What is the test for admitting or denying an expert's opinion in a medical malpractice case?

"Expert testimony may also be premised on material that is not admitted into evidence so long as it is material of a type that is reasonably relied upon by experts in the particular field in forming their opinions. [Citations.] Of course, any material that forms the basis of an expert's opinion testimony must be reliable. [Citation.] For 'the law does not accord to the expert's opinion the same degree of credence or integrity as it does the data underlying the opinion. Like a house built on sand, the expert's opinion is no better than the facts on which it is based.' [Citation.] [] So long as this threshold requirement of reliability is satisfied, even matter that is ordinarily inadmissible can form the proper basis for an expert's opinion testimony." (People v. Gardeley (1996) 14 Cal.4th 605, 618.)