I had sclerotherapy with foam three weeks ago. Should I be concerned of a blood clot/DVT and/or PE? (photo)

Some bruising and occasional tenderness are normal findings after sclerotherapy.  The tenderness could be an area of trapped blood in a large reticular vein with phlebitis (inflammation ) of a treated vein. Warm compresses and over the counter anti-inflammatory medicines like ibuprofen can help. A DVT would not be visible from the surface because it is a deep vein and unlikely from treatment of small superficial veins. If you have ankle swelling and/or deeper generalized pain then ruling out a DVT prior to your flight with ultrasound would definitely be advisable and your treating physician should be able to do so.  You should definitely wear your stockings during your flight. 

Follow up with vein specialist after any treatment for any concern specially in the area you are worried about because of changes and symptoms. Your physician using ultrasound can scan your leg(s) to determine if there is any complication like deep vein thrombosis (DVT) or evaluate you for superficial thrombophlebitis. If any symptoms worsen or you develop new symptoms such as shortness of breath or increased pain seek immediate medical attention. 
Best, 

This are that you are showing most likley represents seepage of blood products from the sclerosed vein and not likely to be deep vein clot. The only way to diagnose a deep vein clot is by venous Doppler ultrasound.You need to use some adjunctive measures after sclerotherapy to reduce post inflammatory hyperpigmentation. it is those that don't follow such instructions immediately following sclerotherapy, starting on day1 (next day) that seem to get retained hyperpigmentation. I recommend 2 creams: Scleroquin plus and Sclerovase and also Scler-X supplement to reduce post inflammatory hyperpigmentation (see link below) as well as compression stockings for at least 2 weeks.
H Karamanoukian MD FCS
certified vein specialist from ABVLM and double board certified surgeon
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The picture looks like normal findings post sclerotherapy.  The tender area may be an area of superficial phlebitis. There is nothing here to suggest DVT or PE.  Symptomatic treatment with ibuprofen and wet heat to the area should help.  If your are leaving the country, then having a venous ultrasound would confirm any DVT(which seems very unlikely).