The approach to heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a prothrombotic drug reaction caused by platelet-activating antibodies that recognize multimolecular complexes of platelet factor 4 (PF4) bound to heparin. HIT is an intense hypercoagulability state (increased thrombin generation in vivo) that is complicated more often by venous thromboembolism (deep vein thrombosis, pulmonary embolism) than by arterial thrombosis. HIT is a risk factor for coumarin-induced microthrombosis, particularly affecting acral regions of limbs with deep vein thrombosis (venous limb gangrene). Coumarins (e.g., warfarin) are therefore contraindicated during the acute (thrombocytopenic) phase of HIT. Venous thromboembolism can occur early during an episode of HIT, sometimes even before HIT-associated platelet count declines become clear. Recognition of HIT may be facilitated through the use of a clinical scoring system, the 4Ts ( Thrombocytopenia, Thrombosis, Timing, and o Ther explanations). Anti-PF4/polyanion enzyme-immunoassays (EIAs) and washed platelet activation assays readily detect HIT antibodies, and thus have high diagnostic sensitivity; however, only the platelet activation assays have high diagnostic specificity, suggesting that HIT is likely to be overdiagnosed in settings where EIAs are used exclusively for diagnosis. Treatment of HIT emphasizes substitution of heparin with an alternative nonheparin anticoagulant, such as a direct thrombin inhibitor (lepirudin, argatroban), or an indirect (antithrombin-mediated) inhibitor of factor Xa (danaparoid, fondaparinux?).     

An overview of the heparin-induced thrombocytopenia syndrome

Heparin-induced thrombocytopenia (HIT) is caused by heparin-dependent, platelet-activating IgG antibodies that increase thrombin generation in vivo, producing a prothrombotic phenotype. In addition to platelet activation, there is in vitro evidence that activation of endothelium and monocytes occurs, perhaps directly by HIT antibodies, but more likely through activated platelet (or microparticle)-endothelial-leukocyte interactions. Patients with cardiac disease receiving heparin present important diagnostic and therapeutic issues when unexpected thrombocytopenia arises. Concomitant vascular disease burden and intravascular catheter use further increase risk of HIT-associated arterial thrombosis in this patient population. Whether arterial thrombosis simply reflects the "hypercoagulability state" of HIT interacting with diseased or injured arteries, or whether arterial "white clots" reflect additional prothrombotic effects of HIT via endothelial and monocyte activation, remains uncertain. Patients with HIT can also develop deep-vein thrombosis, which can progress to limb loss if coumarin (warfarin) leads to severe protein C depletion (coumarin-induced venous limb gangrene). Therapy for patients strongly suspected to have HIT should focus on inhibiting thrombin (or its generation) pharmacologically. Two direct thrombin inhibitors (lepirudin, argatroban) are approved for treating HIT. When using these agents, coumarin anticoagulation should be delayed pending substantial resolution of thrombocytopenia, before cautiously introducing overlapping coumarin therapy.     

Heparin-induced thrombocytopenia with pulmonary embolism and disseminated intravascular coagulation associated with low-molecular-weight heparin

Heparin-induced thrombocytopenia (HIT) is a severe adverse effect of heparin therapy. Although most cases occur in patients receiving unfractionated heparin, HIT can arise in venous thrombosis prophylaxis with a low-molecular-weight heparin (LMWH). We report an uncommon case of HIT in a postoperative orthopedic patient associated with LMWH (nadroparin), complicated by deep venous thrombosis, pulmonary embolism, and disseminated intravascular coagulation, treated successfully with recombinant hirudin and immunoglobulin therapy.     

Heparin-induced thrombocytopenia in hemodialysis. Case report and review of the literature

Thrombocytopenia is a potential complication of heparin therapy. There are two forms of heparin-induced thrombocytopenia (HIT). Type-I HIT is characterized by a mild decrease in platelet count that occurs within the first 2-4 days after heparin initiation. The platelet count often returns to normal without stop heparin treatment. The mechanism of thrombocytopenia appears to be due to a direct effect of heparin on platelet activation. The second form (type-II) is an immune-mediated disorder characterized by severe thrombocytopenia, which may include both arterial and venous thrombosis. We present a case of type-II HIT occurred in a hemodialysis patient resulting in acute pulmonary embolism and peripheral venous thrombosis, and review the literature.     

