What is the test for opinion testimony by an expert in a medical malpractice case?

Opinion testimony by an expert witness must be based on matter "perceived by or personally known to the witness or made known to him at or before the hearing, whether or not admissible, that is of a type that reasonably may be relied upon by an expert in forming an opinion upon the subject to which his testimony relates...." (Evid.Code, 801, subd. (b).) An expert should be allowed to testify to all the facts upon which he bases his opinion, including relevant declarations to him. (People v. Brown (1958) 49 Cal.2d 577, 585, 320 P.2d 5.) The statements are admissible not as proof of the facts stated but to enable the expert to explain and the jury to appraise the basis of his opinion. (Id. at 586.)