**3.1.1 Idiopathic VTE**

Venous thromboembolism is the result of a complex interaction between the circulating proteins, cells/platelets and the endothelium (Collen and Hoylaerts 2005). There is no known provoking or identifiable precipitating factor in idiopathic VTE. A recent study looked at the interactions between the MP of various origin - platelet, endothelial and monocyte and endothelial derived MPs were found to be elevated in association with VTE. One report suggests that the combination of total circulating MP, P-selectin levels and Ddimer levels may help predict VTE (Rectenwald*, et al* 2005). This approach had a sensitivity of 73% indicating the need for further refinement for application in clinical practise.

In another larger investigation no association was found between levels of total circulating MP and risk of recurrent VTE (Ay*, et al* 2009). Interestingly, a study comparing patients with cancer who had VTE and those with idiopathic VTE found raised tissue factor bearing MP only in cancer patients (Thaler*, et al*). In another report, plasma levels of tissue factor MP were not raised in those with pulmonary embolism suggesting that that perhaps other subtypes of MP may have to be studied in more detail to explain the relationship found in experimental models (Garcia Rodriguez*, et al* 2010). Owen and co authors looked at the recurrence of VTE and found that the procoagulant activity but not number of MP was increased in cases of recurrence (Owen*, et al*).

The role of MP in predicting thrombosis in those with heritable thrombophilia has also been explored. It has been found that total circulating MP levels were increased in subjects with heterozygote factor V Leiden status but there was no difference between those who had had VTE and those without (Enjeti*, et al* 2010). This finding and other studies seems to suggest that although total microparticles have been shown to be increased in those with VTE or those prone for VTE, there appears to be no convincing data that MP help to predict or monitor VTE. However, in a recent study that investigated this issue further, looked at MP levels by a different approach by comparing percentiles of MP measured in a retrospective case-control fashion. In those with circulating MP above the 90th percentile of the control population's distribution, a five fold increased risk was observed (Bucciarelli, 2011). They found that elevated MP were indeed an independent risk factor for VTE and this warrants a confirmation in a prospective cohort study.

The draw back of the studies in this area of VTE include the variability of type of MP studied, the techniques employed for measurement of MP and retrospective nature of investigations undertaken.
