See also: BLOOD ALCOHOL TESTS; BREATH TESTS
FIELD SOBRIETY TESTS; INTOXICATION
URINE TESTS.

1.  Generally

Blood-alcohol concentration (BAC) is the most widely used measure of a subject’s level of intoxication.{footnote}See ch. 95 1/2 Ill. Rev. Stat. § 11-501.2 (admissibility in drunk driving cases).

Cf. People v. Keith, 148 Ill. 2d 32, 169 Ill. Dec. 276, 279, 591 N.E.2d 449 (Sup. Ct. 1992)(inadmissible in reckles homicide case).{/footnote} Blood alcohol concentration (BAC) is usually expressed as the percentage, by weight, of alcohol in a given volume of blood.{footnote}S. Brent & S. Stiller, Handling Drunk Driving Cases, § 3:1, at 26 (1985).{/footnote}  Most states provide for BAC measurement by testing breath, blood or urine.{footnote}See BLOOD ALCOHOL TESTS; BREATH TESTS; URINE TESTS.
See also FIELD SOBRIETY TESTS; HORIZONTAL GAZE NYSTAGMUS; INTOXICATION.{/footnote}  A court may take judicial notice of the reliability of widely accepted blood-alcohol testing methods and devices.  See also JUDICIAL NOTICE; SCIENTIFIC EVIDENCE–Judicial Notice of Reliability. 

2.  The Effect of Delay in Testing

Under most drunk driving laws, it is necessary for the prosecution to prove that the driver’s BAC level was above a certain level at the time he or she was driving. Because there is necessarily a significant lapse of time before a chemical test can be administered, and because a subject’s BAC level changes over time, it becomes necessary to infer from the test results what the driver’s BAC level was when driving.  The inference may be drawn with the aid of expert testimony, or through presumptions. This is a controversial area because while one might intuitively assume that a driver’s BAC level will decline over time, research shows that this is not always the case.  Test results may actually reflect a higher BAC level than existed when the subject was driving.

2(a).  Understanding Changes in BAC Levels Over Time

The manner in which a particual subject’s BAC level changes over time is based on the subject’s alcohol absorption rate (the rate at which ingested alcohol is absorbed into the blood stream) and elimination rate (the rate at which blood alcohol is eliminated from the blood stream).  It takes anywhere from 45 minutes to 3 hours for the body to absorbe alcohol into the bloodstream, depedning upon, among other things, how much food is in the stomach and the type of food.  Experts generally agree that alcohol is eliminated from the body at an average rate of .015% per hour, and this figure has been accepted by courts.{footnote}Ring v. Taylor, 141 Ariz. 56, 69, 685 P.2d 121, 134 (1984); Tyner v. State, 503 N.E.2d 444, 446 (Ind.  Ct. App. 1987).{/footnote} 

The graph below shows the BAC levels resulting from the combined effect of absorption and elimination of alcohol on an empty stomach (Curve A) and after a meal (Curve B).{footnote}Fitzgerald & Hume, The Single Chemical Test for Intoxication: A Challenge to Admissibility, 66 Mass. L. Rev. 23, 29 (1981) (reprinted by permission).
See also Jennifer L. Pariser, Note: In Vino Veritas: The Truth About Blood Alcohol Presumptions In State Drunk Driving Law,  64 N.Y.U.L. Rev. 141, 147 (1989) (reproducing and discussing this graph).{/footnote}  Both curves rise initially as alcohol is absorbed into the system faster than it is being eliminated. The BAC levels peak when the alcohol is being eliminated as fast as it is being absorbed.  As the body begins eliminating alcohol faster than it is absorbing it, the curves begin to slope down.

[INSERT GRAPH HERE]

Most experts agree on the average length of time it takes for a subject’s BAC level to reach its peak: forty-five to ninety minutes on an empty stomach (Curve A, supra); two to three hours if the alcohol is consumed with or after a meal (Curve B, supra).{footnote}Jennifer L. Pariser, Note: In Vino Veritas: The Truth About Blood Alcohol Presumptions In State Drunk Driving Law,  64 N.Y.U.L. Rev. 141, 147 (1989).{/footnote}  In either case, a driver who fails a BAC test may have had a BAC level within legal limits at the time he or she was driving.{footnote}See generally Jennifer L. Pariser, Note: In Vino Veritas: The Truth About Blood Alcohol Presumptions In State Drunk Driving Law,  64 N.Y.U.L. Rev. 141, 151 (1989) (“A defendant’s BAC can easily rise from a legal level to a criminal level in the time it takes to administer a test.”), citing Fitzgerald & Hume, The Single Chemical Test for Intoxication: A Challenge to Admissibility, 66 Mass. L. Rev. 23, 29 (1981).
See also Holt, Stewart, Adam & Heading, Alcohol Absorption, Gastric Emptying and a Breathalyzer, 9 Brit. J. Clinical Pharmacology 205 (1980).{/footnote}

