The mistake with a test like plethysmography is to assume the name explains the result. It explains the method. The result is for a clinician to read.
What the source says the test does
medlineplus.gov says: "Plethysmography measures changes in volume in different parts of the body." That is the whole description, and it says nothing about what any one result means.
You may do some things alone. You can read this description, learn the term, and ask why the test has been proposed for you. You may not judge your own result.
Related exams worth reading about
Two entries in the MedlinePlus Medical Encyclopedia describe other exams you may hear about. The MedlinePlus entry on Doppler ultrasound of an arm or leg covers a limb scan, and the MedlinePlus entry on carotid duplex covers a scan of the neck arteries.
A third source shows how a different vascular condition is checked. The NHLBI page on how aortic aneurysm is diagnosed is useful for seeing how tests fit together in a work-up.
Where reading stops
None of these pages replaces a clinician's reading of your own numbers.
The conservative choice
If you are in Tinley Park, Illinois, and this test has been suggested, ask who will explain the result.