The Prothrombin Time (PT) test monitors the adequacy of the following factor levels:
Table 1. Extrinsic (and Common) Pathway.| Factor | Pathway |
| Factor I (Fibrinogen) | Common |
| Factor II (Prothrombin) | Common |
| Factor V | Common |
| Factor VII | Extrinsic |
| Factor X | Common |
If one or more of the factors above are deficient, the PT test will be prolonged. This may indicate the need for a PT mixing study to determine the cause.
