What is the legal test for allowing an expert to testify in a medical malpractice case?

Thus, in general, the law favors permitting experts to testify in malpractice cases if they demonstrate sufficient knowledge of the subject that their opinions will be helpful to the jury in the search for the truth. (Id. at p. 645, 114 Cal.Rptr. 128, 522 P.2d 688.) If a witness has passed this threshold, the question of the degree of the witness' knowledge goes to the weight of the testimony rather than to its admissibility. (Id. at p. 643, 114 Cal.Rptr. 128, 522 P.2d 688 [quoting Seneris v. Haas, supra, 45 Cal.2d at p. 833, 291 P.2d 915].)