What is the test for using the discovery rule or "foreign object" exception?

"[T]here is, under the facts of this case, no basis for the invocation of the discovery rule or 'foreign object' exception. Plaintiff entered defendant's health care facility with a traumatically introduced glass fragment already embedded in her right hand. Thus, the glass fragment retrieved in 1980 was not introduced into plaintiff's body as the result of any affirmative act on the part of one of defendant's employees. Rather, the failure to detect and remove the fragment was 'founded exclusively upon diagnostic judgment or discretion' [citation omitted]." (Garrett v. Brooklyn Hosp., supra, [171 Cal.App.3d 1049] 471 N.Y.S.2d at p. 622. See also Garrett v. Brooklyn Hosp. (1982) 115 Misc.2d 933 [454 N.Y.S.2d 637, 639].)