Title: P. v. McNeal
Citation: 46 Cal. 4th 1183
Docket Number: S157565
State: California
Issuer: California Supreme Court
Date: July 9, 2009

1 
Filed 7/9/09 
 
 
 
IN THE SUPREME COURT OF CALIFORNIA 
 
 
 
THE PEOPLE, 
) 
 
 
) 
 
Plaintiff and Respondent, 
) 
 
 
) 
S157565 
 
v. 
) 
 
 
) 
Ct.App. 4/2 E041226 
TIMMIE LANCE McNEAL, 
) 
 
) 
San Bernardino County 
 
Defendant and Appellant. 
) 
Super. Ct. No. TRE038083 
 
____________________________________) 
 
 
A defendant accused of driving under the influence of alcohol can be 
charged under two separate code sections.  The “generic DUI” provision prohibits 
driving “under the influence” of alcohol.  (Veh. Code, § 23152, subd. (a) 
(hereafter § 23152(a).)1  The “per se DUI” provision prohibits driving with a 
blood-alcohol level of 0.08 percent or more.  (§ 23152, subd. (b) (hereafter 
§ 23152(b).)  This case concerns how a generic DUI charge can be proven, or 
defended, at trial. 
 
The Vehicle Code requires all drivers who are lawfully arrested for DUI to 
submit to chemical testing of the blood or breath to determine the alcohol content 
of their blood.  (§ 23612, subd. (a)(1)(A).)2  Whereas a blood test directly 
measures the subject‟s blood-alcohol level, a breath sample must be converted to 
derive a blood-alcohol percentage.  The conversion factor, known as a “partition 
                                              
1  
All unspecified statutory references are to the Vehicle Code. 
2  
If blood and breath tests are not available, the driver‟s blood-alcohol 
content is determined from a urine test.  (§ 23612, subds. (a)(1)(A), (d)(2).) 
2 
ratio,” reflects the relationship between alcohol measured in a person‟s breath and 
alcohol in the blood.  Breath-testing machines in California incorporate a partition 
ratio of 2,100 to 1, which means the amount of alcohol in 2,100 milliliters of 
breath is considered equivalent to the amount of alcohol in 1 milliliter of blood.  It 
is undisputed, however, that partition ratios can vary widely, both in the general 
population and within an individual. 
 
Defendant was charged with generic and per se DUI after he produced a 
breath sample indicating a blood-alcohol concentration of 0.10 percent.  By 
statute, if a chemical test within three hours of driving measures a driver‟s blood 
alcohol at 0.08 percent or more, the driver is presumed to have been driving 
“under the influence” of alcohol.  (§ 23610, subd. (a)(3) (hereafter § 23610(a)(3).)  
Defendant claims he was wrongly prevented from introducing evidence about 
partition ratio variability to rebut this presumption.  In People v. Bransford (1994) 
8 Cal.4th 885, 887-888 (Bransford), we confronted a similar claim in the context 
of the per se DUI offense.  We concluded evidence about partition ratio variability 
is irrelevant in those cases because the Legislature incorporated a 2,100-to-1 
partition ratio within its definition of the offense.  (Id. at pp. 892-893.) 
 
However, a generic DUI charge is defined differently, and the presumption 
is not part of that definition.  A generic DUI charge requires proof that the 
defendant‟s ability to drive safely was impaired because he had consumed alcohol.  
We conclude this difference is significant and hold that competent evidence about 
partition ratio variability may be admitted to defend against a generic DUI charge.  
Reversal is not required, though, because any error in this case was harmless under 
People v. Watson (1956) 46 Cal.2d 818, 836. 
BACKGROUND 
 
Defendant was stopped after an officer saw him drive through two red 
lights.  Defendant‟s eyes were watery and bloodshot, his speech was slurred, he 
smelled of alcohol, and he admitted he had consumed a beer.  Defendant was 
arrested and given a breath test about an hour after the initial stop.  Defendant 
3 
blew into the machine five times but produced only two samples sufficient for 
testing.3  Both valid samples registered a blood-alcohol level of 0.10 percent. 
 
Defendant was charged with both generic and per se DUI.4  At trial, a 
prosecution expert testified that a person is too impaired to operate a motor vehicle 
safely if he displays slurred speech and bloodshot, watery eyes, commits traffic 
infractions, performs poorly on field sobriety tests, and records a blood-alcohol 
level of 0.10 percent in a breath test.  A defense expert agreed that scientific 
authorities consider a person with a blood-alcohol level over 0.08 percent to be 
under the influence of alcohol.  However, he considered defendant‟s test results 
unreliable because the machine‟s repeated failure to register sufficient samples 
suggested it was not functioning properly.  The defense expert also testified that 
breath-testing machines have a margin of error of 0.02 percent. 
 
After both sides had rested but before closing arguments, defense counsel 
moved to reopen to present expert testimony about partition ratio variability in 
connection with the generic DUI charge.  The record does not disclose whether 
defendant intended to introduce evidence of his own partition ratio or evidence 
about the variability of partition ratios in the general population.  The court denied 
the motion and instructed the jury regarding the statutory presumption of 
intoxication.  (CALJIC No. 12.61.)  Specifically, with regard to the generic DUI 
count, the jury was instructed:  “If the evidence establishes beyond a reasonable 
doubt that at the time of the chemical analysis of the defendant‟s blood, breath or 
urine there was .08 percent or more, by weight, of alcohol in the defendant‟s 
                                              
3  
The test was administered using a Draeger Alcotest 7110 machine.  To 
produce a valid sample for this machine, the subject must exhale at least 1.5 liters 
of breath over at least 4.5 seconds.  (Taylor & Tayac, Cal. Drunk Driving Defense 
(4th ed. 2008) Forensic Chemist: Blood-Alcohol, § 12.40, p. 874 (hereafter Taylor 
& Tayac).)  If the subject fails or refuses to exhale a sufficient volume of breath, 
the sample is considered invalid.  (Ibid.) 
4  
He was also cited for two infractions for failing to stop at a red light.  
(§ 21453, subd. (a).) 
4 
blood, you may, but are not required [to,] infer that the defendant was under the 
influence of an alcoholic beverage at the time of the alleged offense.” 
 
