**4. Laboratory diagnostics**

**3. Clinical diagnosis**

104 Anticoagulant Drugs

T1

T2

T3

T4

sequelae

Other cause of thrombocytopenia

Low, 0–3; Moderate, 4–5; High, 6–8.

according to Lo and Warkentin [6].

decline

Thrombo-cytopenia

Timing od platelet count

Thrombosis or other

30–100 days)

heparin

heparin exposure)

unconfirmed suspicion of thrombosis

Clinicians should assess whether the platelet count decrease is the result of anti-heparin antibodies or underlying disease. They should also be cautious not to over diagnose HIT, because some HIT antibodies are not pathogenic and will not necessarily lead to the clinical HIT syndrome. Before the specific laboratory evidence of anti-heparin antibodies, the probability of HIT should be determined using clinical-laboratory indicators. In heparin-treated patients, platelet count should be monitored before and during the course of therapy. According to the 2006 British Haematology Standards, platelet count should be determined in all patients on the day of the start of heparin therapy. In patients who received heparin within the previous 100 days, platelet count should be determined on the day of the start of heparin therapy and 24 h later. In patients receiving UFH, platelet count should be measured daily from day 4 to day 14, and every 2–4 days between day 4 and day 14 in patients receiving LWMH [10–13].

The clinical scoring system used for determining the probability of a HIT is the so-called "*4 T score*" - thrombocytopenia, timing of onset, thrombosis, and absence of other causes of thrombocytopenia (**Table 1**) [6]. Each of these symptoms is scored from 0 to 2 points. The total score of 0–3 indicates a low probability of HIT II, 4–5 indicates moderate, and 6–8 indicates a high

**4Ts Score**

(a) Occurrence of thrombocytopenia 5–10 days of initial heparin administration or <1 day (with previous exposure within 30 days)

(b) Occurrence of thrombocytopenia >10 days of initial heparin administration or unknown or <1 day (with previous exposure within

(b) Progressive recurrent thrombosis, erythematous skin lesion,

**Table 1.** Clinical assesement of Heparine induced thrombocytopenia (HIT) by use of modified 4T scoring system

(c) Occurrence of thrombocytopenia at <4 days (without previous recent

(a) New thrombosis, skin necrosis, acute systemic reaction following bolus

(c) No other causes of thrombocytopenia 0

(a) No other cause of thrombocytopenia 2 (b) Presence of other possible causes of thrombocytopenia 1 (c) Definitive other cause of thrombocytopenia is present 0

(a) Platelet count decline by >50%, with lowest value of 20 × 10/9/L 2 (b) Platelet count by 30–50%, with lowest value of 10–19 × 10/9/L 1 (c) Platelet count decline by <30%, with lowest value below 10 × 10/9/L 0

2

1

0

2

1

Laboratory investigation of HIT includes two categories of tests: immunologic assays for detecting circulating anti-PF4/heparin antibodies usually of IgG class and functional assays which detect antiplatelet antibodies capable to induce heparin-dependent platelet activation and thrombogenic potential (**Table 2**) [10–12].

In laboratory investigations of HIT II, anti-heparin-PF4 antibody tests are most commonly used in immunologic assays and serotonin-release assay (SRA) to determine anti-heparin antibody-induced platelet activation. In addition to, SRA heparin-induced platelet activation/ aggregation assay (HIPA) is used when the thrombogenic potential of the present antibodies should be determined or *in vitro* effectiveness of heparin, alternative should be estimated. Enzyme-immunologic (EIA) method and gel method are most commonly used for immunologic assays. EIA is performed on a microtiter plate, and heparin-PF4 antigen complex is applied to the plate wells. Gel test is performed on gel-filled microcolonies, and the heparin-PF4 complex is added into a microparticle suspension. These tests have a similar sensitivity (80–90%) and specificity (89–97%). The most important advantage of gel test (quick screening test) is a high negative predictive value (>95%) for the exclusion of HIT II. In the other group


H-PF4, heparin-platelet factor 4 complex; EIA, enzyme immuno assay; HPLC, high presure liquid cromatography; SRA, serotonin release assay; HIPA, heparin-induced platelet activation/aggregation; HIMA, heparin-induced platelet activation/multilate aggregation; NPV, negative predictive value; PPV, positive predictive value; H-PF4, heparin-platelet factor 4 complex.

