What is the test for proving impairment by alcohol?

In Kittle v. R. (1979), 12 C.R.(3d) 120, Walker, D.C.J. (as he then was), held at p. 124 of the report: "… Once there is before me evidence of impairment, which I accept, and evidence of alcohol in the blood, which I accept, and no explanation whatsoever from the accused or other evidence, it seems to me that a conviction may follow. Whether the rather small amount of alcohol found in the blood of the accused combined with fatigue, the 'something' dealt with in police evidence or something else again to produce the impairment is not of the essence. . . . . . "… Here, the starting point is a finding of impairment, and some alcohol, 30 milligrams, is found in the blood, making it necessary to consider the effect that the amount of alcohol found, albeit not great by usual standards, may have had, and whether it may have combined with some other factor (fatigue, age, indifferent health, drugs) to produce the impairment established in fact. Alcohol always acts within a particular body. Alcohol always combines with some physical characteristics or conditions in a particular body. If there is no other evidence bearing on this puzzling impairment found, the alcohol found, on all the evidence that there is, is a basis for finding that impairment by alcohol is proved."