This immune-mediated drug reaction manifests as thrombocytopenia with a paradoxical hypercoagulable state that can result in life-threatening thrombosis. It is imperative to ensure cessation of heparin-based products as soon as HIT is identified. Traditional treatment options include argatroban, bivalirudin, fondaparinux, and danaparoid with a transition to warfarin upon platelet recovery. These anticoagulants are notwithstanding limitations including parenteral administration and routine laboratory monitoring leading to prolonged hospitalizations, emphasizing the need for new therapies. Direct oral anticoagulants (DOACs) have been increasingly investigated for the management of HIT and may overcome the aforementioned challenges of current therapies. The objective of this narrative review is to summarize the current HIT guidelines, discuss limitations to contemporary treatment options, provide insight into the emerging evidence for the DOACs rivaroxaban, apixaban, and dabigatran, and conclude with a clinical summary for their use in this setting. The PubMed, Google Scholar, and MEDLINE databases were searched for peer-reviewed literature from January 1, 2012, to June 31, 2018. Twenty-seven articles met inclusion criteria for review: 1 prospective trial, 5 retrospective cohort studies, and 21 case reports totaling 104 patients treated with a DOAC for HIT. The DOACs prevented new and recurrent thrombosis in 98% (n=102) of cases, and bleeding complications occurred in 3% (n=3). While current literature remains limited, it is suggestive of a potential role of DOACs for HIT, which has led to their integration into the 2018 American Society Hematology Guidelines with a conditional recommendation.

Laboratory diagnosis of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a clinical-pathological disorder; thus, laboratory testing for the pathogenic platelet-activating antiplatelet factor 4 (PF4)/heparin antibodies is central for diagnosis. The "iceberg" model summarizes the inter-relationship between platelet activation assays and PF4-dependent immunoassays, with platelet-activating antibodies comprising a subset of anti-PF4/heparin antibodies. The platelet serotonin-release assay (SRA), performed by reference laboratories, has high sensitivity and specificity for HIT (~95% each), and is especially suited for detecting highly pathogenic HIT sera containing both heparin-dependent and heparin-independent platelet-activating antibodies; this latter subgroup of antibodies explains "autoimmune HIT" disorders (delayed-onset, persisting, spontaneous, heparin "flush," fondaparinux-associated). Recently, SRA-negative HIT has become recognized, in which serum from some HIT patients contains subthreshold levels of platelet-activating antibodies (by SRA) that become detectable using a PF4-enhanced platelet activation assay. Unusual immunologic features of HIT include early antibody detectability (at onset of platelet count fall) and antibody transience (seroreversion). Widely available PF4-dependent enzyme immunoassays (EIAs) have high sensitivity but poor specificity for HIT, although specificity is enhanced with IgG-specific EIAs and strong positive results; unfortunately, EIA results are usually not available in real time. Automated rapid immunoassays, such as the chemiluminescence immunoassay (CLIA) and latex immunoturbidimetric assay (LIA), facilitate real-time laboratory diagnosis. Recently available likelihood ratio (LR) data for positive (LR+) and negative (LR-) test results allow clinicians to adjust their pretest probabilities for HIT, using Bayesian analysis, into real-time posttest probabilities that are dramatically increased (test positive) or decreased (test negative). Moreover, (semi-)quantitative CLIA- and LIA-positive results (weak, moderate, strong positive) can further refine the posttest probability of HIT.

Autoimmune Heparin-Induced Thrombocytopenia: Treatment Obstacles and Challenging Length of Stay

Patient: Male, 55

Final Diagnosis: Auto-immune heparin thrombocytopenia -Treatment obstacles and challenging length of stay

Symptoms: Thrombocytopenia • thrombosis

Medication: —

Clinical Procedure: IVIG

Specialty: Hematology

Argatroban-refractory, Heparin-induced Thrombocytopenia after Coronary Intervention with Radial Artery Occlusion

