I cannot emphasise enough just how crucial it is to carry out patch tests on your client. Even though our adhesive is medical grade (meaning it has been tested and is safe to use on humans), it is possible for clients to experience an allergic reaction. This can range from a mild reaction that can be treated with antihistamines to full blown swelling, itchy, hot and weeping eyes. This kind of reaction is awful and scary. Several years ago I experienced a reaction like this (not to lashes initially) and you do not want your client having this sort of reaction.
Apart from the ethical considerations of knowing that there is a chance they could react and still choosing to risk it, you could also find yourself facing legal action. I am based in the UK (so I can't say for other countries) but over here most insurance companies insist that a patch test is carried out. There's no getting round it, it is an inconvenience and there will always be those clients that want lashes done there and then and I know it's hard to turn work away. Yes, you might lose out on £(whatever you charge) but that will be nothing compared to what you could lose if you get sued and your insurance is void because you didn't patch test.
Unfortunately, even if your client has had lashes multiple times there is always a chance that they could develop an allergy to the adhesive. If this happens the best course of action is to seek medical advice. If it's not too severe a reaction you can remove the lashes but make sure the remover does not come into contact with the skin.
In my experience, regular clients tend not to get this full blown reaction out of nowhere (if the lashes are done correctly). The reaction tends to build up. In my career I've seen this happen 3 times which really isn't much considering how many clients I've seen. They all had lashes regularly. One had no problems at all until she had her carpet cleaned. It seemed that she experienced an allergic reaction to the chemicals used which then made her sensitive to the lash adhesive. Another got a graze on her eyelid and noticed that the day after each infill her eyes would swell a little. She didn't tell me because she knew I would stop doing them but the reaction built up so much that she ended up not being able to have them anymore.
A third experienced no problems with her extensions at all until she couldn't fit in with me one day and had someone else do them. They were done badly with the glue being applied directly to the skin. Her eyelids were red, hot and swollen with masses of adhesive coating all her lashes and the lashes themselves were scratching at her eyelid. I managed to remove them and she had a break. We tried again hoping that the reaction was from the physical trauma but she is now allergic and is having a six month break to see if the allergy will leave her system.
The allergy may go away if the client has a break. When I experienced my reaction, which was initially due to a facial product, I became allergic to a lot of things I was exposed to while the reaction was occurring. This included acrylic and gel nails, a lot of soaps, washing up liquid, oranges and tomatoes and unfortunately lash extensions. I wore cotton gloves whilst lashing for a while as the fumes would cause contact dermatitis on my hands but after about 6 months the reaction stopped. Because of my experience I would advise that you don't lash anyone who is experiencing any sort of reaction where histamine is produced. If I get clients that get severe pollen allergies I recommend that they have a break when those allergies act up. Not only because of the chance of reaction but also because if their eyes are watering and they are rubbing them the extensions aren't going to last anyway!
How to Patch Test
Cyannoacrylate is an irritant and as such it should never come into contact with the skin. I know that if I get any of the adhesive on my skin I get a reaction but I can lash without any problems. There's more of a chance of a client reacting if you do the test on the skin but they may not react with a set of lashes as the adhesive shouldn't be touching the skin. I find the best way to patch test is to either attach a few lashes to the outer corners of the client's eyes or to take a cocktail stick and swipe a few lashes with the adhesive. Always check with your insurer to find out how they expect you to be patch testing.