When a TCA peel results in a full frost, which layer of the skin has been reached?

A full frost during a TCA (trichloroacetic acid) peel indicates that the acid has penetrated deeply enough to affect the papillary dermis. This level of penetration is characterized by the complete whitening of the skin, which signifies that a significant portion of the epidermis and the top layer of the dermis (the papillary dermis) has been reached.

The epidermis, while affected, does not account for the full frost condition, as that only reflects effects within the epidermal layer without reaching the dermis. The reticular dermis, being deeper, is not typically reached unless the peel is extraordinarily deep, which would exceed the parameters of standard TCA peels that achieve full frost. The subcutaneous layer is positioned even deeper and is not involved in the peeling process. Therefore, the correct answer is that a full frost indicates penetration to the papillary dermis, which is essential for achieving the desired results from the peel.