The jury convicted on the generic DUI charge but hung on the per se count.  
Defendant moved for a new trial, arguing he should have been permitted to 
introduce partition ratio evidence to rebut the presumption that he was under the 
influence of alcohol.  (§ 23610(a)(3).)  Defense counsel said his expert would 
testify that 30 percent of the population has a partition ratio other than 2,100 to 1.  
When the new trial motion was denied, defendant sought relief in the superior 
court‟s appellate division, claiming the exclusion of his partition ratio evidence 
was reversible error.  The appellate division concluded partition ratio evidence is 
relevant and admissible but found the error harmless given the strength of the 
evidence supporting the jury‟s verdict. 
 
The Court of Appeal transferred the case to itself on its own motion.  That 
court distinguished between evidence about the variability of partition ratios in the 
general population and evidence showing the defendant had a nonstandard ratio.  
It concluded evidence challenging the validity of the statutory 2,100-to-1 ratio was 
irrelevant, but evidence that this particular defendant had a different partition ratio 
should have been admitted.  The court reasoned that if the defendant‟s own ratio 
differed significantly from the standard ratio, this fact could support an inference 
that the defendant was not actually impaired at the time of the offense.  The court 
therefore held such personal partition ratio evidence is relevant and admissible in 
generic DUI cases.  Although it found the defendant‟s offer of proof insufficient to 
determine the precise nature of the partition ratio evidence he sought to introduce, 
the Court of Appeal concluded that even assuming defendant intended to present 
evidence about his own ratio, and assuming the issue was preserved for review, 
any error was harmless under People v. Watson, supra, 46 Cal.2d at page 836. 
 
The People and defendant separately petitioned for review.  We granted 
both petitions to address the admissibility of partition ratio evidence in 
section 23152(a) cases. 
5 
DISCUSSION 
I. 
The Science of Alcohol Testing 
 
Alcohol affects the central nervous system.  When ingested, it is absorbed 
into the blood and carried through the carotid arteries to the brain.  (State v. 
Downie (1990) 117 N.J. 450 [569 A.2d 242, 245] (Downie); State v. Brayman 
(1988) 110 Wn.2d 183 [751 P.2d 294, 297] (Brayman).)  After passing through the 
brain, alcohol travels through venous blood to the liver and heart, and from there, 
to the lungs, where it diffuses into alveolar air space and is exhaled in the breath.  
(Downie, at pp. 245-246.)  As a practical matter, it is impossible to measure the 
amount of alcohol in a person‟s carotid arteries or brain.  (Id. at p. 246; Taylor & 
Tayac, Forensic Chemist: Blood-Alcohol, supra, § 12.2, p. 712.)  However, most 
experts agree that measurements of alcohol in venous blood or breath give a good 
indication of the amount of alcohol in the brain during the post-absorptive phase.5  
(Downie, at p. 246.) 
 
When a subject blows into a breath-testing machine, the device measures 
the amount of alcohol vapor expelled into alveolar spaces deep in the lungs.  From 
this measurement of breath alcohol, a blood-alcohol percentage can be computed 
using a mathematical constant.  The conversion from breath alcohol to blood 
alcohol is based on the chemistry principle of “Henry‟s law,” which holds that 
there is “a constant ratio between the concentration of alcohol in the blood and the 
concentration of alcohol in the alveolar air of the lungs.”  (Taylor & Tayac, 
Forensic Chemist: Blood-Alcohol, supra, § 12.19, p. 770.)  Breath-testing 
                                              
5  
As the body absorbs ingested alcohol, blood-alcohol levels rise until they 
reach a peak.  The rate of absorption depends on many variables, including the 
amount of food in the stomach, the amount of alcohol consumed, the time over 
which it was consumed, and numerous individual factors.  (Downie, supra, 569 
A.2d at pp. 245-246; Taylor & Tayac, Forensic Chemist: Blood-Alcohol, supra, 
§ 12.2, pp. 712-714.)  During active absorption, blood-alcohol levels are highest in 
arterial blood.  (Downie, at p. 246; Taylor & Tayac, § 12:2, p. 712.)  The post-
absorptive phase occurs after the blood-alcohol level has peaked and begun to 
decline. 
6 
machines in California use a conversion factor of 2,100 to 1, meaning “the amount 
of alcohol in 2,100 milliliters of alveolar breath is equivalent to the amount of 
alcohol in 1 milliliter of blood.”  (Cal. Code Regs., tit. 17, § 1220.4, subd. (f); see 
People v. McDonald (1988) 206 Cal.App.3d 877, 880.)  This ratio is used, 
apparently without exception, in breath-testing devices throughout the United 
States.  (See Brayman, supra, 751 P.2d at p. 297; State v. McManus (1989) 152 
Wis.2d 113 [447 N.W.2d 654, 656].) 
 
Nevertheless, courts here and across the country have long recognized that 
actual partition ratios vary, both among members of the population and within a 
single individual.  In Bransford, supra, 8 Cal.4th at page 889, we noted that an 
individual‟s ratio of breath-alcohol to blood-alcohol concentration can be 
influenced by many variables, including “body temperature, atmospheric pressure, 
medical conditions, sex, and the precision of the measuring device.  [Citations.]”  
(See also Brayman, supra, 751 P.2d at p. 297 [mentioning, in addition to these 
factors, hematocrit level and elapsed time between drinking and breath-alcohol 
measurement].)  Experts have also described a wide range of variability in 
partition ratios throughout the general population.  In People v. McDonald, supra, 
206 Cal.App.3d at page 880, for example, the people‟s expert testified a person‟s 
ratio could be as high as 2,700 to 1 or as low as 1,550 to 1, and in Downie, supra, 
569 A.2d at page 247, the court noted that partition ratios measured in a group of 
experimental subjects ranged from a low of 1,706 to 1 to a high of 3,063 to 1. 
 
