MTHFR and deep vein thrombosis: We also assessed the combined effect of the four polymorphisms on the risk of DVT; But we found no statistically significant difference (P > 0.05 for all the 6 possible combination, including FV Leiden/ PAI-1, FV Leiden/ MTHFR C677T, FV Leiden/ MTHFR A1298C, PAI-1/ MTHFR C677T, PAI-1/ MTHFR A1298C, and MTHFR C677T/ MTHFR A1298C).