Yes, APHERESIS is a Scrabble word!

Letter values in Scrabble US: A 1, P 3, H 4, E 1, R 1, E 1, S 1, I 1, S 1.

Is it a valid word?

APHERESIS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(linguistics) omission at the beginning of a word as in `coon' for `raccoon' or `till' for `until'
a procedure in which blood is drawn and separated into its components by dialysis; some are retained and the rest are returned to the donor by transfusion