What is the test for establishing a cause of action for negligence?

"The mere breach of a professional duty, causing only nominal damages, speculative harm, or the threat of future harm--not yet realized--does not suffice to create a cause of action for negligence (citation). Hence, until the client suffers appreciable harm as a consequence of his attorney's negligence, the client cannot establish a cause of action for malpractice. Prosser states the proposition succinctly, 'It follows that the statute of limitations does not begin to run against a negligence action until some damage has occurred.' " (Budd v. Nixen (1971) 6 Cal.3d 195, 200-201, 98 Cal.Rptr. 849, 491 P.2d 433.)