Hill, J.

Defendant was convicted after a trial by jury of driving while under the influence of intoxicating liquor (DUI) in violation of 23 V.S.A. § 1201(a)(2), and subsequently filed a timely notice of appeal. We affirm.

At approximately 11:15 p.m. on October 21, 1980, defendant and a companion left the scene of an all day strike protest at the Waterbury Plastics Plant in defendant's van. While the two were driving towards the village of Randolph, Vermont, a police officer patrolling in the vicinity noticed that the taillights on defendant's van were not working. The officer stopped the van, and after conversing with defendant for a short while, detected a strong odor of alcohol on her breath. The officer testified at trial that, when asked if she had been drinking, defendant responded that she had consumed five or six beers. In addition, the officer testified that defendant displayed poor coordination while attempting to perform several dexterity tests, and that her gait was unsteady. The first officer's testimony was buttressed by the testimony of a second officer, who had occasion to observe defendant shortly thereafter at the Randolph police station. His testimony was that defendant's eyes were watery and bloodshot, and that her speech was slurred. Based upon the above observations, and despite defendant and her companion's contentions to the contrary, the first officer formed the opinion that defendant had been driving while under the influence of intoxicating liquor, and requested her presence at the Randolph police station for DUI processing.

At the station, defendant submitted to a breath test to measure the percent of alcohol by weight in her blood. The results of the test, which was administered approximately forty-five minutes after the initial stop, indicated a .21 percent blood-alcohol concentration at the time of testing. At trial, since the State made no attempt to relate back the test results to the time of the initial stop, defendant objected to their admission into evidence. The trial court ruled, on the basis of our recent decision in State v. Dacey, 138 Vt. 491, 418 A.2d 856 (1980), that the test results were admissible to corroborate the above noted testimony of the police officer, but that in the absence of expert testimony relating said results back to the initial stop, the State could not avail itself of the permissive inference instruction embodied in 23 V.S.A. § 1204(a)(3).

On appeal, defendant has briefed two exceptions for our consideration, the first of which is whether the trial court erred when it admitted into evidence defendant's breath test results for the purpose of corroborating the testimony of the two officers. Defendant insists that the State had an affirmative obligation to relate back the test results to the time of the initial stop and, in the absence of such evidence, the test results were inadmissible. The essence of her argument is that the relationship between a person's blood-alcohol concentration and her degree of impairment, if any, is a matter beyond the ordinary experience and knowledge of the average lay person. Thus, she contends that in the absence of expert testimony explaining the significance of the .21 percent alcohol concentration, the jury was left to speculate as to the importance of the .21 test result. By permitting such speculation, defendant concludes that she was irreparably prejudiced. We disagree.

In State v. Dacey, supra, a case involving a DUI conviction in violation of § 1201(a)(2), we held that a prosecution under 23 V.S.A. § 1204(a)(3) " permits but does not compel a jury finding that defendant was under the influence of intoxicating liquor while operating a motor vehicle upon proof of .10% or more blood-alcohol content by weight at the time of operation." Id. at 496, 418 A.2d at 859 (emphasis in original). In order to take advantage of this inference, however, the State must produce "expert testimony relating the blood-alcohol test results back to the time of operation." Id. at 497, 418 A.2d at 859. Claiming that this holding stood for the proposition that all blood-alcohol test results were inadmissible absent expert testimony relating back the results to the time of operation, a claim now raised again by defendant, the State in Dacey filed a motion for reargument. We specifically denied that motion, and unanimously held, without elaboration, that "[i]t [was] obvious that we [had] not ruled blood-alcohol test results inadmissible if otherwise relevant." Id. (emphasis added).

The relevancy of evidence hinges on whether it has some probative value; that is, whether "it tends to make the existence of the fact at issue more or less probable." State v. Rollins, 141 Vt. 105, 110, 444 A.2d 884, 887 (1982) (citing State v. Patnaude, 140 Vt. 361, 370, 438 A.2d 402, 405 (1981)). If found to be relevant, the evidence may still be inadmissible "if its probative value is outweighed by the danger of unfair prejudice." State v. Picknell, 142 Vt. 215, 230, 454 A.2d 711, 718 (1982) (citing State v. Bevins, 140 Vt. 415, 419, 439 A.2d 271, 272 (1981)). The decision to admit relevant but prejudicial evidence rests within the trial court's discretion. Discretionary decisions are not "`subject to revision here unless it clearly and affirmatively appears that such discretion has been abused or withheld.'" State v. Polidor, 130 Vt. 34, 39, 285 A.2d 770, 773 (1971) (quoting State v. Goyet, 120 Vt. 12, 19, 132 A.2d 623, 630 (1957)).

In the present case, defendant was charged with operating a motor vehicle while under the influence of intoxicating liquor. The State produced scientific evidence indicating traces of alcohol in defendant's blood forty-five minutes after her initial stop. Since she had not had anything to drink between the time of her initial stop and her subsequent DUI processing, the test results were unquestionably relevant to the State's case. Moreover, we fail to see how the State's decision not to relate back the test results to the time of operation resulted in prejudice to defendant. They were as relevant as personal observations at that time. Absent the relating back evidence, the test results simply established the fact that defendant had consumed some amount of intoxicating liquor before being stopped. Although the evidence corroborated the testimony of the two officers, the jury still had to determine whether the ultimate effect of such consumption was "a loss of full control over the faculties of mind and body." State v. Carmody, 140 Vt. 631, 638, 442 A.2d 1292, 1295 (1982).

Accordingly, the trial court properly exercised its discretion in admitting the test results, and we see no reason to modify our holding in State v. Dacey, supra. We wish to make clear, however, that nothing we have said today in any way affects our holding in State v. Rollins, supra. Thus, in a case involving a charge of operating a motor vehicle on a public highway while there is .10% or more by weight alcohol in an individual's blood, 23 V.S.A. § 1201(a)(1), such relating back evidence is "indispensible to the prosecution's case." Id. at 110, 444 A.2d at 887.

Finally, defendant asserts that the trial court's charge to the jury unfairly favored the State. When reviewing a jury charge, "we are guided by the familiar rule that instructions to the jury may not be isolated into small segments and considered piecemeal, but must be measured by their full context." State v. Girouard, 135 Vt. 123, 139, 373 A.2d 836, 847 (1977); see also State v. St. Amour, 139 Vt. 99, 102, 422 A.2d 937, 939 (1980). "If the charge as a whole breathes the true spirit of the law, without misleading the jury, it ought to stand." State v. Rathburn, 140 Vt. 382, 387, 422 A.2d 452, 455 (1981) (citing State v. Gokey, 136 Vt. 33, 36, 383 A.2d 601, 602 (1978)).

We have carefully reviewed the trial court's charge in its entirety, and consider defendant's exception to be a piecemeal attack on a charge that "as a whole was full and fair to the defendant and to the State." State v. Girouard, supra, 135 Vt. at 140, 373 A.2d at 847. Since "[n]one of the objections raised by the defendant . . . demonstrates any error of law so prejudicial as to require reversal," id. (citing State v. Oakes, 129 Vt. 241, 259, 276 A.2d 18, 29 (1971)), the judgment of the trial court is affirmed.

Affirmed.