PAI-1 stands for plasminogen activator inhibitor type 1, an increase of this can lead to venous and arterial thrombosis. PAI-1 is responsible for inhibiting two proteins that play an important role in resolving a thrombus. An increased PAI-1 level therefore leads to a greater inhibition of the proteins and thus to a slower breakdown of the blood clot. Two polymorphisms in the PAI-1 gene are associated with an elevated PAI-1 level.

By insertion or deletion, 4 or 5 guanine nucleotides can be present at position 675. If the 4G allele is present, there is an increased risk of venous thrombosis. An increased risk of miscarriages and heart attacks has also been found for this genotype. This PCR kit detects the increased expression of PAI-1, because an increased expression is present when the 4G allele is present.