Wash. Rev. Code § 9A.88.040
In any prosecution for prostitution under RCW 9A.88.030, it is an affirmative defense that the actor committed the offense as a result of being a victim of trafficking, RCW 9A.40.100, promoting prostitution in the first degree, RCW 9A.88.070, or trafficking in persons under the trafficking victims protection act of 2000, 22 U.S.C. Sec. 7101 et seq. Documentation that the actor is named as a current victim in an information or the investigative records upon which a conviction is obtained for trafficking, promoting prostitution in the first degree, or trafficking in persons shall create a presumption that the person's participation in prostitution was a result of having been a victim of trafficking, promoting prostitution in the first degree, or trafficking in persons.
RCW 9A.88.040