Heparin-induced thrombocytopenia: a review

Immune heparin-induced thrombocytopenia (HIT) is a relevant adverse drug reaction consisting in a hypercoagulable state caused by an anticoagulant agent. The incidence is approximately 6.5% in patients receiving unfractionated heparin after orthopedic surgery, and is equal to or lower than 1% in other settings. HIT occurrence is a function of heparin type, duration of heparin administration, patient population, and gender. The pathogenesis is due to an antibody response to the complex heparin/platelet factor 4 in most cases, with secondary activation of platelets and coagulation, and finally increased thrombin generation. Thrombocytopenia, venous or arterial thrombosis, heparin-induced skin necrosis, adrenal hemorrhage, and transient amnesia can characterize the clinical course of HIT. Platelet monitoring in patients receiving heparin is indicated to early detect HIT. A thrombotic event can be the first manifestation of HIT. Laboratory demonstration of heparin-dependent platelet activation confirms the clinical diagnosis; antigenic or functional assays are available. Once HIT is highly likely or confirmed serologically, immediate heparin cessation is mandatory and an alternative therapeutic anticoagulant is needed due to the high risk (or the presence) of thrombotic events. The available nonheparin anticoagulants aim to reduce thrombin generation. Lepirudin, argatroban, and bivalirudin (direct thrombin inhibitors) and danaparoid and fondaparinux (factor Xa inhibitors) represent the current treatment options for HIT. Vitamin K antagonists can be used safely only after a stable platelet count has been obtained.     

No difference in risk for thrombocytopenia during treatment of pulmonary embolism and deep venous thrombosis with either low-molecular-weight heparin or unfractionated heparin: a metaanalysis

BACKGROUND: Low-molecular-weight heparin (LMWH) is a popular alternative to unfractionated heparin (UH) for the treatment of pulmonary embolism (PE) and deep vein thrombosis (DVT), in part based on the perception of a lower risk for heparin-induced thrombocytopenia (HIT). To investigate the evidence supporting this perception, we performed a metaanalysis to compare the incidence of thrombocytopenia between LMWH and UH during PE and/or DVT treatment. METHODS: Randomized trials comparing LMWH with UH for PE and/or DVT treatment were searched for in the MEDLINE database, bibliographies, and by correspondence with published investigators. Two reviewers independently selected high-quality studies and extracted data regarding heparin-associated thrombocytopenia (HAT), HIT confirmed by laboratory testing, and heparin-induced thrombocytopenia with thrombosis (HITT). Outcome rates between LMWH and UH were compared using a binomial, generalized linear mixed model with a logit link and Gaussian random effects for study. RESULTS: Thirteen studies involving 5,275 patients met inclusion criteria. There were no statistically significant differences in HAT rates between the two treatments (LMWH, 1.2%; UH, 1.5%; p = 0.246). The incidence of documented HIT and HITT was too low to make an adequate comparison between groups. CONCLUSIONS: Our review disclosed no statistically significant difference in HAT between LMWH and UH and insufficient evidence to conclude that HIT and HITT rates were different between them. There was no evidence from randomized comparative trials to support the contention that patients receiving treatment for PE or DVT with UH are more prone to these complications than those receiving LMWH.     

A case of pulmonary embolism due to heparin-induced thrombocytopenia after the placement of hepatic arterial infusion catheter

A 68-year-old woman developed acute pulmonary embolism after hepatic arterial infusion therapy for advanced hepatocellular carcinoma. Because the platelet count was significantly reduced, heparin-induced thrombocytopenia (HIT) due to heparin usage in hepatic arterial infusion therapy was clinically suspected. Subsequently, the patient tested positively for HIT antibodies, and a definitive diagnosis was obtained. Antithrombotic therapy with heparin was discontinued and treatment with argatroban was started. After the heparinized hydrophilic catheter was removed, the platelet count improved immediately. HIT should be considered when a decrease in platelet count and thrombosis are involved with the usage of heparin.     

