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Heparin Induced Thrombocytopenia

Heparin induced thrombocytopenia (HIT) is an antibody mediated adverse effect of heparin that is strongly associated with venous and arterial thrombosis. Major risk factors include exposure to unfractionated high molecular weight heparin for more than 5 days, undergoing surgery and female gender. Diagnostic criteria include:

• Thrombocytopenia
• Onset 5 to 12 days after exposure to heparin
• Exclusion of other causes of thrombocytopenia
• Positive laboratory test for HIT

The incidence of HIT is 2.6% of patients exposed to unfractionated heparin and 0.2% exposed to low molecular weight heparin. Approximately 25% of these patients will develop thrombosis within 30 days. Most cases arise in patients who have not been previously exposed to heparin. In this situation, thrombocytopenia usually occurs 5 to 12 days after heparin initiation. In patients who have been previously sensitized to heparin, platelet counts may decrease within the first three days or even hours after re-exposure.  Platelet counts usually decrease more than 50% from baseline and typically fall to 20,000 - 150,000/uL.The nadir is usually reached 5 days after onset of the decline. The thrombocytopenia associated with HIT is not as severe as other drug induced thrombocytopenia and is seldom associated with spontaneous bleeding.

Venous thrombosis is 3 times more common than arterial, except for patients undergoing cardiovascular surgery.  Venous thrombosis most often results in deep vein thrombosis and pulmonary embolism. Arterial thrombosis may cause limb gangrene, myocardial infarction or stroke. Skin necrosis is associated with heparin dependent platelet antibodies but usually not thrombocytopenia.

Any heparin compound can induce antibody formation, but those forms with the highest molecular weight and highest degree of sulfation are associated with the highest incidence of HIT. The types of heparin reported to cause HIT in order of decreasing frequency are bovine heparin> porcine heparin> low molecular weight heparin> heparinoids.  Low molecular weight heparin appears to induce antibody formation about one fourth as often as bovine heparin and seldom causes thrombocytopenia.  HIT can be induced by any dose or route of heparin administration, including heparin flushes and heparin coated intra-arterial lines.  High dose IV heparin is more likely to induce antibody formation than low dose subcutaneous heparin.  Long duration of heparin administration is more likely to cause HIT, but the syndrome can occur after a single bolus.

The antibody that causes this syndrome is not directed against heparin alone, but a complex of heparin with platelet factor 4 (PF4).  PF4 is a heparin-neutralizing protein contained in the alpha granules of platelets that is released upon platelet activation. When a patient is treated with heparin, complexes of heparin and circulating PF4 form within the bloodstream. Complex formation causes a conformational change in the PF4 molecule that exposes a neoantigen, which stimulates an immune response in some patients. Unlike traditional immune responses, IgM, IgA and IgG antibodies are produced simultaneously, but only IgG is associated with thrombosis. In a subset of susceptible patients, immune complexes consisting of heparin, PF4 and antibody bind to the Fc receptor on platelet membranes, causing platelet activation and inducing platelet aggregation. Aggregated platelets are removed prematurely from the circulation leading to thrombocytopenia and the generation of platelet microparticles, which stimulate thrombin generation and thrombosis.

The human platelet factor 4 (H/PF4) IgG antibody immunoassay is an enzyme-linked immunosorbent assays (ELISA) using microwells precoated with an antigen complex of PF4 and polyanionic heparinoid substitute (PHS). Patient serum is incubated in the wells, and binding of antibodies to this complex is detected by binding of a second phosphatase-conjugated antihuman IgG antibody. Color is generated when this bound conjugate cleaves a chromogenic phosphate substrate and is quantitated by light absorption at 405 nm.

Addition of excess heparin (100 U/mL) to patient serum prior to testing inhibits the reaction between heparin-dependent antibodies and the PF4-PHS complex, and produces a negative result. This procedure is used to confirm that a positive screening result is caused by heparin-dependent antibodies. Results are calculated as the percent heparin inhibition of the reactivity of the antibody. Elisa results <0.40 units are considered negative.

Laboratory tests that detect IgG PF4 antibodies are the most sensitive (96-100%). A negative result strongly suggests the absence of HIT, but a positive test does not necessarily mean that a patient has antibodies capable of activating platelets. Many more patients form antibodies than have thrombosis. The strength of the reaction is predictive of the risk of thrombosis, as seen in the following table.

 

A PF4 antibody result of >2.00 is most often associated with antibodies capable of activating platelets.  The platelet activation capability of intermediate-strength PF4 antibodies can be determined by a confirmatory serotonin release assay (SRA).

In the SRA, washed healthy donor platelets are loaded with ¹⁴C-labelled serotonin prior to incubation with patient plasma. If heparin-PF4 antibody complexes are present, they activate platelets and release ¹⁴C-serotonin, which is detected as an increase in supernatant radioactivity. Each patient’s sample is tested in the presence of both high (100 U/mL) and low (1.0 U/mL) concentrations of heparin. Serotonin release in the presence of low, but not high, heparin concentration is considered to be a true positive result. Platelet activation in the presence of both low and high heparin concentration is considered a false positive result. Platelet activation is decreased at high concentrations of heparin, since the excess heparin disrupts formation of heparin, antibody and PF4 immune complexes. SRA is difficult to perform and only available at a few reference laboratories but has the highest specificity (94-100%) for clinically significant antibodies.

If a patient's serum plus low dose heparin induces release of serotonin from platelets in the SRA, but reactivity is not significantly inhibited by high dose heparin the results are not consistent with, HIT. One explanation is that patient's serum contains non-heparin dependent antibodies (e.g. anti-HLA), which can cause platelet activation resulting in heparin-independent release of serotonin.

Heparin should be discontinued in any patient with a clinical presentation consistent with HIT.  Heparin flushes and heparin coated catheters should also be avoided.  After discontinuing heparin, platelet counts begin to rise within 2-3 days and usually return to normal within 10 days.  PF4 antibody decreases to undetectable levels within 2-3 months after cessation of heparin therapy. Platelet transfusions are contraindicated because they may contribute to thrombus formation or extension. 

A baseline platelet count and a repeat platelet count should be performed within 24 hours of starting unfractionated (UFH) or low molecular weight heparin (LMWH) on all patients who have received heparin within the past 100 days or who have an uncertain history of previous exposure.  Platelet counts should be repeated at least once every two to three days from day 4 to day 14.

If a patient experiences acute inflammatory, cardiorespiratory, neurologic or other unusual signs or symptoms within 30 minutes of IV UFH infusion, a platelet count should be performed immediately and compared to a recent prior result.

Specimen requirement is one red top tube of blood is required.