What is the test for reversal of a jury's verdict in a medical malpractice case?

In Henderson v. Harnischfeger Corp. (1974) 12 Cal.3d 663, 670, 117 Cal.Rptr. 1, 5, 527 P.2d 353, 357, the court stated: "Generally speaking if it appears that error in giving an improper instruction was likely to mislead the jury and thus to become a factor in its verdict, it is prejudicial and ground for reversal. (Citation.) To put it another way, '(w)here it seems probable that the jury's verdict may have been based on the erroneous instruction prejudice appears and this court "should not speculate upon the basis of the verdict. " ' (Citations.)" In addition, however, the Henderson court added: " 'The determination whether, in a specific instance, the probable effect of the instruction has been to mislead the jury and whether the error has been prejudicial so as to require reversal depends on all the circumstances of the case, including the evidence and [81 Cal.App.3d 47] the other instructions given. No precise formula can be drawn.' " (Id. at 670-671, 117 Cal.Rptr. at 5, 527 P.2d at 357.)