Heparin induced thrombocytopenia (HIT) is serious disorder that occurs in a small percentage of patients following exposure to heparin. HIT can further be classified into two types: HIT type 1 and type 2. Type 2 HIT is a potentially life threatening with clinically significant outcomes. It presents with thrombocytopenia and evidence of thrombus formation in the presence of antibody formation. Additionally, several variations of HIT exist, including delayed onset HIT and refractory HIT, known collectively as autoimmune HIT (aHIT). Here we present a case of delayed onset and refractory HIT in a patient with little heparin exposure, discovered only after cardiac intervention for suspected STEMI. Significant thrombotic events occurred thereafter, including radial artery stenosis and intracardiac thrombus. Treatment with argatroban was ineffective. Significant resolution of thrombocytopenia was seen several weeks after infusion with IVIG, thus depicting further suspicion for refractory HIT. IVIG for aHIT treatment is traditionally chosen only if the disease process is refractory to other anticoagulation efforts due to the potential risk for increasing thrombotic risk with IVIG infusion. Our case illistrate the rare presentation of aHIT and the use if IVIG to successfully treat thrombocytopenia in refractory HIT.

Non-vitamin K antagonist oral anticoagulants for heparin-induced thrombocytopenia. A systematic review of 54 reported cases

Supplementary Material to this article is available at www.thrombosis-online.com.

Autoimmune heparin-induced thrombocytopenia

Autoimmune heparin-induced thrombocytopenia (aHIT) indicates the presence in patients of anti-platelet factor 4 (PF4)-polyanion antibodies that are able to activate platelets strongly even in the absence of heparin (heparin-independent platelet activation). Nevertheless, as seen with serum obtained from patients with otherwise typical heparin-induced thrombocytopenia (HIT), serum-induced platelet activation is inhibited at high heparin concentrations (10-100 IU mL-1 heparin). Furthermore, upon serial dilution, aHIT serum will usually show heparin-dependent platelet activation. Clinical syndromes associated with aHIT include: delayed-onset HIT, persisting HIT, spontaneous HIT syndrome, fondaparinux-associated HIT, heparin 'flush'-induced HIT, and severe HIT (platelet count of < 20 × 109  L-1 ) with associated disseminated intravascular coagulation (DIC). Recent studies have implicated anti-PF4 antibodies that are able to bridge two PF4 tetramers even in the absence of heparin, probably facilitated by non-heparin platelet-associated polyanions (chondroitin sulfate and polyphosphates); nascent PF4-aHIT-IgG complexes recruit additional heparin-dependent HIT antibodies, leading to the formation of large multimolecular immune complexes and marked platelet activation. aHIT can persist for several weeks, and serial fibrin, D-dimer, and fibrinogen levels, rather than the platelet count, may be helpful for monitoring treatment response. Although standard anticoagulant therapy for HIT ought to be effective, published experience indicates frequent failure of activated partial thromboplastin time (APTT)-adjusted anticoagulants (argatroban, bivalirudin), probably because of underdosing in the setting of HIT-associated DIC, known as 'APTT confounding'. Thus, non-APTT-adjusted therapies with drugs such as danaparoid and fondaparinux, or even direct oral anticoagulants, such as rivaroxaban or apixaban, are suggested therapies, especially for long-term management of persisting HIT. In addition, emerging data indicate that high-dose intravenous immunoglobulin can interrupt HIT antibody-induced platelet activation, leading to rapid platelet count recovery.

Treatment of refractory delayed onset heparin-induced thrombocytopenia after thoracic endovascular aortic repair with intravenous immunoglobulin (IVIG)

Delayed onset heparin induced thrombocytopenia (HIT), is characterized by a late nadir due to persistent platelet-activating IgG antibodies. It typically begins or worsens 5 or more days after heparin is discontinued with complications such as thrombosis up to 3 weeks after exposure to heparin.1-3 In 50% of cases, the platelet count can decrease to very low numbers (<20 000/μL), which is not usual for typical HIT. Here we report 2 cases of post-operative delayed onset HIT manifesting as severe thrombocytopenia that persisted despite cessation of heparin and initiation of argatroban. Key Clinical Question: Is intravenous immunoglulin beneficial in severe refractory delayed-onset HIT?