Despite this recognized variability, most scientists agree that the 2,100-to-1 
ratio roughly approximates or even underestimates the ratio of most people.  In the 
late 1980‟s, the New Jersey Supreme Court considered evidence presented by ten 
experts at a hearing addressing partition ratios and the reliability of breath test 
results.  (Downie, supra, 569 A.2d at pp. 243-244.)  These experts “generally 
agreed” that breath-testing machines using the 2,100-to-1 partition ratio “will 
usually underestimate the amount of alcohol in the blood” for several reasons.  (Id. 
at p. 247.)  “First, most people‟s partition ratios may be closer to 2300:1 than to 
7 
2100:1.  Second, the breathalyzer results are truncated, or the third decimal 
position is dropped when read.  If a person reads .099 on the breathalyzer, the 
results will be shortened to read .09, thereby underestimating the breath alcohol.  
Third, a suspect may not provide enough deep breath to register all of the alcohol 
present in the alveolar air.  Fourth, the breathalyzer‟s scale is set .003 below the 
start line and this gives suspects an added benefit.”  (Ibid.)  Several experts opined 
that the standard partition ratio is set artificially low, and the true mean ratio in the 
population is closer to 2,300 to 1.  (Id. at p. 247.)  Dr. Robert Borkenstein, 
inventor of the breathalyzer machine, stated that “breathalyzer researchers and 
members of the National Safety Council adopted the 2100:1 partition ratio instead 
of the more accurate 2300:1 ratio because they wanted to err on the low side and 
have almost no errors on the high side.”  (Id. at p. 247.)6 
II. 
Development of California Law Regarding Partition Ratio Evidence 
 
California‟s first drunk driving statute, enacted in 1913, prohibited any 
“intoxicated person” from driving a motor vehicle on a public highway.  (Stats. 
1913, ch. 326, § 17, p. 646; see Burg v. Municipal Court (1983) 35 Cal.3d 257, 
262.)  The prohibition was later redefined as driving “under the influence” of 
alcohol.  “To be „under the influence‟ within the meaning of the Vehicle Code, the 
liquor or liquor and drug(s) must have so far affected the nervous system, the 
brain, or muscles as to impair to an appreciable degree the ability to operate a 
                                              
6  
See also People v. Lepine (1989) 215 Cal.App.3d 91, 94 (defense expert 
testified that the average partition ratio is 2,286 to 1); People v. Pritchard (1984) 
162 Cal.App.3d Supp. 13, 16 (people‟s expert testified that 95 percent of the 
population falls within the range of 2,100 to 1, plus or minus a 10 percent margin 
of error); People v. Gustafson (1990) 194 Ill.App.3d 910 [551 N.E.2d 826, 829] 
(state‟s expert testified “he believed the 2,100:1 ratio was applicable to at least 95 
[percent] of the population”); State v. McManus, supra, 447 N.W.2d at pages 656-
657 (noting “the 2,100:1 ratio has been shown to either correctly estimate or 
underestimate a person‟s corresponding blood alcohol concentration with 94 
percent accuracy”); Brayman, supra, 751 P.2d at page 300 (citing studies 
indicating breath tests underestimate blood-alcohol levels 80 to 91 percent of the 
time and overestimate them only 5 to 6 percent of the time). 
8 
vehicle in a manner like that of an ordinarily prudent and cautious person in full 
possession of his faculties.  [Citations.]”  (Byrd v. Municipal Court (1981) 125 
Cal.App.3d 1054, 1058, italics omitted.)  In 1969 the Legislature enacted a 
statutory presumption that a driver was under the influence if the driver‟s blood 
contained 0.10 percent or more, by weight, of alcohol.  (Stats. 1969, ch. 231, § 1, 
p. 565; Burg v. Municipal Court, at p. 263.)  However, “[e]ven these laws, which 
considerably assisted the prosecution of „driving under the influence‟ cases, 
proved inadequate in many respects.  Under them, the ultimate question was 
defined in terms of the defendant‟s subjective behavior and condition:  „Was the 
defendant under the influence at the time he drove?‟  Celerity and certainty of 
punishment were frustrated by the ambiguity of the legal criteria; no matter what 
his blood-alcohol level, a defendant could escape conviction merely by raising a 
doubt as to his intoxication.  [Citations.]”  (Burg v. Municipal Court, at p. 263.)  
These difficulties led the Legislature to create a new crime.  Section 23152(b), 
added in 1981, made it unlawful for a person to drive with a blood-alcohol level of 
0.10 percent or more, by weight, and specified that, “percent, by weight, of 
alcohol shall be based upon grams of alcohol per 100 milliliters of blood.”  (Stats. 
1981, ch. 940, § 33, p. 3578.)  To secure a conviction for this new “per se DUI” 
offense, the prosecution no longer had to prove the accused driver was actually 
impaired at the time of the offense, but only that he drove with a blood-alcohol 
level at or exceeding 0.10 percent.  (Burg v. Municipal Court, at p. 265.)  In 1989, 
the Legislature further strengthened our state‟s DUI laws by lowering the 
punishable blood-alcohol threshold from 0.10 percent to 0.08 percent.  (Stats. 
1989, ch. 1114, § 27, p. 4080; see People v. Ireland (1995) 33 Cal.App.4th 680, 
689.) 
 
Thus, after 1981 there were two parallel statutes making it a crime to drive 
while intoxicated.  The generic DUI provision (§ 23152(a)) retained the historical 
approach, requiring proof that the defendant was actually impaired by his 
drinking.  The per se DUI statute (§ 23152(b)) simply required proof that the 
9 
defendant had been driving with a blood-alcohol level over the legal limit.  If the 
limit was exceeded, the statute was violated, and no additional proof of the 
defendant‟s impairment was required. 
 