2(b).  Extrapolation Testimony

Both the prosecution and the defense may offer expert testimony extrapolating from the test results to estimate the driver’s blood-alcohol level when driving. The expert generally must take into account several factors: the rate of alcohol absorption and elimination, how much time passed between driving and testing, and how much time passed between the driver’s last drink and driving the vehicle.{footnote}John D. LaRocca, Recent Case, 62 Temp. L.Q. 757 n.3 (1989).{/footnote}   Particularly in close cases, expert extrapolation testimony will be excluded if is deemed speculative.{footnote}E.g., Commonwealth v. Jarman, 601 A.2d 1229 (Pa. 1992); Commonwealth v. Gonzalez, 546 A.2d 26 (Pa. 1988) (where results from blood test conducted three hours after accident showed BAC of 0.09%, expert extrapolation testimony that the defendant’s BAC level may have been as high as 0.125% at time of accident too speculative to be admissible with respect to charge of driving with BAC over 0.10%, but admissible to prove separate offense of driving under the influence).{/footnote}

2(c).  Presumptions

States have created several types of presumptions in order to simplify the proof as well as the defense of a charge of drunk driving.  Under laws which require proof that the driver was “intoxicated” or was “under the influence,” the requisite level of intoxication is often presumed from proof that the driver’s BAC was at or above a certain level at the time of testing, typically 0.10%.{footnote}E.g., Ala. Code § 32-5A-191(a)(1) (Supp. 1988); Alaska Stat. § 28.35.033 (1996); Cal. Veh. Code § 23155 (1996) (0.08%); Ill. 625 ILCS 5/11-501.2(b)(3) (1996); Ky. Rev. Stat. Ann. § 189.520(3) (Michie/Bobbs-Merill Supp. 1986); Md. Cts. & Jud. Proc. Code Ann. § 10-307 (Supp. 1988).
See Jennifer L. Pariser, Note: In Vino Veritas: The Truth About Blood Alcohol Presumptions In State Drunk Driving Law,  64 N.Y.U.L. Rev. 141, 142 (1989); Thompson, The Constitutionality of Chemical Test Presumptions of Intoxication in Motor Vehicle Statutes, 20 San Diego L. Rev. 301 (1983); 16 ALR3d 748; 46 ALR2d 1176. {/footnote}  Generally, the length of delay affects the weight but not the admissibility of the blood test results.{footnote}Commonwealth v. Gonzalez, 546 A.2d 26 (Pa. 1988) (results from blood test conducted three hours after accident showing BAC of 0.09% admissible in prosecution for driving under the influence and driving with BAC over 0.10%); Commonwealth v. Tylwalk, 393 A.2d 473 (Pa. Super. Ct. 1978) (four and one-half hour delay affected weight but not admissibility).
See also Commonwealth v. Jarman, 601 A.2d 1229, 1230-31 (Pa. 1992) (while a positive test result shortly after driving creates a strong inference of guilt, the inference weakens the greater the time which elapses between driving and testing).{/footnote}

