Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction associated with thrombocytopenia and a high risk for thrombosis.
In contrast to other drug-induced thrombocytopenia, significant spontaneous bleeding is rare with HIT even when thrombocytopenia is severe. This makes the diagnosis of HIT very perplexing. Clinical picture alone will not provide enough clues for the diagnosis. A careful history about the use and allergic reactions to medications, diseases that necessitate heparin use, and date and duration of heparin use should be obtained from the patient. Ancillary procedures should supplement history and physical examination in making a diagnosis.
HIT can be classified into three subtypes:
- The typical HIT presents on days 5 to 14 of heparin therapy in previously heparin-unexposed patients.
- Rapid-onset HIT, which is defined as a platelet count drop within hours of heparin administration, occurs on patients with pre-existing anti-PP4/heparin antibodies . The presence of pre-existing antibodies suggests that the patient has one of the following: recent heparin therapy (generally within the last month), previous heparin treatment or major surgery
- HIT that occurs 1 to 2 weeks after heparin administration is referred to as delayed-onset HIT. This is often associated with disseminated intravascular coagulation (DIC) .
Jaw & Teeth
Patients undergoing cardiopulmonary bypass had the highest rates of heparin-induced thrombocytopenia (7702 [0·63%; SE 0·03] of 1 230 362), followed by those undergoing haemodialysis (23 012 [0·47%; 0·01] of 4 908 100), those with gingival or periodontal [ncbi.nlm.nih.gov]
Furthermore, bacterial sepsis is associated with formation of these antibodies in a mouse model [ Krauel et al. 2011b ], and in humans there is an association between the presence of PF4/heparin antibodies and periodontitis, a chronic infection in which [doi.org]
Complete Blood Count
HIT commonly presents as a decrease in platelet count.
The thrombocytopenia is defined as:
- Platelet count drop of not less than 100,000/L
- Platelet count drop of >50% from the baseline platelet count.
The diagnosis of HIT can only be established by using enzyme-linked assays to detect antibodies against heparin-PF4 complexes or heparin-dependent platelet activation.
The current gold standard among the different enzyme-linked assays is the serotonin release assay (SRA). Other tests such as the polyspecific antigen assays (IgG/A/M) and the IgG-specific enzyme-immunoassay (EIA) are at par with SRA in terms of sensitivity (99%) in detecting HIT antibodies . What distinguishes SRA from other tests is its 95-99% diagnostic specificity compared to polyspecific IgG/A/M (50-75%) and IgG-EIA (55-90%) .
The SRA works by utilizing washed platelets loaded with serotonin to detect the presence of HIT antibodies in a patient’s serum. Heparin addition to the platelets should trigger platelet activation and subsequent serotonin release when HIT antibodies are present.
Stop all heparin administration
The offending agent should be removed immediately to stop further release of HIT antibodies.
Switch to an alternative anticoagulant
The following are some choices of anticoagulant to be used:
- Direct thrombin inhibitors (DTIs)
DTIs bind directly to thrombin and do not require binding to plasma cofactors such as antithrombin. This characteristic makes it appealing to use in HIT. Lepirudin and Agatroban are DTIs that are clinically proven to safely treat HIT.
The recommended dose is 75mcg/kg/hr and the dosage should be titrated based on aPTT monitoring. The recommended aPTT level is 45-70 seconds. It is important to note that lepirudin is cleared by the kindneys. It should not be used by patients with renal insufficiency.
The recommended dose is 1 mcg/kg/hr that is titrated based on aPTT monitoring. The level of aPTT should be maintained at 45-70 seconds. It is important to remember that agatroban is metabolized in the liver. Patients with hepatic problems should not use this drug.
Fondaparinux is a synthetic analogue of the unique pentassachardie sequence in heparin. However, fondaparinaux binds only to antithrombin and not thrombin. It also does not bind to plasma proteins; therefore, it does not activate platelets and cause release of and bind to PF4. The recommended dosage for fondaparinux in treatment of HIT is 2.5mg OD given subcutaneously.