Throughout this time, both the per se DUI offense and the presumption of 
intoxication applicable to the generic DUI offense were defined based on 
measurements of blood alcohol.  As a result, whenever a defendant elected to have 
a breath test instead of a blood draw, it was necessary to convert the breath results 
into an equivalent blood-alcohol percentage.  (Bransford, supra, 8 Cal.4th at 
pp. 888-889; People v. Ireland, supra, 33 Cal.App.4th at p. 689.)  A Department 
of Health Services regulation required that this conversion be made using a 
partition ratio of 2,100 to 1.  (Cal. Code Regs., tit. 17, § 1220.4, subd. (f).)7 
 
The mandated use of a standard partition ratio, in the face of scientific 
knowledge that such ratios vary greatly, provided fertile ground for defense 
arguments challenging the reliability of breath test results.  Initially, courts 
permitted defendants to show only that their personal partition ratio differed from 
the norm.  (Bransford, supra, 8 Cal.4th at p. 889.)  Applying the judicially created 
“rule of convenience,” these courts placed the burden on the defendant to produce 
evidence of a nonstandard personal ratio because this fact was considered to be 
peculiarly within the defendant‟s knowledge.  (People v. Pritchard, supra, 162 
Cal.App.3d at p. Supp. 16; People v. Gineris (1984) 162 Cal.App.3d Supp. 18, 23; 
People v. Herst (1987) 197 Cal.App.3d Supp. 1, 3-4.) 
 
Later courts questioned this application of the rule of convenience when 
confronted with evidence showing that an individual‟s partition ratio can vary 
from time to time due to the influence of numerous external factors.  (See, e.g., 
People v. McDonald, supra, 206 Cal.App.3d at p. 883.)  This evidence directly 
                                              
7  
“A breath alcohol concentration shall be converted to an equivalent blood 
alcohol concentration by a calculation based on the relationship: the amount of 
alcohol in 2,100 milliliters of alveolar breath is equivalent to the amount of 
alcohol in 1 milliliter of blood.”  (Cal. Code Regs., tit. 17, § 1220.4, subd. (f).) 
10 
contradicted something the Pritchard line of cases had apparently assumed: “that 
one‟s partition ratio is constant and can be measured by the defendant.”  (Ibid.; see 
also People v. Lepine, supra, 215 Cal.App.3d at pp. 97-99.)  Because partition 
ratios may vary depending on many factors, reliable measurements may be 
difficult and costly to acquire.  Thus, some courts reasoned that defendants did not 
have substantially better access to evidence of their personal partition ratios and 
could not be expected to carry the burden of production on the issue.  (People v. 
McDonald, at p. 883; People v. Lepine, at pp. 99-101; People v. Thompson (1989) 
215 Cal.App.3d Supp. 7, 13.)  Further, having freed defendants from the rule of 
convenience, these courts saw no reason to exclude general partition ratio 
evidence describing the variability of partition ratios among the general 
population.  (People v. Lepine, at p. 100; People v. Thompson, at pp. Supp. 13-14; 
People v. Cortes (1989) 214 Cal.App.3d Supp. 12, 18.)  After citing a string of 
out-of-state decisions permitting such evidence, one court reasoned:  “[I]t seems 
clear from the evidence submitted in this case and from a host of opinions in this 
and other states, that the partition ratio may vary from time to time and from 
individual to individual.  This being the case it is appropriate a jury be allowed to 
consider that fact.  We trust in the general rules of evidence, the preparation of 
counsel and the good judgment of trial judges to insure that this question of 
partition ratio variability is presented to jurors in a proper, complete and 
understandable form.”  (People v. Lepine, at p. 100, fn. omitted.) 
 
The Legislature responded swiftly to these developments.  In April 1989, 
legislators amended a pending Senate Bill on a related topic to specify for 
purposes of the per se DUI offense and the presumption of intoxication that the 
percentage of alcohol in a person‟s blood is to be based upon grams of alcohol per 
100 milliliters of blood “or grams of alcohol per 210 liters of breath.”  (Sen. 
Amend. to Sen. Bill No. 1119 (1989-1990 Reg. Sess.) Apr. 27, 1989, p. 23; Stats. 
1989, ch. 1114, §§ 24-25, pp. 4078-4079 [amending § 23152(b)]; Sen. Amend. to 
Sen. Bill No. 1119 (1989-1990 Reg. Sess.) Apr. 27, 1989, p. 26; Stats. 1989, 
11 
ch. 1114, §§ 34-35, pp. 4085-4086 [amending former § 23155, subd. (b)].)  In so 
doing, the Legislature codified 2,100 to 1 as the partition ratio to be used in 
converting breath test results into blood-alcohol levels. 
 
As we observed in Bransford, supra, 8 Cal.4th at page 890, scant 
explanation for these amendments appears in the legislative history of Senate Bill 
No. 1119, but more illumination can be found in the history of Assembly Bill 
No. 4318 (1989-1990 Reg. Sess.), enacted as Statutes 1990, chapter 708, section 1, 
page 3289.  The purpose of Assembly Bill No. 4318 was to accelerate the effective 
date of the partition ratio amendments to section 23152(b) that had been enacted 
by passage of Senate Bill No. 1119.  (Assem. Com. on Public Safety, com. on 
Assem. Bill No. 4318 (1989-1990 Reg. Sess.) as introduced May 15, 1990; see 
People v. Ireland, supra, 33 Cal.App.4th at p. 691.)  Committee reports 
concerning this bill evince the Legislature‟s desire to prohibit challenges to breath 
test results based on the partition ratio.  For example, the Assembly Committee on 
Public Safety observed that “[a]ttacks on the partition ratio may result in 
expensive and time consuming evidentiary hearings and undermine successful 
enforcement of driving under the influence laws.”  (Assem. Com. on Public 
Safety, com. on Assem. Bill No. 4318 (1989-1990 Reg. Sess.) as introduced May 
15, 1990, p. 2.)  The express purpose of Assembly Bill No. 4318 was to 
“[e]liminate the need for conversion of a breath quantity to a blood concentration 
of alcohol by statutorily defining driving under the influence in terms of the 
concentration of alcohol found in the breath when breath analysis is used.”  (Ibid.; 
see also Sen. Rules Com., Off. of Sen. Floor Analyses, Rep. on Assem. Bill 
No. 4318 (1989-1990 Reg. Sess.) Aug. 9, 1990, p. 1.) 
 
