The degree of burn is diagnosed clinically.
It's a clinical diagnosis and it is based on your observation and clinical experience in diagnosing these types of injuries. The tricky part of diagnosing clinically burns between second and third degree is trying to determine the level at which the second degree burn has occurred.
There are three different levels of second degree burn, superficial, intermediate and deep. The intermediate and deep aspects of a second degree burn often times may require surgical intervention, but it is a clinical observation that usually takes approximately anywhere between 48 to 72 hours to be able to definitively make that diagnosis. The natural progression of a burn is that it tends to extend microscopically over that period of time. That is 48 to 72 hours.
Patients can often times come in with what appears to be superficial second degree burns, but because of the natural progression of a burn injury, microscopically it could end up being a third degree burn within a 72 hour period of time. That's why it's super important to have all burns evaluated medically so that if these burns were evaluated medically it can be determined what depth that burn is over a 48 to 72 hour period.