TT #80 · Parshas Tazria · Aliyah 1 · Passuk 3
English: The priest shall examine the affection on the skin: if hair in the affected patch has turned white and the affection appears to be deeper than the skin, it is a leprous affection on the skin; and if the priest sees that it is a leprous affection, the priest shall pronounce the person impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:3
Torah Temimah cites Sifra (Toras Kohanim) for assessing the affected skin together with the surrounding flesh.
Sifra — Vayikra 13, Midrash Sefer
English: “On the skin of the flesh” — he must see all the flesh with it, all at once.
Holistic examination
The examiner must assess the affected area together with the surrounding flesh as a whole, rather than isolating a small patch from its context.