Do not give platelet transfusions
Thrombocytopenia is a result of the circulating HIT antibodies causing intravascular coagulation. Therefore, the underlying cause (heparin) should be removed rather than give an unnecessary transfusion. Platelet transfusion will only be rendered useless since the platelets will become activated and participate in coagulation as it reaches the circulation.
Do not give warfarin until the platelet count reaches the lower normal limit of 150 x 109/L
HIT causes thrombin generation that consumes protein C. Warfarin is a vitamin K antagonist that depletes the protein C (a vitamin K-dependent anticoagulant) further. The end result is skin necrosis if the protein C in the body becomes insufficient.
Warfarin should only be given once platelet count is normal. It should be overlapped for 5 days with the alternative anticoagulants to maintain the half-lives of all the clotting factors.
The thrombocytopenia usually becomes corrected after 7 days in majority of patients. The circulating HIT antibodies will be cleared by the body after approximately 6-8 weeks unless there is re-exposure to heparin therapy.
The main etiology of HIT is a history of treatment with heparin, particularly unfractionated heparin (UFH).
- UFH easily binds to platelets. Binding between the two causes a release of PF4, which together with heparin is the main presenting antigen of HIT.
- UFH possesses a strong affinity to PF4. Their binding produces conformational changes within the PF4 . These changes invoke an immunologic response led by IgG antibodies.
The risk of HIT is dependent on the type of heparin used for treatment.
- Use of low-molecular weight heparin (LMWH) is preferred over unfractionated heparin (UFH) because of the lower risk for HIT .
- LMWH’s affinity to bind with plasma proteins, platelets, and PF4 is significantly lower than UFH’s. Therefore, the chances of triggering an IgG response from platelet activation and PF4-heparin complex decreases as well.
- The use of LMWH significantly decreases the risk of HIT by 5 to 10 times   .
- The only reason behind the recent increase in incidence of LMWH-induced HIT is that the use of LMWH is increasing in popularity too.One confounder to the risk of developing HIT with LMWH is prior exposure to UFH therapy. Previously exposed patients have a higher risk with LMWH treatment .
HIT is reported to occur in 0.2% to 5% of heparin-treated adults.
- This is a rough approximate because the challenge of clinical recognition of HIT is a very daunting task .
The typical detection of HIT occurs in between days 5 to 14 of heparin therapy.
- The range is variable due to the varying amounts of heparin available in the circulation of each individual. People who are known to receive UFH treatment can have an earlier course of HIT.
It is very rare to see thrombocytopenia below the value of 100,000/L in patients with HIT. Baseline levels should be obtained prior to heparin treatment. A drop of 50% from the baseline should provide a high index of suspicion among patients with heparin use .
HIT is frequently seen in surgical patients with comparison to medical patients.
- Major surgery induces inflammation and the activation of platelets. Activated platelets then promote the release of PF4. Consequently, PF4 will bind with endogenous non-heparin factors (e.g. an inflammatory process) to form PF4-non-heparin complexes. The body responds to these complexes by eliciting a B-cell mediated antibody response . Memory cells are formed with the inflammatory process. These cells can become active again when heparin is introduced. The PF4-heparin complexes may trigger the same memory cells to mount an immunologic response.
The type of surgery also plays a factor in HIT.
- A cardiopulmonary bypass increases the risk of HIT by 2 to 3 % compared to other major surgical procedures. The underlying cause is an increase in IgG antibody formation rate against the PF4-heparin complexes after operation. The rate increases significantly increases between the post-operative days 5 to 14 .
Females are more likely to be affected with HIT than males .
- The odds ratio of female sex to suffer from HIT is 2.37 .
A late complication of HIT is thrombosis. Thrombosis that is venous in origin is more common than arterial  with a ratio of 2:1 .