The Legislature accomplished this purpose by defining the offense of per se 
DUI in terms of a prohibited level of blood alcohol or breath alcohol.  As we 
explained in Bransford, supra, 8 Cal.4th at page 890, the amendment of 
section 23152(b) providing for an alternative measurement based upon breath 
changed the definition of the offense.  In the per se DUI statute, the Legislature 
12 
has set a legal limit on permissible blood alcohol and has defined how that limit is 
to be measured in a breath sample.  If the limit, measured as the statute sets out, is 
exceeded, the statute has been violated.  Because section 23152(b) now defines the 
offense of per se DUI as the presence of a prohibited level of alcohol in either 100 
milliliters of blood or 210 liters of breath, a conversion from breath to blood-
alcohol concentration is no longer required to establish guilt.  Accordingly, 
evidence attacking the standard partition ratio is no longer relevant in a per se DUI 
prosecution because the Legislature has codified the 2,100-to-1 ratio as part of the 
offense.  (Bransford, at pp. 890-892.) 
 
Assembly Bill No. 4318 did not amend former section 23155, however.8  
The bill amended only the per se DUI offense (§ 23152(b)) and a similar offense 
of per se DUI causing bodily injury (§ 23153, subd. (b)).  (Stats. 1990, ch. 708, 
§§ 1-4, pp. 2870-2872.)  Thus, committee statements reflecting the Legislature‟s 
desire to end the conversion of breath results to blood alcohol were all made in 
regard to changing the definition of the per se DUI offense.  No legislative history 
clarifies exactly what the Legislature intended when it amended the presumption 
of intoxication applicable to generic DUI cases 
III. 
Admissibility of Partition Ratio Evidence in Generic DUI Cases 
 
Despite our holding in Bransford that partition ratio evidence is 
inadmissible in per se DUI cases, defendant argues partition ratio variability 
evidence should be admissible in generic DUI cases to rebut the presumption that 
a person who produces a certain breath result is actually “under the influence” of 
alcohol.  (§§ 23152(a); 23610(a)(3).)  This issue was not presented in Bransford, 
and we specifically declined to consider it.  (Bransford, supra, 8 Cal.4th at p. 893, 
fn. 10.)  Defendant‟s claim does involve the same statutory language we 
interpreted in Bransford:  “Percent, by weight, of alcohol in the person‟s blood 
                                              
8  
As a result, partition ratio amendments to the statutory presumption of 
intoxication did not go into effect until January 1, 1992, the operative date set 
forth in Senate Bill No. 1119. 
13 
shall be based upon grams of alcohol per 100 milliliters of blood or grams of 
alcohol per 210 liters of breath.”  (§ 23610(b); see Bransford, at p. 890, quoting 
§ 23152(b).)  However, in Bransford this language was used in the definition of a 
criminal offense.  Here, it is part of a rebuttable presumption.  The question is 
whether a defendant who has a blood-alcohol concentration of 0.08 percent or 
more measured by breath is entitled to rebut the presumption that he was “under 
the influence” in a section 23152(a) case by introducing evidence that use of a 
2,100-to-1 partition ratio may have produced an inaccurate measure of his blood 
alcohol. 
 
The Legislature passed section 23152(b) to facilitate the prosecution of 
drunk drivers.  The creation of a per se DUI offense did away with the need to 
prove the defendant was actually impaired.  However, impairment must still be 
proven for a charge of generic DUI under section 23152(a).  In a generic DUI 
case, section 23610 creates a rebuttable presumption that the defendant was under 
the influence of alcohol if a chemical test measures his blood-alcohol 
concentration at 0.08 percent or higher.  As relevant here, section 23610 states: 
 
“(a) Upon the trial of any criminal action, or preliminary proceeding in a 
criminal action, arising out of acts alleged to have been committed by any person 
while driving a vehicle while under the influence of an alcoholic beverage in 
violation of subdivision (a) of Section 23152 or subdivision (a) of Section 23153, 
the amount of alcohol in the person‟s blood at the time of the test as shown by 
chemical analysis of that person‟s blood, breath, or urine shall give rise to the 
following presumptions affecting the burden of proof:  [¶] . . .[¶] 
 
“(3) If there was at that time 0.08 percent or more, by weight, of alcohol in 
the person‟s blood, it shall be presumed that the person was under the influence of 
an alcoholic beverage at the time of the alleged offense.[9] 
                                              
9  
Deleted subdivisions state that a blood-alcohol level between 0.05 and 0.08 
gives rise to no presumption, and a blood-alcohol level under 0.05 gives rise to a 
presumption that the driver was not under the influence.  (§ 23610(a)(1)-(2).) 
14 
 
“(b) Percent, by weight, of alcohol in the person‟s blood shall be based 
upon grams of alcohol per 100 milliliters of blood or grams of alcohol per 210 
liters of breath. 
 
“(c) This section shall not be construed as limiting the introduction of any 
other competent evidence bearing upon the question of whether the person 
ingested any alcoholic beverage or was under the influence of an alcoholic 
beverage at the time of the alleged offense.” 
 
Although section 23610 applies to chemical testing of the driver‟s blood, 
breath, or urine, the presumption of intoxication arises only if the testing shows 
the presence of 0.08 percent or more of alcohol in the driver‟s blood.  
(§ 23610(a)(3).)  The statute is not written to invoke a presumption based on a 
certain blood result or breath result; it is framed in terms of blood alcohol only.  
Thus, if the prosecution is relying on the result of a breath test, the breath result 
must be converted into a blood-alcohol equivalent for the section 23610 
presumption to apply.  Subdivision (b) sets this conversion factor at a ratio of 210 
liters of breath to 100 milliliters of blood. 
 
The logic of the presumption proceeds as follows.  The Legislature has 
concluded that most people with a blood-alcohol concentration at or above 
0.08 percent are too impaired to operate a vehicle safely.  Breath tests are a 
convenient way to measure a suspect‟s alcohol consumption, and they can be 
converted into an equivalent blood-alcohol level by applying a mathematical ratio.  
A conversion ratio of 2,100 to 1 is a safe standard to use because, for most people, 
it will produce an accurate or slightly low measure of alcohol in the blood.  (See, 
e.g., People v. Lepine, supra, 215 Cal.App.3d at p. 94; Downie, supra, 569 A.2d at 
p. 247; Brayman, supra, 751 P.2d at p. 300.)  Thus, if a driver‟s breath test shows 
a converted blood-alcohol level of 0.08 percent or more, the measurement is 
generally accurate and may fairly be used to prove the driver was actually under 
the influence, as the generic DUI statute requires.  Other evidence of actual 
impairment may include the driver‟s appearance, an odor of alcohol, slurred 
15 
speech, impaired motor skills, slowed or erratic mental processing, and impaired 
memory or judgment.  Conversely, absence of these manifestations may indicate 
that the driver was not impaired. 
 
