Yes, HEMATOCRITS is a Scrabble word!

Letter values in Scrabble US: H 4, E 1, M 3, A 1, T 1, O 1, C 3, R 1, I 1, T 1, S 1.

Is it a valid word?

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

the ratio of the volume occupied by packed red blood cells to the volume of the whole blood as measured by a hematocrit
a measuring instrument to determine (usually by centrifugation) the relative amounts of corpuscles and plasma in the blood

Also: packed cell volume.

Not playable in Scrabble US: HAEMATOCRIT.