In Carson v. Fine, 123 Wn.2d 206, 867 P.2d 610 (Wash. 1994), the issue before the Supreme Court of Washington was the "admissibility of adverse opinion evidence offered by a treating physician against the plaintiff, his former patient in a malpractice action filed against another physician." 867 P.2d at 613.
That court held that "a plaintiff's waiver of the physician-patient privilege extends to all knowledge possessed by the plaintiff's doctors, be it fact or opinion." 867 P.2d at 616.
The court reasoned that "there is no basis in reason, the common law, or in statutory law to draw a distinction between the types of testimony a treating physician may offer once the physician-patient privilege has been waived." 867 P.2d at 616-617.