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Institute of Environmental Sciences and Research Limited, P O Box 30-547, Lower Hutt, New Zealand
Breath alcohol testing has been used for over 15 years by the New Zealand Police in order to enforce drink driving laws. The results presented in this paper were obtained in the course of normal road traffic law enforcement over the last 6 years using either Intoxilyzer R 5000 VA or Seres Ethylometre 679T evidential breath testing devices. The field stability of these devices has been established by monthly calibration checks for selected devices. At least 80% of all duplicate results obtained using either instrument agreed to within 10% of the lower result, and at least 88% agreed to within 15% of the lower result. A database containing about 5,000 breath and subsequent blood analysis results has been established. The relationship between the mean breath alcohol analysis result (BrAC), the subsequent blood alcohol analysis result (BAC) and the delay time between breath and blood sampling has been examined in detail for both devices. The distribution of the results of evidential breath tests provides an indication of the state of intoxication of drinking drivers detected by the New Zealand Police.
Blood alcohol analysis is used throughout the world in the enforcement of traffic safety laws. Breath alcohol analysis can provide an indirect measure of Blood Alcohol Concentration (BAC) but is such that the BAC determined will seldom be as reliable as the one determined directly. The main advantage of breath alcohol analysis is that it is non-invasive and it can be performed with ease and speed.
The current New Zealand (NZ) legal limits are 80 and 30 mg% BAC, 400 and 150 mg/L Breath Alcohol Concentration (BrAC).
The law also provides for 'conclusive evidential breath tests' which are conclusive in the sense that the driver has no right to demand a blood test after being informed of a positive breath alcohol test result. The criteria for these tests are that both duplicate test results are greater than 600 µg/L (with the higher being no more than 15% greater than the lower) and that the results be obtained using an Intoxilyzer ® 5000 VA of Seres Ethylometre 689T, within the one testing cycle.
In addition to these approved 'conclusive' devices the following devices have been used in New Zealand over the last 16 years: Draeger R/80 Alcotest, Intoximeter Alco-sensor II, Lion Alcolmeter S-L2 ANZ and the Alcotech AR1005.
The process of detection, screening and production of an evidential breath test result for a drinking driver can follow a number of events of which the following are typical:
If a driver refuses any step of the series of tests, he/she then proceeds automatically to the next (higher) step; a complete refusal results in an automatic drink driving conviction.
Over a period of six years the Intoxilyzer and Seres devices have been used to breath test approximately 150,000 suspected drinking drivers in New Zealand. The distribution of results for some randomly selected devices is given in Table 1.
Breath Test Distribution
Both devices have been programmed to display and record measured BrAC's in the ranges 400-439 µg/L and 150-169 µg/L as 400 and 150 µg/L respectively. Outside these "windows", actual analytical results are displayed and recorded.
We have examined the agreement between random duplicate breath test results (Tables 2-3).
Intoxilyzer ® 5000 VA
Seres Ethylometre 679T
This data indicates:
At BrAC's less than 400 µg/L, the standard deviation of the devices will become increasingly significant; typical standard deviations (at 400 µg/L) for Intoxilyzer and Seres devices are 3-8 and 2-5 µg/L respectively.
All devices used by the New Zealand Police (except Alcotest) are regularly serviced and recalibrated at ESR:Forensic. The field stability of the Intoxilyzer and Seres devices has been proven by in-field calibration checks or selected devices over a two year period and routine calibration checking of the devices as they are received for servicing at ESR:Forensic. In every instance the calibration has remained as set or has lowered by no more than 5%. These devices are serviced and calibrated at least annually. Alco-sensor II, Alcolmeter and AR1005 devices are all serviced and recalibrated at least 6 monthly. Generally, only minor adjustment of calibration is required.
Most suspected drinking drivers provide satisfactory breath test result(s) which have not been challenged in the majority of cases. However, since 1989 over 5,000 breath tested drivers have subsequently had blood tests. The results that are not affected by the 400 and 150 µg/L "windows" allow us to examine the correlation between BrAC and BAC results. BAC results have also been regressed against the mean of duplicate BrAC results and the delay time (DEL, in hours). A similar regression analysis has also been performed for the pairs where a single BrAC result was obtained. These trivariate regressions result in the following equations:
BAC = -3.55 + (0.2442 x BrAC) - (14.2 x DEL) [4868 Intoxilyzer duplicate]
BAC = -3.55 + (0.2442 x BrAC) - (14.2 x DEL) [205 Seres duplicate]
BAC = +3.36 + (0.2340 x BrAC) - (17.7 x DEL) [476 Intoxilyzer single]
BAC = +13.34 + (0.2271 x BrAC) - (13.1 x DEL) [71 Seres single)
BAC = +9.9 + (0.2140 x BrAC) - (12.9 x DEL) [71 Seres single]
These equations provide indications of the blood : breath partition coefficient (2140 to 2442) and also the mean blood alcohol clearance rates (12.9 to 17.7 mg% per hour). The blood : breath ratio is close to that determined by others under field conditions (Harding, 1990).
Almost all BrAC's over 400 µg/L correspond to estimated BAC values above 80 mg% at the time of breath testing. Ignoring the effect of delay time on BAC results, we examined the frequency of reporting a BAC result at or below the 80 mg% limit, while the lower of the corresponding breath test result(s) was over 400 µg/L. This category comprised about 1.5% of the total database. This reduced to 0.02%, a single result, when the reported BAC result was corrected for delay time, as determined above. Of the drivers affected by the 150 µg/L limit there was only one who would have been disadvantaged if he had not requested a blood test. All of the drivers whose BrAC result(s) exceeded 600 µg/L had reported BAC values greater than 111 mg%.
Our results show that despite the time delay between breath and blood sampling and the variations in the quality of the supplied breath sample(s), a New Zealand driver who fails a breath test and requests a blood test is more likely to be convicted on the basis of that blood test than to avoid prosecution. When the respective BAC and BrAC limits were set in New Zealand, it was considered necessary to allow for the inherently large natural variation in BrAC measurements (Simpson, 1987). Clearly, if large numbers of drivers pass a blood test after having previously failed a breath test, it would become common knowledge which would increase the requests for blood tests.
The results of our study show that there is little chance of an unfair conviction resulting from the acceptance of a breath test result under current New Zealand law enforcement practice.
Harding P M, Laessig R H, Field P H. Field performance of the Intoxilyzer 5000: a comparison of blood- and breath-alcohol results in Wisconsin drivers. J Forens Sci 1990 Sept; 35(5):1022-1028.
Jones A W. Physiological aspects of breath-alcohol measurements. Alcohol Drugs and Driving 1990 Apr-Jun; 6(2) 1-25.
Simpson G. Accuracy and precision of breath-alcohol measurements for a random subject in the postabsorptive state. Clin Chem 1987; 33(2):261-268.