467 U.S. 479 (1984)
In unrelated incidents in 1980 and 1981, California law enforcement officers stopped each of the respondents on suspicion of driving while under the influence of alcohol on state highways.1 Each respondent submitted to an Intoxilyzer breath test.2 The test produced two closely correlated measurements of blood-alcohol concentration substantially higher than 0.10 percent.3 Each respondent was charged with driving while intoxicated under the Vehicle Code provisions then in force.4
The Intoxilyzer requires officers to purge the device with clean air until a zero reading is obtained.5 Officers must have the subject blow alveolar air at constant pressure for several seconds to fill the chamber for infrared analysis.6 They conduct two such tests that must register within 0.02 of each other.7 Officers purge and recheck the chamber after each test.8 The machine is calibrated weekly and those calibration results and samples are made available to defendants.9 Although a separate device approved by the Department of Health can preserve breath samples in indium tubes for later laboratory analysis, California officers did not use any preservation method in these cases.10
Prior to trial in Municipal Court, each respondent moved to suppress the Intoxilyzer results on the ground that the officers had failed to preserve breath samples that could have been used to challenge the test readings.11 The motions were denied.12 Respondents Ward and Berry then submitted their cases on the police records and were convicted.13 Respondents Trombetta and Cox appealed the suppression rulings.14
The California Court of Appeal consolidated the cases. The court accepted that breath-sample preservation was technically feasible.15 It ordered new trials for Ward and Berry while barring admission of the Intoxilyzer results against Trombetta and Cox.16 After the California Supreme Court denied the State's petition for review, the State petitioned for certiorari.17 The United States Supreme Court granted certiorari.18
Whether the Due Process Clause of the Fourteenth Amendment requires law enforcement agencies to preserve breath samples of suspected drunken drivers in order for the results of breath-analysis tests to be admissible in criminal prosecutions?19
The Due Process Clause imposes a duty to preserve evidence only when the evidence possesses an exculpatory value that was apparent before destruction, and the defendant cannot obtain comparable evidence by other reasonably available means.20
No. California authorities did not destroy respondents' breath samples in bad faith. They followed their normal practice of not preserving the samples.21 The samples lacked apparent exculpatory value.22 The Intoxilyzer procedures, including dual tests that must correlate within 0.02 and weekly calibrations available to defendants, make inaccurate results unlikely.23 Respondents could challenge the test results through inspection of calibration data, evidence of external interference such as radio waves, or cross-examination of the operator without needing preserved samples.24
The Due Process Clause of the Fourteenth Amendment does not require that law enforcement agencies preserve breath samples to introduce the results of breath-analysis tests at trial.25
Related opinions on this issue
Justice O’Connor wrote separately to concur in the judgment. She stressed that rules concerning preservation of evidence are generally matters of state, not federal constitutional, law.26 O’Connor explained that the failure to preserve breath samples does not render a prosecution fundamentally unfair. Thus the tests cannot be rendered inadmissible as evidence against the accused.27
She added that the failure to employ alternative methods of testing is of no due process concern. Persons are presumed to know their rights under the law. The existence of tests not used does not affect the fundamental fairness of the convictions actually obtained.28