**3. Clinical diagnosis**

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].

probability of HIT II. To confirm the diagnosis of HIT II, laboratory evidence of anti-heparin antibodies is needed. Anti-heparin antibodies may be confirmed in about half of the patients with clinically suspected HIT requiring laboratory investigation. The frequency of positive results depends on the clinical 4 T score and sensitivity and specificity of the test used [6–9].

Heparin-Induced Thrombocytopenia (HIT) http://dx.doi.org/10.5772/intechopen.78024 105

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

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

**Method Sensitivity Specificity NPV PPV**

(a) Gel-columns-mycro-particle assay High Moderate High Low (b) Lateral flow immunodiffudion assay-IgG High Moderate High Moderate (c) EIA-IgG High Moderate High Moderate

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

Low/moderate High High

**4. Laboratory diagnostics**

and thrombogenic potential (**Table 2**) [10–12].

Anti H-PF4 assays (antigen-antygody assays):

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

Funtional assays: (a) SRA-cr (b) SRA-HPLC (c)SRA-EIA (d) HIPA (e) HIMA

factor 4 complex.

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


**Table 1.** Clinical assesement of Heparine induced thrombocytopenia (HIT) by use of modified 4T scoring system according to Lo and Warkentin [6].

probability of HIT II. To confirm the diagnosis of HIT II, laboratory evidence of anti-heparin antibodies is needed. Anti-heparin antibodies may be confirmed in about half of the patients with clinically suspected HIT requiring laboratory investigation. The frequency of positive results depends on the clinical 4 T score and sensitivity and specificity of the test used [6–9].
