TT #157 · Parshas Tazria · Aliyah 3 · Passuk 20
English: The priest shall examine it. If its appearance is lower than the skin and its hair has turned white, the priest shall pronounce it unclean; it is a tzaraas affliction that has broken out in the boil.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:20
Sifra (Toras Kohanim).
Sifra — Vayikra 13, Midrash Sefer
English: The wording includes an appearance level with the skin or higher, not only one visibly depressed.
Textual derivation
The diagnostic rule is not restricted to a visibly depressed spot.