Hi Can somebody clarify the following to me?

1. At what dose of UH will it affect the Prothrombin time assay?
2. How (mechanism or action) does UH affect/influence/react with PT assay to give it a prolonged value?
3.How can I convert the UH dosage given to patient in the heparin activity in the lab testing. Example if patient is given 20,000 U/ 500ml,is this equivalent to 2 IU/ml of UH? How to calculate/convert?

Thanks a lot