Direct oral anticoagulants for treatment of HIT: update of Hamilton experience and literature review

Direct oral anticoagulants (DOACs) are attractive options for treatment of heparin-induced thrombocytopenia (HIT). We report our continuing experience in Hamilton, ON, Canada, since January 1, 2015 (when we completed our prospective study of rivaroxaban for HIT), using rivaroxaban for serologically confirmed HIT (4Ts score ≥4 points; positive platelet factor 4 [PF4]/heparin immunoassay, positive serotonin-release assay). We also performed a literature review of HIT treatment using DOACs (rivaroxaban, apixaban, dabigatran, edoxaban). We focused on patients who received DOAC therapy for acute HIT as either primary therapy (group A) or secondary therapy (group B; initial treatment using a non-DOAC/non-heparin anticoagulant with transition to a DOAC during HIT-associated thrombocytopenia). Our primary end point was occurrence of objectively documented thrombosis during DOAC therapy for acute HIT. We found that recovery without new, progressive, or recurrent thrombosis occurred in all 10 Hamilton patients with acute HIT treated with rivaroxaban. Data from the literature review plus these new data identified a thrombosis rate of 1 of 46 patients (2.2%; 95% CI, 0.4%-11.3%) in patients treated with rivaroxaban during acute HIT (group A, n = 25; group B, n = 21); major hemorrhage was seen in 0 of 46 patients. Similar outcomes in smaller numbers of patients were observed with apixaban (n = 12) and dabigatran (n = 11). DOACs offer simplified management of selected patients, as illustrated by a case of persisting (autoimmune) HIT (>2-month platelet recovery with inversely parallel waning of serum-induced heparin-independent serotonin release) with successful outpatient rivaroxaban management of HIT-associated thrombosis. Evidence supporting efficacy and safety of DOACs for acute HIT is increasing, with the most experience reported for rivaroxaban.

Functional Assays in the Diagnosis of Heparin-Induced Thrombocytopenia: A Review

A rapid and accurate diagnosis in patients with suspected heparin-induced thrombocytopenia (HIT) is essential for patient management but remains challenging. Current HIT diagnosis ideally relies on a combination of clinical information, immunoassay and functional assay results. Platelet activation assays or functional assays detect HIT antibodies that are more clinically significant. Several functional assays have been developed and evaluated in the literature. They differ in the activation endpoint studied; the technique or technology used; the platelet donor selection; the platelet suspension (washed platelets, platelet rich plasma or whole blood); the patient sample (serum or plasma); and the heparin used (type and concentrations). Inconsistencies in controls performed and associated results interpretation are common. Thresholds and performances are determined differently among papers. Functional assays suffer from interlaboratory variability. This lack of standardization limits the evaluation and the accessibility of functional assays in laboratories. In the present article, we review all the current activation endpoints, techniques and methodologies of functional assays developed for HIT diagnosis.

Clinical picture of heparin-induced thrombocytopenia (HIT) and its differentiation from non-HIT thrombocytopenia

HIT is an acquired antibody-mediated disorder strongly associated with thrombosis, including microthrombosis secondary to disseminated intravascular dissemination (DIC). The clinical features of HIT are reviewed from the perspective of the 4Ts scoring system for HIT, which emphasises its characteristic timing of onset of thrombocytopenia. HIT antibodies recognize multimolecular complexes of platelet factor 4 (PF4)/heparin. However, a subset of HIT sera recognise PF4 bound to platelet chondroitin sulfate; these antibodies activate platelets in vitro and in vivo even in the absence of heparin, thus explaining: delayed-onset HIT (where HIT begins or worsens after stopping heparin); persisting HIT (where HIT takes several weeks to recover); spontaneous HIT syndrome (a disorder clinically and serologically resembling HIT but without proximate heparin exposure); and fondaparinux-associated HIT (four distinct syndromes featuring thrombocytopenia that begins or worsens during treatment with fondaparinux), with a new patient case presented with ongoing thrombocytopenia (and fatal haemorrhage) during treatment of HIT with fondaparinux, with fondaparinux-dependent platelet activation induced by patient serum ("fondaparinux cross-reactivity"). Ironically, despite existence of fondaparinux-associated HIT, this pentasaccharide anticoagulant is a frequent treatment for HIT (including one used by the author). HIT can be confused with other disorders, including those with a) timing similar to HIT (e. g. abciximab-associated thrombocytopenia of delayed-onset); b) combined thrombocytopenia/thrombosis (e. g. symmetrical peripheral gangrene secondary to acute DIC and shock liver); and c) both timing of onset and thrombosis (e. g. warfarin-associated venous limb gangrene complicating cancer-associated DIC). By understanding clinical and pathophysiological similarities and differences between HIT and non-HIT mimicking disorders, the clinician is better able to make the correct diagnosis.