Some drunk driving laws (so-called “per se  laws”) make it illegal to drive while having a blood-alcohol content above a certain level.{footnote}E.g., Minn. Stat. Ann. § 169.121(1)(c)-(d) (West Supp. 1987); 75 Pa. Cons. Stat. Ann. § 3731(a)(4).{/footnote} Most states with such laws have created a presumption that a driver’s BAC when driving was equal to or higher than his BAC at the time of the test.{footnote}Cal. Veh.  Code § 23152(b) (West 1985); Ind. Code Ann. § 9-11-4-15 (Burns Supp. 1987); Iowa Code Ann. § 321J.2(7) (West Supp. 1988); Nev. Rev. Stat. § 484.381(1) (1987).
Buchanan v. Auburn, 512 So. 2d 145, 147 (Ala. Crim. App.), rev’d on other grounds sub nom. Hays v. Jacksonville, 518 So. 2d 892 (Ala. Crim. App.  1987); Erickson v. Municipality of Anchorage, 662 P.2d 963, 965 (Alaska Ct. App. 1983); Ex rel.  Williams, 487 P.2d 766, 768 (Ariz. Ct. App. 1971); State v. Garrity, 552 A.2d 452, 456 (Conn. Ct. App. 1989); State v. Rucker 297 A.2d 400, 402 (Del. Super. Ct. 1972); State v. Sutliff, 547 P.2d 1128, 1129-30 (Idaho 1976); People v. Kappas, 120 Ill.  App. 3d 123, 129, 458 N.E. 2d 140, 143-44 (Ill. Ct. App. 1983); State v. Tripp, 180 A.2d 601, 603 (Me. 1962); People v. Kozar, 221 N.W.2d 170, 173 (Mich. App. 1974); State v. Hvistendahl, 405 N.W.2d 273, 277 (Neb. 1987); State v. Gallant, 227 A.2d 597, 598 (N.H. 1967); People v. Mertz, 497 N.E.2d 657, 662 (N.Y. 1986); State v. Ulrich, 478 N.E.2d 812, 821-22 (Ohio App. 1984); Commonwealth v. Speights, 509 A.2d 1263, 1266-67 (Pa. Super. 1986); State v. McDonald, 421 N.W.2d 492, 494 (S.D. 1988); Forte v. State, 707 S.W.2d 89, 95 (Tex. Crim. App. 1986); State v. Bence, 627 P.2d 1343, 1346 (Wash. Ct. App. 1981); State v. Vick, 312 N.W.2d 489, 494-95 (Wis. 1981).
See also State v. Larson, 429 N.W.2d 674, 676-77 (Minn. 1988) (expert extrapolation testimony unnecessary "[a]s long as the delay between apprehension and testing is reasonable.").{/footnote}  Other states have created a permissible inference to the same effect.{footnote}Cooley v. Municipality of Anchorage, 649 P.2d 251, 255 (Alaska Ct. App.  1982); State v. Vick, 312 N.W.2d 489, 497 (Wis. 1981).{/footnote}  Based on the research discussed supra, these presumptions and inferences have been criticized as scientificaly unsound and unconstitutional.{footnote}State v. Conway, 707 P.2d 618, 619 (Or. App. 1985) (error to instruct jury that it may infer BAC at time of driving no lower than BAC at time of test).
Jennifer L. Pariser, Note: In Vino Veritas: The Truth About Blood Alcohol Presumptions In State Drunk Driving Law,  64 N.Y.U.L. Rev. 141 (1989).{/footnote}  In any event, a presumption as to a driver’s level of intoxication when driving is rebuttable.{footnote}Logan v. Brown, 725 P.2d 1130, 1136 (Ariz. 1986).{/footnote}

In order to avoid this entire controversy, some states criminalize driving by a person whose BAC is 0.10% or greater at the time of testing, so long as the test was performed within a certain period (such as three hours) after driving.{footnote}Colo. Rev.  Stat. § 42-2-1202(1.5)(a) (Supp. 1988); Ga. Code Ann. § 40-6-391(4) (Supp.  1987); Kan. Stat.  Ann. § 8-1567(1) (Supp. 1987); Minn. Stat. Ann. § 169.121(1)(e) (West Supp.  1989); N.C.  Gen. Stat. § 20-138.1 (Supp. 1985); N.D. Cent. Code § 39-08-01 (1987) (two hours); Okla. Stat. Ann. tit. 47, § 11-902 (Supp.  1989); 75 Pa. Cons. Stat. Ann. 3731(a)(5) (three hours or a reasonable additional time where necessary).
See also State v. Tischio, 527 A.2d 388 (N.J. 1987), appeal dismissed, 108 S. Ct. 768 (1988) (construing New Jersey’s statute, N.J. Stat. Ann. § 39:4-50.1, (West Supp. 1987) as making the relevant time for a BAC the time of the test).{/footnote}  Because these laws do not create a presumption but merely redefine the crime, some scholars have argued that they are constitional.{footnote}Jennifer L. Pariser, Note: In Vino Veritas: The Truth About Blood Alcohol Presumptions In State Drunk Driving Law,  64 N.Y.U.L. Rev. 141, 164 (1989).
Contra Myles A. Kauffman, The Coming Of Subsection (A)(5) Of Pennsylvania’s Drunk Driving Law: "A Statute With A Face Only A Prosecutor Could Love", 4 Widener J. Public L. 493 (1995).

{/footnote}

Bibliography

S. Brent & S. Stiller, Handling Drunk Driving Cases (1985).
Jennifer L. Pariser, Note: In Vino Veritas: The Truth About Blood Alcohol Presumptions In State Drunk Driving Law,  64 N.Y.U.L. Rev. 141 (1989).
Love Phipps, Case Note: Constitutional Law — Self-Incrimination — Caution!  Refusal To Take A Blood-Alcohol Test May Be Hazardous To Your Trial — South Dakota v. Neville, 12 Fla. St. U.L. Rev. 167 (1984).
Jay M. Zitter, Annnotation, Admissibility In Criminal Case Of Evidence That Accused Refused To Take Test Of Intoxication, 26 A.L.R.4th 1112 (19__).