From JWfacts' site:
Watchtower 2000 October 15 p.31
“Occasionally, a doctor will urge a patient to deposit his own blood weeks before surgery (preoperative autologous blood donation, or PAD) so that if the need arises, he could transfuse the patient with his own stored blood. However, such collecting, storing, and transfusing of blood directly contradicts what is said in Leviticus and Deuteronomy. Blood is not to be stored; it is to be poured out—returned to God, as it were. Granted, the Mosaic Law is not in force now. Nevertheless, Jehovah’s Witnesses respect the principles God included in it, and they are determined to ‘abstain from blood.’ Hence, we do not donate blood, nor do we store for transfusion our blood that should be ‘poured out.’ That practice conflicts with God’s law.”
It occurred to me that "respecting the principles" of the law in this case actually amounts to following it to the letter!
One could say that about any of the Mosaic laws. For example, with this reasoning they could say that one should refrain from eating fat due to the principle behind Lev 7:23.