With regard to the inference to be drawn from a breath result, the question 
of whether a defendant‟s converted blood-alcohol level is accurate depends upon 
the validity of the 2,100-to-1 partition ratio.  If the defendant‟s own partition ratio 
is the same as or higher than this standard ratio, his converted blood-alcohol result 
is valid or skewed in his favor.  If, however, the defendant‟s own partition ratio at 
the time of testing is lower than the standard ratio, conversion of the breath result 
using the statutory formula produces an artificially high measure of his blood 
alcohol. 
 
Evidence that a defendant has a comparatively low partition ratio would 
thus tend to show that a blood-alcohol concentration calculated with the standard 
2,100-to-1 ratio overstates the actual amount of alcohol in his blood.  For someone 
with an extremely low ratio of 1,100 to 1, for example, use of the 2,100-to-1 
partition ratio would overstate blood-alcohol content by almost 50 percent.  (See 
State v. Burling (1987) 224 Neb. 725 [400 N.W.2d 872, 876-877].)10  Evidence 
showing the defendant had a low partition ratio, and thus a lower concentration of 
blood alcohol than was reported, could also support an inference that he was not 
under the influence in violation of the generic DUI law.  In addition, evidence 
about the variability of partition ratios in the general population is relevant to raise 
a reasonable doubt about the accuracy of a defendant‟s converted blood-alcohol 
level.  Because partition ratios vary among individuals and the 2,100-to-1 ratio 
used by breath-testing machines is based on an average, there is a possibility that 
                                              
10  
Burling‟s holding that a breath test result must be reduced to 52.38 percent 
of the machine‟s reading (to reflect this potential margin of error) has been 
overruled by Nebraska‟s high court.  (State v. Baue (2000) 258 Neb. 968 [607 
N.W.2d 191, 200-201].) 
16 
the blood-alcohol level such a machine produces for a given defendant is 
inaccurate. 
 
The People contend the Legislature rendered partition ratio evidence 
irrelevant when it amended section 23610(b) to state that blood-alcohol 
percentages “shall be based upon grams of alcohol per 100 milliliters of blood or 
grams of alcohol per 210 liters of breath.”  According to the People‟s reasoning, 
because the Legislature has placed a partition ratio in section 23610, no evidence 
can be offered to contradict or question that ratio in a particular case.  Evidence 
that the defendant may have a different ratio is irrelevant, the People assert, 
because the Legislature has spoken and established 2,100 to 1 as the ratio to be 
used in every case.  Our reasoning was similar when we construed the partition 
ratio amendments to section 23152(b) in Bransford, supra, 8 Cal.4th at page 892, 
but the People‟s attempt to apply Bransford here overlooks an important 
difference in context.  As noted, in Bransford, we were construing language in a 
statute that defines an offense.  Section 23512(b) makes it a crime to drive with a 
blood-alcohol content of 0.08 percent or more as measured in grams of alcohol per 
100 milliliters of blood or per 210 liters of breath.  The crime itself is defined in 
terms of a prohibited blood or breath result.  Here, we are considering language in 
a statute that describes a rebuttable presumption.  Section 23610 permits, but does 
not require, the jury to infer that the defendant was under the influence if he had a 
blood-alcohol level of 0.08 percent or more.  (See People v. Milham (1984) 159 
Cal.App.3d 487, 505; see also CALJIC No. 12.61; CALCRIM No. 2110.)  
Although section 23610(b) incorporates the same conversion factor for breath test 
results as appears in section 23152(b), here this factor is not part of the definition 
of an offense and, like the rest of the presumption, it is rebuttable. 
 
Moreover, section 23610 specifically mandates that it “shall not be 
construed as limiting the introduction of any other competent evidence bearing 
upon the question of whether the person ingested any alcoholic beverage or was 
under the influence of an alcoholic beverage at the time of the alleged offense.”  
17 
(§ 23610, subd. (c).)  Thus, even in light of the presumption, a defendant charged 
under the generic 23152(a) provision is entitled to offer “other competent 
evidence” relevant to whether he was actually under the influence of alcohol.11 
 
Competent evidence that the 2,100-to-1 ratio does not accurately reflect the 
partition ratio for all people, or that the defendant‟s partition ratio may have been 
lower, is relevant to this question.  “ „Relevant evidence‟ means evidence . . . 
having any tendency in reason to prove or disprove any disputed fact that is of 
consequence to the determination of the action.”  (Evid. Code, § 210.)  The central 
disputed fact in a generic DUI prosecution is whether the defendant was under the 
influence of alcohol while driving.  The chemical test result is circumstantial 
evidence that supports an inference regarding that disputed fact.  Specifically, 
when a defendant‟s breath test result is equivalent to 0.08 percent or more of blood 
alcohol, section 23610 permits the jury to infer he was indeed under the influence 
of alcohol.  The defendant is entitled to challenge this inference and can do so by, 
among other things, raising a reasonable doubt as to whether the test result was an 
accurate measure of his blood-alcohol level.  Evidence casting doubt on the 
accuracy of the breath-to-blood conversion ratio is just as relevant as other 
evidence rebutting the presumption of intoxication from a breath test result, such 
                                              
11  
The People assert that by “other competent evidence” the statute is referring 
to evidence “other” than the partition ratio set forth in subdivision (b).  This overly 
narrow reading does not follow from the plain language of the statute.  Clearly, the 
import of subdivision (c) is to preserve a defendant‟s ability to rebut the 
presumption, not to limit the type of evidence that can be introduced.  Moreover, 
the People‟s interpretation of subdivision (c) makes little sense given that the 
subdivision existed in essentially its present form before the breath provision was 
added to subdivision (b).  (Former § 23126, subd. (c), added by Stats. 1969, 
ch. 231, § 1, p. 565 [“The foregoing provisions shall not be construed as limiting 
the introduction of other competent evidence bearing upon the question whether 
the person was under the influence of intoxicating liquor at the time of the alleged 
offense”].)  The reference to “other competent evidence” cannot have meant 
“other than the partition ratio” when no partition ratio was specified. 
18 
as evidence that the defendant had a high tolerance for alcohol or performed well 
in field sobriety testing. 
 