Spontaneous HIT syndrome post-knee replacement surgery with delayed recovery of thrombocytopenia: a case report and literature review

Recently published reports have established a heparin-induced thrombocytopenia (HIT)-mimicking thromboembolic disorder without proximate heparin exposure, called spontaneous HIT syndrome. Although the pathophysiology remains unclear, anti-platelet factor 4 (PF4)/heparin antibodies possibly triggered by exposure to knee cartilage glycosaminoglycans or other non-heparin polyanions found on bacterial surfaces and nucleic acids have been postulated. We present a 53-year-old female receiving antithrombotic prophylaxis with aspirin following right total knee replacement surgery (without perioperative or any previous lifetime heparin exposure) who acutely presented with high-risk pulmonary embolism (PE) and right great saphenous vein thrombophlebitis on postoperative day (POD) 14; her platelet count at presentation was 13 × 10⁹/L. Prior to diagnostic consideration of spontaneous HIT syndrome, the patient briefly received unfractionated heparin (UFH) and one dose of enoxaparin. The patient’s serum tested strongly positive for anti-PF4/heparin antibodies by two different PF4-dependent enzyme-linked immunosorbent assays (ELISAs) and by serotonin release assay (SRA). Failure of fondaparinux anticoagulation (persisting HIT-associated disseminated intravascular coagulation) prompted switching to argatroban. Severe thrombocytopenia persisted (platelet count nadir, 12 × 10⁹/L, on POD21), and 9 days after starting argatroban symptomatic right leg deep-vein thrombosis (DVT) occurred, prompting switch to rivaroxaban. Thereafter, her course was uneventful, although platelet count recovery was prolonged, reaching 99 × 10⁹/L by POD45 and 199 × 10⁹/L by POD79. The patient’s serum elicited strong serotonin release in the absence of heparin (seen even with 1/32 serum dilution) that was enhanced by pharmacological concentrations of UFH (0.1 and 0.3 IU/mL) and fondaparinux (0.1–1.2 μg/mL, i.e., in vitro fondaparinux “cross-reactivity”). Ultimately, platelet count recovery was associated with seroreversion to a negative SRA (documented at POD151). Our literature review identified joint replacement surgery, specifically knee replacement, to be a relatively common trigger of spontaneous HIT syndrome. Further, including our patient case, 5 of 7 patients with spontaneous HIT syndrome post-orthopedic surgery who received treatment with argatroban developed new and/or progressive lower-limb DVT or recurrent PE despite anticoagulation with this parenteral direct thrombin inhibitor, suggesting that this patient population is at high risk of breakthrough thrombotic events despite treatment with this HIT treatment-approved anticoagulant. Our case also illustrates successful outcome with rivaroxaban for treatment of spontaneous HIT syndrome, consistent with emerging literature supporting safety and efficacy of direct oral anticoagulant therapy for treatment of acute HIT.     

心脏直视手术后肝素诱导性血小板减少症的诊断与治疗经验

目的 探讨肝素诱导性血小板减少症（heparin-induced thrombocytopenia,HIT）的诊断及治疗经验.方法 自 1993年3月至2013年12月,高州市人民医院心脏外科共施行心脏直视手术18 156例,6例患者诊断疑似HIT.其中,男4例,女2例;年龄49～65岁;单纯人工机械瓣膜置换术2例,其中1例为二尖瓣再次置换;人工机械瓣膜置换术合并冠状动脉旁路术移植术2例;单纯冠状动脉旁路移植术2例,均为不停跳冠状动脉旁路移植术.症状出现时间为术后5～12 d,1例患者首要症状表现为血小板减少,其他5例患者均表现为肢体末梢淤黑.2例患者在本次术前存在肝素接触史,其他4例患者不能确定是否有肝素暴露史.第1例至第4例患者误诊为弥散性血管内凝血（DIC）,治疗方案错误.第5、6例获得及时诊断及治疗.诊断依据为Greinacher评分诊断系统,并重点与弥散性血管内凝血进行鉴别诊断;对于Greinacher评分达到6-8分的患者给予停用治疗用肝素及冲管用肝素盐水,停用双香豆素类药物,未进一步使用直接凝血酶抑制剂（DTI）.结果 第1例至第4例患者死亡,其中第1至第3例患者死于多脏器功能衰竭,第4例患者死于肺栓塞、低心排血量综合征、多脏器功能衰竭;第5例、第6例痊愈,病死率为66.7％.2例生存患者分别随访3.5年及1年,无血栓栓塞相关性疾病发生.结论 心脏直视手术后HIT是一种罕见但后果严重的并发症,只有对其有充分的理论认识,及时发现、早期干预才有可能挽救患者的生命,否则它将带来高病死率和并发症发生率.     

