5. About thrombocytopenia in general

The normal value of platelet count is between 150 and 300 � 109 /L. Platelet number between 100 and 150 � <sup>10</sup><sup>9</sup> /L is considered as normal in some studies. That may be the cause of some controversial research data. Decreased thrombocyte number might be present because of


thrombocytopenic purpura (ITP). No clinical significance or role in mechanisms of thrombocytopenia of aPL antibodies was found [36]. In an earlier study, about 30% of ITP patients had a positive aCL test at the time of diagnosis [37]. In case of Evans syndrome (thrombocytopenia

The pathogenesis of thrombocytopenia related to aPL antibodies is still unclear. It is possibly caused by direct binding of anti-β2-GPI antibodies or anti-β2-GPI-β2-GPI complexes on activated platelets and promotes their aggregation and thrombus formation, so thrombocytopenia

On the other hand, thrombocytopenia in APS may be due to immune-mediated clearance of

intervention. In most of the cases, the main significance of thrombocytopenia is that it can be a sign, and when noticed it, the presence of aPL antibodies can be found. Severe thrombocyto-

Interestingly, the presence of thrombocytopenia in patients with APS is not typically associated with haemorrhagic complications; rather it can trigger thrombotic events. Even more, it has been proven that the more severe the thrombocytopenia, the higher the probability of future

In a retrospective study 138 patients were enrolled with positive aPL without fulfilling clinical criteria for APS, after a mean follow-up of 146 60.3 months, 29.4% with thrombocytopenia developed thrombosis. They concluded that aPL-positive patients who develop thrombocyto-

Platelet activation plays an essential role in the development of atherosclerosis. In case of arterial thrombosis, the role of platelets is also essential. Continuous platelet activation in patients with APS may be involved, among other factors, in accelerated atherosclerosis. Moreover, atherosclerosis and its thrombotic complications may be mediated by local secretion of molecular effectors embedded or packed into microvesicles from the platelet surface [40].

A fundamental role of platelets and platelet activation in the process of thrombosis generation of APS patients has been supported by several data. These data suggest that aPL antibodies do not interact with circulating platelets in these patients. Instead, anti-β2-GPI-β2-GPI complexes bind exclusively to the platelet thrombus and not to the endothelium, a phenomenon leading

/L) may be seen in 5–10% of patients [37].

/L) and does not require clinical

Antiphospholipid Syndrome and Thrombocytopenia http://dx.doi.org/10.5772/intechopen.72509 59

and haemolytic anaemia) aPL autoantibodies are also frequently present.

7. Pathogenesis of thrombocytopenia in APS

8. Characterisation of thrombocytopenia in APS

Thrombocytopenia in APS is usually mild (70–<sup>120</sup> 109

9. Clinical significance of thrombocytopenia in APS

penia have a potential risk of developing thrombosis [39].

is a consequence of consumption of platelets.

platelets, as in case of ITP [38].

penia (platelet count <50 109

thrombosis.

CAPS: catastrophic antiphospholipid syndrome; HELLP: hemolysis, elevated liver enzymes, and low platelet count; aHUS: atypical hemolytic uremic syndrome; TTP: thrombotic thrombocytopenic purpura.

Table 2. Mechanisms of thrombocytopenia in antiphospholipid antibody (aPL)-positive patients [34].

several reasons. The two main categories are the decreased thrombopoiesis in the bone marrow and the increased destruction of platelets in the peripheral blood [32].

Consequences depend on the degree of thrombocytopenia: under 20 109 /L it is considered severe, between 20 and 50 109 /L moderate and above 50 109 /L mild thrombocytopenia [33].

It is not only the number but also the thrombocyte function has a great importance in respect of consequences of thrombocytopenia: even a few thrombocytes can provide a satisfactory performance if the underlying disease does not diminish thrombocyte function. For example, in case of acute leukaemia patient may have bleeding even with a higher platelet number than that patient with immune thrombocytopenic purpura (ITP) who will be fine with a much lower thrombocyte count.

In case of the so-called consumptive thrombocytopenia, the basic phenomenon is the activation of platelets. Activation leads to thrombosis generation, factors of blood coagulation such as platelets are utilised and the thrombocyte number will decrease. As a secondary process bleeding occurs (Table 2).

### 6. Thrombocytopenia in APS

Thrombocytopenia is frequently found in patients with the APS and is usually mild (70– <sup>120</sup> <sup>10</sup><sup>9</sup> /L) and benign, with no intervention required. In a few cases it can be severe and aggressive treatment may be required. Low platelet counts usually appear associated with other APS manifestations, but sometimes it may be the only sign of APS [35].

The same working group has compared the frequency of thrombocytopenia in different subgroups of APS. They found no differences in the occurrence of low platelet number in patients with primary or secondary APS [35].

Thrombocytopenia is a frequent phenomenon of SLE. Increased concentrations of aPL antibodies have been found to be common in patients not only with SLE, but also with immune thrombocytopenic purpura (ITP). No clinical significance or role in mechanisms of thrombocytopenia of aPL antibodies was found [36]. In an earlier study, about 30% of ITP patients had a positive aCL test at the time of diagnosis [37]. In case of Evans syndrome (thrombocytopenia and haemolytic anaemia) aPL autoantibodies are also frequently present.