Heparin’s high-molecular weight is largely in part to its content of at least 18 saccharide units. These saccharide units enable heparin to bind to antiithrombin. However, the saccharide units are not exclusive to antithrombin alone. They also bind to platelets, which release PF4 upon activation, and plasma proteins such as PF4, which carries a great affinity for heparin. A heparin-PF4 complex, which is fortified by forming linear mulitimolecular clusters , can carry presenting antigens. An immune-mediated IgG antibody response ensues to neutralize the antigens by binding to the F domain of PF4. The antibody-antibody interaction promotes platelet aggregation and coagulation  by activating platelets which release thrombin-generating microparticles. Therefore, HIT’s thrombocytopenic manifestation is a result of intravascular platelet activation  caused by the immunologic response to the PF4-heparin antigen. The PF4-heparin complex antibodies also activate monocytes  and neutrophils. This leads to a prothrombotic platelet—monocyte—neutrophil linkage that further augments the formation of thrombosis in HIT.
HIT is an immunologic response to the administration of heparin. Careful switch to LMWH or other alternative anticoagulants must be discussed with the patient, especially high-risk patients, to ensure safety. These drugs may be more expensive than heparin but the cost-effectiveness of their administration in terms of safety is invaluable.
Heparin is a frequently used anticoagulant that works by activating antithrombin. A unique pentassacharide sequence found in heparin wraps around antithrombin to activate it. The activated antithrombin directly inhibits factor Xa and binds to thrombin. The newly formed antithrombin-thrombin complex renders thrombin inactive.
Unfortunately, heparin does not exclusively bind to thrombin and factor Xa. It also attaches to the endothelium which affects the half-life of the drug. It also binds to other plasma proteins such acute phase reactants and platelet factor 4 (PF4) . PF4 is released by activated platelets in the presence of thrombus and is highly attracted to heparin.
Binding of heparin to PF4 produces a PF4-heparin complex that is perceived as antigen. This triggers a cascade of IgG antibodies that are directed against the complex’s antigen. The interaction of the antigen with the antibody promotes activation of platelets and subsequent generation of platelet microparticles. These microparticles exhibit a prothrombotic effect by binding clotting factors and generating thrombin formation. The antibody-mediated process is known as Heparin-Induced Thrombocytopenia (HIT). Despite the thrombocytopenia, the major clinical complication of HIT is venous and/or arterial thrombosis  caused by the release of platelet microparticles.
It is important to note that HIT is a clinicopathologic syndrome . Clinical presentation alone will not provide sufficient evidence to diagnose HIT. The suspected HIT patient must be check for the presence of circulating IgG antibodies against the PF4-heparin antigen. The main limiting factor with ancillary diagnosis is that the antibodies present in HIT do not present in a typical primary (initially IgM followed by IgG) or secondary response fashion (persistence of IgG) . Therefore, timing is key to solve the HIT mystery. The IgG antibodies typically rise between days 4-10 after the first heparin dose . Therefore, prompt diagnostic procedures during these days should be done as soon as HIT is suspected to ensure accurate diagnosis and early intervention with alternative anticoagulants. A timely diagnosis will prevent life-threatening complications from arising.
Heparin-induced thrombocytopenia (HIT) is a sudden drop in platelet count after the use of heparin for treatment. This is mainly due to an immunologic or allergic response to some contents produced as heparin enters the blood. It occurs from 0.2-5% of patients receiving heparin and is very variable since each individual has a different response to heparin treatment.
- Females are more likely to suffer from HIT.
- Previous surgery prompts a person to make antibodies (defense cells). These cells may trigger HIT once heparin comes into contact with the defense cells.
Previous Heparin Treatment
- Previous or recent treatment with heparin may trigger a delayed immune system response to heparin and cause HIT.
- Below 100,000/L
- Greater than 50% lower from the baseline value
It is best to consult your physician for monitoring if you are undergoing heparin treatment. This holds true especially for first timers to ensure safety since HIT can be very tricky to diagnose. A timely diagnosis is key to preventing unwanted complications of heparin such as deep vein thrombosis (DVT), pulmonary embolism(PE), myocardial infarction (MI), or stroke.
Treatment of HIT should be done under the supervision of a physician. Blood monitoring is done throughout the treatment to make sure that bleeding or complications do not take place.
The allergic response to heparin usually clears out totally after 6-8 weeks. The platelet count should return to normal after a week of stopping heparin treatment.
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