Accordingly, we hold that evidence about partition ratio variability is 
relevant in generic DUI cases to rebut the presumption of intoxication in 
section 23610.  Although the Court of Appeal distinguished between “general” 
and “personal” partition ratio evidence, concluding only the latter is potentially 
admissible, this distinction does not survive close scrutiny.  Both types of evidence 
challenge the accuracy of a defendant‟s reported blood-alcohol level, and therefore 
both can be used to support an inference that, despite a high breath test result, the 
defendant was not under the influence.  Although the inference is less direct when 
the evidence is not tied to the defendant, and concerns only variability of partition 
ratios in the population at large, indirectness alone is not a reason to exclude 
evidence that is logically relevant to a defense. 
 
We do agree with the Court of Appeal, however, that partition ratio 
evidence may not be used to negate the basic fact triggering the section 23610 
presumption, and thereby defeat operation of the presumption itself.  Because 
section 23610 expressly incorporates a 2,100-to-1 partition ratio, the defendant 
may not argue the presumption does not apply because a different ratio should 
have been used.  The result of the statutorily mandated test remains admissible, 
and the jury is still properly instructed on the presumption.  Defense evidence is 
relevant to rebut the presumption that the defendant was intoxicated, but not to 
remove the presumption altogether. 
 
Our decision is in accord with those of the two other state courts that have 
discussed the admissibility of partition ratio evidence in generic DUI cases.  In 
State v. Hanks (2001) 172 Vt. 93, 96 [772 A.2d 1087, 1089] (Hanks), the Vermont 
Supreme Court considered the effect of a statute that defined “alcohol 
concentration” for purposes of the state‟s DUI laws to mean grams of alcohol per 
100 milliliters of blood or per 210 liters of breath.  The defendant in Hanks 
conceded this statute may have made evidence about partition ratio variability 
19 
inadmissible in cases alleging a violation of the state‟s per se DUI law, but he 
argued such evidence was still relevant in cases alleging the defendant drove 
“under the influence of intoxicating liquor” in violation of the generic DUI law.  
(Id. at pp. 1088, 1091.)  The Vermont Supreme Court agreed.  (Id. at p. 1091.)  
The court observed that “any evidence raising a doubt as to the defendant‟s 
condition, which is the ultimate question in a generic DWI prosecution . . . is 
relevant and admissible.”  (Id. at p. 1092, italics added.)  Like California, Vermont 
has a statutory presumption that permits the jury to infer a person with a blood-
alcohol concentration of 0.08 percent or more was under the influence of alcohol 
at the time of the offense.  (23 Vt. Stat. Ann. § 1204, subd. (a)(2); see Hanks, at 
p. 1088.)  The Hanks court explained that evidence about partition ratio variability 
is relevant to rebut this inference because it could cast doubt on the accuracy of 
the breath test result as an indicator of the defendant‟s impairment.  (Hanks, at 
p. 1092.)  Finally, the court observed, where the “defendant is charged with 
driving while under the influence rather than driving with an alcohol concentration 
exceeding the statutory limit, admitting scientifically accepted evidence 
concerning the variability of partition ratios will not negate a statutory offense or 
even an element of a statutory offense; rather, it will merely allow defendant to 
challenge the permissive inference and the State‟s charge that he was impaired.”  
(Id. at pp. 1092-1093.) 
 
An Arizona appellate court considered the same question, in light of similar 
statutes, and concluded evidence about partition ratio variability “is not relevant to 
a prosecution for per se DUI, but may be relevant to a prosecution for traditional 
DUI if, in the latter instance, the State introduces evidence of the defendant‟s 
breath alcohol level to support conviction.”  (Guthrie v. Jones  (Ariz.Ct.App. 
2002) 202 Ariz. 273, 274 [43 P.3d 601, 602] (Guthrie).)  The court reasoned that 
when the prosecution relies on a breath test result to invoke Arizona‟s statutory 
presumption that the defendant was under the influence of alcohol (Ariz. Rev. 
Stat. § 28-1381, subd. (G)), partition ratio evidence casting doubt on that 
20 
presumption is relevant and admissible.  (Guthrie, at p. 604.)  The Guthrie court 
relied in part on statutory language similar to our section 23610, subdivision (c), 
concluding that the statute “does not limit the introduction of any other competent 
evidence bearing on the question of whether or not the defendant was under the 
influence of intoxicating liquor.”  (Ariz. Rev. Stat. § 28-1381, subd. (H); Guthrie, 
at p. 604.) 
 
We reach the same conclusion as the Vermont and Arizona courts.  If the 
defendant in a section 23152(a) case offers competent evidence showing that the 
use of a 2,100-to-1 conversion ratio may have yielded an inaccurate representation 
of his blood-alcohol level, introduction of this evidence is permissible.  In light of 
this decision, we need not consider whether a contrary interpretation of 
section 23610 would raise constitutional concerns.  (See Santa Clara County 
Local Transportation Authority v. Guardino (1995) 11 Cal.4th 220, 230 [the court 
will not decide constitutional questions where other grounds are available and 
dispositive]; see also Lyng v. Northwest Indian Cemetery Prot. Assn. (1988) 485 
U.S. 439, 445.) 
 
Naturally, the proffered evidence must still satisfy standards of competence 
and all other applicable evidentiary requirements.  We do not here consider how a 
party might establish partition ratio variability in general or in regard to an 
individual.  In particular, we express no opinion as to whether evidence of an 
individual‟s personal partition ratio has gained sufficient acceptance in the 
scientific community to be admissible.  (See People v. Kelly (1976) 17 Cal.3d 24, 
30-32.) 
 
