Abstract: A syringe needle thimble cap and its method of use is shown such that the thimble cap has a porous plastic, hydrophilic core captured within a cylindrical cap member for use with an arterial blood gas syringe barrel and needle assembly. The syringe needle thimble cap has a generally thimble-shaped cap with a proximal opening. The porous plastic core material is positioned within the thimble cap distally to the proximal opening, and the core material is held in place by an inwardly extending ring to prevent its removal. An open or hollow space is created distally to the porous material which is otherwise bounded by the interior surfaces of the thimble cap.