Risk of thrombosis in patients with malignancy and heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a common immunological drug reaction. After exposure to heparin, some patients develop heparin dependent antibodies with no evidence of thrombosis, while others are at risk of thrombocytopenia, thrombosis, limb loss, and death. We conducted a retrospective chart review on all patients serologically positive for HIT by HPIA ELISA in a single tertiary-care hospital, to determine whether patients with malignancy had an increased risk of thrombotic complications. Medical records of 55 patients who tested positive for HIT and met clinical criteria for HIT were analyzed. All patients had been treated with unfractionated heparin. Malignancy was diagnosed in 11 patients, either at surgery or post-mortem examination. A higher rate of venous thrombosis and pulmonary embolism was observed in patients with HIT and malignant disease when compared to patients with no underlying malignancy (odds ratio 13.6, 95% CI 2.9-63.8).     

Therapy of deep vein thrombosis with low molecular weight heparin, leg compression and immediate ambulation

BACKGROUND: Traditionally, patients with acute deep vein thrombosis (DVT) are treated with strict bed rest for several days to avoid clots from breaking off and causing pulmonary emboli. The purpose of this study is to give a precise estimate of short term complications like pulmonary embolism, bleeding, heparin-induced thrombocytopenia (HIT) and death in a cohort of consecutive patients who were admitted because of acute symptomatic DVT, all treated by compression and walking exercises instead of conventional bed-rest and nearly all by low-molecular-weight heparin. PATIENTS AND METHODS: In 1289 consecutive patients the following five endpoints were registered for the period of hospital-stay: 1. Frequency of pulmonary embolism (PE) at admission (V/Q lung scan), 2. Frequency of new PE's after 10 days (second lung scan), 3. Fatal events (autopsy), 4. Frequency of malignant disease, 5. Bleeding complications and HIT. RESULTS: 1. 190/356 (53.4% of iliofemoral, 355/675 (52.6%) of femoral and 84/239 (35.1%) of lower leg vein thrombosis showed PE (difference iliofemoral and femoral versus lower leg DVT p < 0.001). Two thirds of these PE were asymptomatic. 2. New PE after 10 days in comparison to the baseline scan occurred in 7.4%, 6.4% and 3.4% respectively. 3. Fatal events, all investigated by autopsy, were caused by PE in 3 patients aged over 76 years (0.23%), by malignant diseases in 12 (0.9%) and due to other causes in 2 (0.15%). 4. 232 patients (18%) had associated malignant diseases, from which 33% were detected by our screening. 5. Non-fatal bleeding complications were seen in 3.3%, including 5 patients (0.4%) with major bleeding. Three patients (0.2%) suffered from HIT II. CONCLUSION: The low incidence of recurrent and fatal pulmonary emboli in this series affirms the value of early ambulation with heavy leg compression in patients with symptomatic acute leg deep venous thrombosis. In addition, the presence of pulmonary emboli in one-third of those with calf vein thrombi emphasizes the importance of fully diagnosing and treating calf clots.     

PF4/heparin-antibody complex induces monocyte tissue factor expression and release of tissue factor positive microparticles by activation of FcγRI

Heparin-induced thrombocytopenia (HIT) is a potentially devastating form of drug-induced thrombocytopenia that occurs in patients receiving heparin for prevention or treatment of thrombosis. Patients with HIT develop autoantibodies to the platelet factor 4 (PF4)/heparin complex, which is termed the HIT Ab complex. Despite a decrease in the platelet count, the most feared complication of HIT is thrombosis. The mechanism of thrombosis in HIT remains poorly understood. We investigated the effects of the HIT Ab complex on tissue factor (TF) expression and release of TF-positive microparticles in peripheral blood mononuclear cells and monocytes. To model these effects ex vivo, we used a murine mAb specific for the PF4/heparin complex (KKO), as well as plasma from patients with HIT. We found that the HIT Ab complex induced TF expression in monocytes and the release of TF-positive microparticles. Further, we found that induction of TF is mediated via engagement of the FcγRI receptor and activation of the MEK1-ERK1/2 signaling pathway. Our data suggest that monocyte TF may contribute to the development of thrombosis in patients with HIT.     

