###Human: What causes blisters on the lip? ###Assistant: Hello. 1. There can be other skin diseases which mimic HSV 1 infection on lips, but still, it is quite characteristic clinically, things which can sometimes look like are paederus dermatitis, hand foot, and mouth disease, but it is seen usually in children and blisters elsewhere are also present. Candida can be there, but it rarely presents as blisters in adults. 2. To conclude the appearance is quite characteristic and your description of vesicles and their evolution looks like HSV. 3. The serology can take up to a month (average 2 to 4 weeks) to become positive after primary infection, HSV IgM will be detectable at that time in serum. So it might be one of the reasons for that. 4. For culture the sensitivity overall is 50 percent, that is 50 out 100 patients culture will grow the virus. Although in primary syphilis sensitivity is up to 80 percent. 5. When lesions start to heal it is difficult for a culture to get a positive result. Also, fluid will be required for it to be positive. Either fluid from blisters or erosions whatever is present. It will not come positive if the sample is taken from the surface of the vesicle. 6. My advice is to repeat serology after 3 to 4 weeks once again, and even if it is positive, you do not need to get frightened so much as HSV 1 serology is usually seen in 50 to 75 percent cases. It does not mean it will come back again, though being a carrier chance of spread is there somewhat.