**Table 2.** Methods for anti-heparin antibodies detection.

of tests, heparin-induced platelet activation (HIPA), and serotonin-release assay (SRA) are the most commonly used tests. They have lower sensitivity, but higher specificity than the first group of tests. In addition to, EIA test result, OD value is also obtained. OD <1.000 indicates the presence of clinically significant antibodies and a high-risk of thromboembolic complications. OD ranging from 0.400 to 0.999 and low clinical 4T indicate low thrombogenic activity of the antibodies and subclinical HIT. Monitoring antibody titres using OD values is used in the preoperative preparation of patients with previously confirmed HIT II; the surgery in which heparin is given is performed after the antibody titter decreases or the test results become negative [14, 15].

Lepirudin is a recombinant hirudin and is a highly specific direct and irreversible inhibitor of thrombin. One molecule of lepirudin binds with one molecule of thrombin. Lepirudin is almost exclusively excreted by the kidneys and hence systemic clearance of lepirudin is dependent on the glomerular filtration rate. The drug should be avoided in hemodialysis patients and those with acute renal failure with creatinine clearance <15 ml min−1 (normally 120 ml min−1) or serum creatinine >528 μmol liter−1. The 2006 British Guidelines recommend lepirudin, recom-

Heparin-Induced Thrombocytopenia (HIT) http://dx.doi.org/10.5772/intechopen.78024 107

Bivalirudin is a direct thrombin inhibitor and an analogue of the peptide fragment hirugen, which is a compound derived from hirudin. Unlike lepirudin, the binding of bivalirudin to thrombin is reversible. Bivalirudin binds specifically to the catalytic site and substrate-binding site of thrombin. The US Food and Drug Administration (FDA) has approved bivalirudin for use in patients undergoing coronary angioplasty with unstable angina who are also on aspirin therapy [18].

Argatroban is a direct competitive synthetic inhibitor of thrombin. It binds reversibly to the thrombin catalytic site and therefore, inhibits reactions that are catalyzed or induced by the presence of soluble and clot-bound thrombin. When given i.v. and is metabolized in the liver by cytochrome P450 enzymes it is 100% bioavailable. Unchanged drug is excreted in the urine (16%) and feces (14%). It is eliminated as its metabolite in the feces (65%), presumably through

It should be taken into account that all non-heparin anticoagulants may also lead to anaphylactic reaction and bleeding. However, unlike heparin, there is no known antidote for these agents and laboratory monitoring of their effects and antibody cross-reactivity is difficult (anti-Xa activity and ellagic time are not standard laboratory tests for hemostasis), in addition

Danaparoid is a low molecular weight heparinoid. It is a mixture of dermatan sulfate, glycosaminoglycans, and chondroitin sulfate. A favorable outcome in 90% of patients with HIT is associated with the use of danaparoid. The main activity of danaparoid is against factor Xa, with the anti-Xa: the anti-IIa ratio of 22:1 resulting in inhibition of fibrin formation. There is a 10–20% cross-reactivity rate with HIT antibodies in vitro although it is less common in vivo. [18].

Coumarin agents (e.g. warfarin) are contraindicated in acute HIT II, because they increase the risk of microvascular thrombosis, necrosis, and gangrene. Replacing UFH with LWMH is also contraindicated in the treatment of HIT II due to the antibody cross-reactivity [12, 13, 18].

Most guidelines suggest the use of argatroban over other nonheparin anticoagulants in patients with HITT and renal insufficiency, and the use of bivalirudin over other nonheparin anticoagulants or heparin plus antiplatelet agents in patients with acute HIT or subacute HIT who

Recently, the most commonly used agent has been fondaparinux, a synthetic and selective factor Xa inhibitor, which rarely causes anti-heparin antibody cross-reactivity. Clinical experience shows it has beneficial effects in the prevention of thromboembolic complications. The fondaparinux results are an important step in developing the recommendations for the use of

alternative agents to heparin, although randomized studies are needed [20, 21].

binant protein, and direct thrombin inhibitor to be used as heparin alternatives [12].

biliary secretion, and in the urine (22%) [17].

require urgent cardiac surgery (Grade 2C) [12, 18, 19].

to the high price of the substance.

To exclude other causes of thrombocytopenia, differential diagnosis should include pseudo thrombocytopenia (artifact), massive pulmonary embolism, disseminated intravascular coagulation (DIK), sepsis, other drug-induced thrombocytopenia (e.g. by GP IIb/IIIa inhibitors), autoimmune or alloimmune thrombocytopenia, post-transfusion purpura, diabetic ketoacidosis, and antiphospholipid syndrome with thrombocytopenia [16].