New Concepts in Heparin-Induced Thrombocytopenia: Diagnosis and Management

Heparin-induced thrombocytopenia (HIT) is a clinicopathologic condition and adverse drug reaction caused by immunoglobulin G (IgG) antibodies directed against the heparin-platelet factor 4 complex. In most patients, the onset of thrombocytopenia begins while the patient is receiving heparin. In less than 5% of patients, the onset of thrombocytopenia begins several days following heparin discontinuation and has been termed “delayed-onset” HIT. This review summarizes the presentation and clinical course of published reports of delayed-onset HIT occurring in 30 patients. The diagnosis of delayed-onset HIT should be considered in all patients presenting with venous thromboembolism (VTE) and all patients with recent heparin exposure (within the past 14 days) who present with a low platelet count. Most patients with HIT are treated with direct thrombin inhibitors and transitioned to warfarin oral anticoagulation. Administration of direct thrombin inhibitors requires close monitoring for bleeding, dose adjustments based upon coagulation monitoring and is costly. Fondaparinux, a synthetic pentasaccharide and indirect-acting factor-Xa inhibitor, has little to no cross-reactivity with the heparin-platelet factor 4 antibody in in vitro testing. This review summarizes dosing, monitoring and outcomes of preliminary reports of fondaparinux successfully administered to 13 patients with subacute HIT and 22 patients with acute HIT. While several reports have described the treatment and prophylaxis of thrombosis in patients with HIT using fondaparinux, clinical trials should be conducted and reported before fondaparinux becomes a therapy of choice for HIT.     

Risk of heparin-induced thrombocytopenia in patients receiving thromboprophylaxis

Heparin-induced thrombocytopenia (HIT) is a clinicopathological syndrome associated with heparin therapy that is characterized by a decrease in platelet counts and/or the development of a new thrombosis. Two types of HIT exist, type I is nonimmune and self-resolves, whereas type II is immune-mediated and clinically important. The formation of antibodies against the platelet factor 4-heparin complexes results in platelet activation and thrombin formation, which lead to an increased risk of thrombosis. Unfractionated heparin is associated with a higher risk of HIT than low-molecular-weight heparins. Surgical patients, particularly those undergoing orthopedic or cardiac surgery, are at higher risk of HIT than medical patients. Treatment of HIT involves heparin cessation together with anticoagulation with direct thrombin inhibitors or indirect factor Xa inhibitors.     

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a syndrome of antibody-mediated activation of platelets and coagulation that confers increased risk for thrombosis. This article reviews recent studies of the pathogenesis, laboratory testing, frequency, and clinical presentation of HIT. Topics discussed include the nature of the neoepitopes on platelet factor 4 recognized by HIT antibodies, the nonspecific nature of the polyanions that support neoepitope formation, activation of monocytes and endothelium, and the development of new animal models. The transient "autoimmune" nature of HIT is highlighted in relation to new data concerning the rapid decline in HIT antibodies, as well as the recent recognition of a syndrome of "delayed-onset HIT" in which thrombocytopenia and thrombosis begin several days after stopping heparin. Detecting only HIT antibodies of the immunoglobulin G class is suggested as one way to increase diagnostic specificity of laboratory testing. Recent reports suggest that HIT could explain approximately 5% of cases of acute adrenal failure caused by bilateral adrenal hemorrhagic infarction.     

Successful liver transplantation in a patient with splanchnic vein thrombosis and pulmonary embolism due to polycythemia vera with Jak2v617f mutation and heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a rare complication of heparin treatment resulting in a severe acquired thrombophilic condition with an associated mortality of about 10 %. We report the first case of successful urgent liver transplantation (LT) in a patient with end-stage liver disease due to a Budd–Chiari syndrome, portal vein thrombosis and pulmonary embolism due to acquired thrombophilia associated to polycythemia vera carrying JAK2V617F gene mutation and HIT in the acute phase. Lepirudin was used to provide anticoagulation in the LT perioperative period that was performed without haemorrhagic and thrombotic complications despite the donor received heparin during liver explantation.     

Heparin-induced thrombocytopenia in a uremic patient requiring hemodialysis: an alternative treatment and reexposure to heparin.

