How do you tell if laser vein removal worked?

It’s important to distinguish between varicose veins and spider veins when choosing between laser therapy, sclerotherapy, and vein surgery. With two board certified vein specialists, our center uses a combination of visual examination, functional flow measurements, ultrasound, transillumination, and infrared light to map out vein networks prior to therapy. Lasers can be used to treat deep varicose vein insufficiency, vein perforators, and varicose veins. Sclerotherapy is ideal for spider veins and reticular veins. Whereas surgery is reserved for severely incompetent varicose vein segments.


Best,
Dr. Karamanoukian
Realself100 Surgeon

Great question but didn't get any idea what part of the body was treated with laser. You are absolutely right that if the vein blanches with pressure its still open. The success of laser depends on the size of the vein and also location (leg veins don't respond well to laser). In addition to that during treatment there are certain things to be careful of like how much pressure is applied to the treatment area. So several things need to be considered. It is very common to require more than one treatment for complete resolution of all the veins since some will go into spasm and then they have to be treated second time. 

Rarely will one laser treatment eradicate all spider veins. From your description some veins are still open.  Each laser treatment injures the vein to seal it and some veins take severas sessions for this to occur.  My experience is that scleortherapy is the best treatment for veins larger enough to accommodate a small needle.

Follow up with your vein specialist for further treatment. Make sure to discuss with your specialist other options if laser is not effective and make sure your specialist also evaluates you for underlying venous disease if symptoms are present such as varicose veins, swelling, pain, cramps, burning sensation, itching of the skin, or other changes in the lower extremities. 
All the best, 

If the vein disappears, it means that the laser procedure was ineffective. You may need additional treatments with the pulse dye laser. BestH Karamanoukian MD FACScertified vein specialist ABVLM and double board certified surgeron

Assuming this question is about spider veins on the legs, if the vessels blanch (go away) when you push on them, then the vessels are still open and carrying blood and are not fully treated. If the treatment was 10 days ago, then you can assume you'll need more treatments. This can occur if the treatment is not appropriate for the vessel in size or color, if the underlying feeder veins were not addressed, or if not enough energy was used to treat the vessel. 
Unless the spider veins on the legs are tiny and red, then PDL is not usually the first choice for treatment. Sclerotherapy with polidocanol (POL) or sodium tetrodecyl sulfate (STS)  is still considered the gold standard for spider vein treatments on the legs. Alternatively 532 KTP or 1064 NdYag lasers are generally thought to be more efficacious for laser treatments of spider veins of the legs.
It is important to have an ultrasound of the legs before any treatment of spider veins is started. This is to be sure the underlying venous network is functioning properly to allow any spider vein treatment to proceed successfully. If the underlying veins are not functioning properly underneath the spider veins, then the chances of treatment success is greatly reduced.