The Concentration of Exogenously Added Lipids and Endogenously Available Microparticles Are Major Determinants of Thrombin Generation in Plasma.

Thrombin generation tests are started by adding coagulation activator (e.g. tissue factor or contact activator) together with lipids.

We evaluated the role of lipids in tests started with a fixed amount of tissue factor (7.16 pM) and addition of either 3.2 (high) or 0.32 (low) μM of lipids (Technothrombin ® TGA assays from Technoclone, Vienna) in both normal plasma and plasma that was ultracentrifuged (30 minutes at 15,000 g) to remove microparticles (MPs). The tests were performed in plasma samples of groups of apparently healthy individuals.

It was observed in 54 healthy volunteers that starting with high or low lipids substantially influenced the total amount of thrombin generated expressed by the area under the curve (AUC) (AUC median 2492, IQR 716 versus AUC median 1154, IQR 652 nM*min, respectively), the rate of thrombin formation or velocity index (VI) (median 53.4, IQR 43.6 and median 11.7, IQR 10.6 nM/min, respectively), and the lag time to the start of thrombin generation (median 10.3, IQR 2.4 versus median 17.5, IQR 5.0 minutes, respectively). It can be concluded that the VI is the most sensitive variable showing approximately a factor of 5 difference between high and low lipid.

The difference of adding high or low lipid on VI was primarily dependent upon the lipid concentration and to a limited extend influenced (univariate) by factor II levels (12%) and factor IX levels (10 %), taking into account practically all known coagulation factors (fibrinogen, II, V, VII, VIII, IX, X, XI, XII, PC, PS, PZ, TFPI, PCI) determined in the 54 plasma’s as potential determinant. In plasma of 36 volunteers microparticles were removed and VI dropped to 19% in comparison to the untreated plasma when tested with the addition of high lipid and to 3.1% with the addition of low lipid. Re-addition of MPs to a specific plasma restored VI dose dependently with an optimum at 2x10⁴ MPs/ml. The same level of VI (63.8 and 62.0 nM/min, respectively) was reached with high and low lipid addition when 2.10⁴ MPs/ml were added.

It is concluded that endogenous MPs play an important role in thrombin generation tests, in particular but not exclusively when the test is performed with low levels of added lipids.