Heparin-induced thrombocytopenia (HIT) is an uncommon but potentially serious complication of hemodialysis, and subsequent reexposure to heparin after the disappearance of antiheparin-PF4 complex antibodies (HIT antibody) has been controversial. We report a 60-year-old woman who was sensitized to unfractionated heparin (heparin) as anticoagulant during hemodialysis (HD) and heparin flush on a nonsession day. The patient suddenly developed acute systemic reactions with acute pulmonary embolism a few minutes after manipulation with heparin flush on day 9, a nonsession day. Although there was no evidence of pulmonary embolism on a pulmonary scintigram on the next day, the fifth HD session was discontinued owing to recurrence of acute systemic reactions and massive clots in the dialyzer 30 min into the session. After confirmation of the presence of HIT antibody and maturation of vascular access fistula, a sixth HD session was carried out with argatroban, a synthetic direct thrombin inhibitor, with a bolus of 10 mg and continuous infusion of 0.5 mg/kg/hr as an alternative to heparin. Optimal dose adjustment of argatroban through activated partial thromboplastin time (APTT) monitoring led to a bolus of 5 mg and continuous infusion of 0.15 mg/kg/hr. The patient's HD treatment at the same doses 3 times a week followed an uneventful course over 6 months. HIT antibody was seronegative about 40 days after the cessation of heparin treatment. Reexposure to heparin was attempted with the monitoring of HIT antibody and platelet counts before and after the sessions on day 210. The titers of HIT antibody compared with before the level of reexposure showed a transient insignificantly small peak, and dialysis with heparin has been maintained to date with no recurrence of HIT. The measurement of HIT antibody titer could be useful in assessing not only the effect of argatroban to replace heparin but also in predicting the recurrence of HIT due to reexposure.     

Antithrombotic therapy in heparin-induced thrombocytopenia: guidelines translated for the clinician

Heparin-induced thrombocytopenia (HIT) is a clinicopathologic syndrome initiated by heparin exposure and characterized by thrombocytopenia and paradoxical thrombophilia. HIT is mediated by the formation of antibodies against the platelet factor 4/heparin complex, which leads to platelet activation, thrombin generation, and potentially fatal thrombotic sequelae. The clinical presentation of HIT is variable and can be easily overlooked. Although a number of functional and antigen-based immunoassays have been developed to detect the presence of HIT antibodies, initial diagnosis is often based on recognition of thrombocytopenia in the appropriate clinical context and later confirmed with immunologic testing. Given the serious clinical consequences of HIT, immediate cessation of heparin products and administration of non-heparin anticoagulants are crucial components of treatment. We provide a review of the clinical syndrome and practical summary of treatment recommendations from the most recent 2012 American College of Chest Physicians evidence-based guidelines for the treatment and prevention of HIT.     

Treatment of heparin-induced thrombocytopenia: a critical review

Heparin-induced thrombocytopenia (HIT) is a serious complication of heparin therapy that has a high rate of morbidity (thrombosis and amputation) and mortality. In the past, a number of different anticoagulants have been used to treat HIT in an attempt to prevent these complications. More recently, direct thrombin inhibitors have become popular. This systematic review summarizes the risk for thrombosis in HIT patients when heparin therapy is stopped; evidence of the efficacy of thrombin inhibitors in patients with HIT with and without thrombosis; evidence supporting the use of thrombin inhibitors in patients with a history of HIT who require a coronary intervention procedure; and the risk for bleeding when antithrombotic agents are used.     

Heparin-induced thrombocytopenia

Summary Heparin-induced thrombocytopenia (HIT), next to bleeding complications, is the most important side-effect of heparin therapy in cardiac patients and the most frequently found thrombocytopenia induced by medication. Two types of HIT are distinguished on the basis of both severity of disease, and pathophysiology: type I HIT is an early, transient, clinically harmless form of thrombocytopenia, due to direct heparin-induced platelet aggregation. Thromboembolic complications are usually not seen. No treatment is required. A normalization of platelet count even if heparin is continued is a usual observation. Type II HIT is more severe than type I HIT and is frequently complicated by extension of preexisting venous thromboembolism or new arterial thrombosis. The thrombocytopenia is caused by a pathogenic heparin-dependent IgG antibody (HIT-IgG) that recognizes as its target antigen a complex consisting of heparin and platelet factor IV. Type II HIT should be suspected when the platelet count falls to less than 100,000 per cubic millimeter or less than 50% of the base line value 5 to 15 days after heparin therapy is begun, or sooner in a patient who received heparin in the recent past. The clinical diagnosis of type II HIT can be confirmed by several sensitive assays. In cases of type II HIT, heparin must be stopped immediately. However, if the patient requires continued anticoagulant therapy for an acute event such as deep venous thrombosis, substitution of an alternative rapid-acting anticoagulant drug is often needed. In the authors experience Danaparoid sodium, a low-sulfated heparinoid with a low cross-reactivity (10%) to heparin, can be regarded as an effective anticoagulant in patients with type II HIT. Preliminary experiences with intravenous recombinant hirudin are also encouraging and suggest that this direct thrombin inhibitor will emerge as a valuable alternative treatment for patients who suffer from HIT.