In addition, like all other evidence, partition ratio evidence must be timely 
offered.  Here, defendant attempted to introduce this evidence only after both sides 
had rested.  His motion to reopen was one addressed to the court‟s sound 
discretion.  (People v. Jones (2003) 30 Cal.4th 1084, 1110; People v. Marshall 
(1996) 13 Cal.4th 799, 836.)  We need not discuss this procedural question further.  
Like the Court of Appeal, we may assume that defendant could have made an 
21 
adequate offer of proof, to be presented by a competent witness, and that the trial 
court would have allowed him to reopen.  Nevertheless, defendant is not entitled 
to a reversal here, because any error in excluding the partition ratio evidence was 
harmless. 
 
“As a general matter, the „[a]pplication of the ordinary rules of evidence . . . 
does not impermissibly infringe on a defendant‟s right to present a defense.‟  
[Citations.]”  (People v. Fudge (1994) 7 Cal.4th 1075, 1102-1103.)  Because the 
trial court merely rejected some evidence concerning a defense, and did not 
preclude defendant from presenting a defense, any error is one of state law and is 
properly reviewed under People v. Watson, supra, 46 Cal.2d at page 836.  (People 
v. Fudge, at p. 1103.)  Having examined all the evidence, we conclude it is not 
reasonably probable defendant would have achieved a more favorable result 
absent the exclusion of partition ratio evidence. 
 
The question for the jury was whether defendant was under the influence of 
alcohol when he drove through two red lights.  Defendant was stopped at 11:00 
p.m.  When asked to produce his driver‟s license, he initially could not find it.  
Asked why he ran the lights, defendant replied, at first, that he was chasing 
someone who had threatened him with a gun.  Later, he said he was looking for his 
wife, or that his wife was following right behind him.  The arresting officer saw no 
one else around.  Defendant‟s eyes were watery and bloodshot, his speech was 
slurred, and a strong odor of alcohol came from the interior of his car and from his 
person.  His face was flushed and he had difficulty answering questions.  When he 
got out of his car, he leaned on the vehicle for support as he made his way to the 
sidewalk.  Defendant said he had consumed one beer and had taken medication 
that morning for diabetes; however, no evidence offered at trial showed defendant 
was diabetic.  Defendant failed every one of the field sobriety tests he tried to 
perform.  A preliminary alcohol screening test given at the scene showed the 
presence of alcohol in his breath.  Based on defendant‟s driving, appearance, and 
22 
performance on the field sobriety tests, the arresting officer formed an opinion that 
defendant was impaired due to the influence of alcohol. 
 
In addition to this significant evidence of defendant‟s intoxication, the 
jury‟s verdict also indicates that the admission of partition ratio evidence was not 
reasonably likely to have produced a more favorable result.  (See People v. 
Watson, supra, 46 Cal.2d at p. 836.)  Although they found him guilty of generic 
DUI, the jury could not agree whether defendant was also guilty of per se DUI.  
They reported an unspecified split of “five to seven” on this issue.  Thus, even 
without partition ratio evidence, nearly half the jury rejected the accuracy of 
defendant‟s breath test result and declined to find that he had exceeded the legal 
limit of 0.08 percent blood alcohol.  Nevertheless, all jurors believed defendant 
was “under the influence” for purposes of section 23152(a).  It is difficult to 
imagine that the jurors who rejected defendant‟s breath result in considering the 
per se DUI charge would have relied on this result, and the section 23610 
presumption, to convict him of the generic DUI charge. 
 
DISPOSITION 
 
The judgment of the Court of Appeal is affirmed. 
 
 
 
 
 
 
 
CORRIGAN, J. 
 
WE CONCUR:   GEORGE, C. J. 
 
 
KENNARD, J. 
 
 
BAXTER, J. 
 
 
WERDEGAR, J. 
 
 
CHIN, J. 
 
 
MORENO, J. 
 
 
 
 
 
 
 
 
See next page for addresses and telephone numbers for counsel who argued in Supreme Court. 
 
Name of Opinion People v. McNeal 
__________________________________________________________________________________ 
 
Unpublished Opinion 
Original Appeal 
Original Proceeding 
Review Granted XXX 155 Cal.App.4th 582 
Rehearing Granted 
 
__________________________________________________________________________________ 
 
Opinion No. S157565 
Date Filed: July 9, 2009 
__________________________________________________________________________________ 
 
Court: Superior 
County: San Bernardino 
Judge: Michael A. Smith 
 
__________________________________________________________________________________ 
 
Attorneys for Appellant: 
 
Jamie L. Popper, under appointment by the Supreme Court, and Linn Davis, under appointment by the 
Court of Appeal, for Defendant and Appellant. 
 
Darryl W. Genis for California DUI Lawyers Association, David Stanford, Marta Gajewski, Lyle Clayton 
Turner and Ronald Audia as Amici Curiae on behalf of Defendant and Appellant. 
 
 
__________________________________________________________________________________ 
 
Attorneys for Respondent: 
 
Michael A. Ramos, District Attorney, Mark A. Vos, Lead Deputy District Attorney, and Astrid G. Alfonso, 
Deputy District Attorney, for Plaintiff and Respondent. 
 
Bonnie M. Dumanis, District Attorney (San Diego); Albert C. Locher, Assistant District Attorney 
(Sacramento); Margaret O‟Malley, Deputy District Attorney (Santa Barbara); and W. Scott Thorpe for 
California District Attorneys Association as Amicus Curiae on behalf of Plaintiff and Respondent. 
 
 
 
 
 
Counsel who argued in Supreme Court (not intended for publication with opinion): 
 
Jamie L. Popper 
555 West Beech Street, Suite 300 
San Diego, CA  92101 
(619) 696-0282 
 
Mark A. Vos 
Lead Deputy District Attorney 
412 Hospitality Lane, First Floor 
San Bernardino, CA  92415 
(909) 382-7629 
 
Margaret O‟Malley 
Deputy District Attorney 
1112 Santa Barbara Street 
Santa Barbara, CA  93101 
(805) 568-2300