Heparin-induced thrombocytopenia

PURPOSE OF REVIEW

Thrombocytopenia and heparin exposure are common in critically ill patients, yet immune heparin-induced thrombocytopenia (HIT), a prothrombotic adverse effect of heparin, rarely accounts for thrombocytopenia in this patient population. The review discusses the clinical and laboratory features that distinguish HIT from non-HIT thrombocytopenia.

RECENT FINDINGS

The frequency of HIT in heparin-exposed critically ill patients is approximately 0.3-0.5% versus at least a 30-50% background frequency of non-HIT thrombocytopenia. Most patients who form anti-PF4/heparin antibodies do not develop HIT, contributing to HIT overdiagnosis. Disseminated intravascular coagulation (DIC), particularly in the setting of cardiogenic or septic shock associated with 'shock liver', can cause ischemic limb gangrene with pulses, mimicking a clinical picture of HIT. However, whereas non-HIT-related DIC with microthrombosis can be treated with heparin, HIT usually requires nonheparin anticoagulation. HIT-associated DIC can result in an elevated INR, which could reflect factor VII depletion because of extrinsic (tissue factor) pathway-mediated activation of coagulation.

SUMMARY

Greater understanding of the various clinical and laboratory features that distinguish HIT from non-HIT thrombocytopenia could help improve outcomes in patients who develop thrombocytopenia and coagulopathies in the ICU.

How I treat patients with a history of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a relatively common prothrombotic adverse drug reaction of unusual pathogenesis that features platelet-activating immunoglobulin G antibodies. The HIT immune response is remarkably transient, with heparin-dependent antibodies no longer detectable 40 to 100 days (median) after an episode of HIT, depending on the assay performed. Moreover, the minimum interval from an immunizing heparin exposure to the development of HIT is 5 days irrespective of the patient’s previous heparin exposure status or history of HIT. This means that short-term heparin reexposure can be safely performed if platelet-activating antibodies are no longer detectable at reexposure baseline and is recommended when heparin is the clear anticoagulant of choice, such as for cardiac or vascular surgery. The risk of recurrent HIT 1 to 2 weeks after heparin reexposure is ∼2% to 5% and is attributable to formation of delayed-onset (or autoimmune-like) HIT antibodies that activate platelets even in the absence of pharmacologic heparin. Some studies suggest that longer-term heparin reexposure (eg, for chronic hemodialysis) may also be reasonable. However, for other antithrombotic indications that involve patients with a history of HIT (eg, treatment of venous thromboembolism or acute coronary syndrome), preference should be given to non-heparin agents such as fondaparinux, danaparoid, argatroban, bivalirudin, or one of the new direct-acting oral anticoagulants as appropriate.

The platelet serotonin-release assay

Few laboratory tests are as clinically useful as The platelet serotonin-release assay (SRA): a positive SRA in the appropriate clinical context is virtually diagnostic of heparin-induced thrombocytopenia (HIT), a life- and limb-threatening prothrombotic disorder caused by anti-platelet factor 4 (PF4)/heparin antibodies that activate platelets, thereby triggering serotonin-release. The SRA's performance characteristics include high sensitivity and specificity, although caveats include indeterminate reaction profiles (observed in ∼4% of test sera) and potential for false-positive reactions. As only a subset of anti-PF4/heparin antibodies detectable by enzyme-immunoassay (EIA) are additionally platelet-activating, the SRA has far greater diagnostic specificity than the EIA. However, requiring a positive EIA, either as an initial screening test or as an SRA adjunct, will reduce risk of a false-positive SRA (since a negative EIA in a patient with a "positive" SRA should prompt critical evaluation of the SRA reaction profile). The SRA also provides useful information on whether a HIT serum produces strong platelet activation even in the absence of heparin: such heparin-"independent" platelet activation is a marker of unusually severe HIT, including delayed-onset HIT and severe HIT complicated by consumptive coagulopathy with risk for microvascular thrombosis.