Washington is a certification only state. Certification gives the holder the right to use certain titles, only. From the law:
18.138.020 - Certification required.
(1) No persons shall represent themselves as certified dietitians or certified nutritionists unless certified as provided for in this chapter.
(2) Persons represent themselves as certified dietitians or certified nutritionists when any title or any description of services is used which incorporates one or more of the following items or designations: "Certified dietitian," "certified dietician," "certified nutritionist," "D.," "C.D.," or "C.N."
So long as you are not using a protected title, or giving the impression that you are in any way a dietitian, nutritionist, or certified, you are free to practice.