Evaluating the hip-flask defence in subjects with alcohol on board: An experimental study

Highlights

• The UAC/BAC ratio was less sensitive to detect a recent intake in subjects with alcohol on board.

• Two consecutive urine samples provided good evidence for investigating the hip-flask defence.

• Small or moderate amounts of recently taken ethanol can go undetected.

Abstract

Driving under the influence of alcohol is a major problem for traffic-safety and a popular defence argument is alleged consumption after driving, commonly referred to as the hip-flask defence. Forensic toxicologists are often called as expert witnesses in drinking and driving cases where the suspect has claimed the hip-flask defence, to assess the credibility of the explanation. Several approaches to help the expert have been introduced but the scientific data used to support or challenge this is solely based on data from controlled single doses of ethanol administered during a short time and in abstinent subjects. In reality, we believe that even in drinking after driving cases, the subject many times has alcohol on board at time of the hip-flask drink. This questions the applicability of the data used as basis to investigate the hip-flask defence. To fill this knowledge gap, we aimed to investigate how blood and urine ethanol kinetics vary after an initial drinking session of beer and then a subsequent hip-flask drink of three different doses of whiskey. Fifteen subjects participated in the study and each provided 10 urine samples and 17 blood samples over 7 h. The initial drink was 0.51 g ethanol/kg and the second was either 0.25, 0.51, or 0.85 g/kg. Our data suggested that the difference between the ethanol concentrations in two consecutive urine samples is a more sensitive parameter than the ratio between urine and blood alcohol to detect a recent intake when ethanol from previous intakes are already present in the body. Twelve subjects presented results that fully supported a recent intake using the criteria developed from a single intake of ethanol. Three subjects showed unexpected results that did not fully support a recent intake.

We conclude that data from one blood sample and two urine samples provide good evidence for investigating the hip-flask defence even if alcohol was on board at the time of the hip-flask drink.

Introduction

Driving under the influence of alcohol (DUI) is a major problem for traffic-safety. Drunk drivers are over-represented in statistics of injuries and deaths on the road. Among alcohol-impaired drivers a popular defence argument is alleged consumption after driving, commonly referred to as the hip-flask defence [1], [2]. In such cases the analysis and interpretation of a person’s blood-alcohol concentration (BAC) and urine alcohol concentration (UAC) are important tasks for forensic toxicologists. Therefore, knowledge about the pharmacokinetics of alcohol is important.

There is a considerable variation in pharmacokinetics of ethanol both within and between subjects [3], [4], [5]. In one study, the variation in mean maximum ethanol concentration (Cmax) was 23% between subjects and 24% within subject when subjects were fasted. Corresponding figures for the mean area under the ethanol concentration-time curve (AUC) were 21 and 22%, respectively [5]. In addition, both the absorption and elimination are affected by a number of factors, e.g. type of beverage, nutritional state, age, sex, body weight and genetic factors [4]. The speed of absorption of ethanol from the gut, which has bearing on the peak concentration of alcohol, depends on e.g. the concentration of ethanol in the beverage and in particular the fed or fasting state of the individual [5], [6]. In the above mentioned study [5], where 0.3 g ethanol/kg was given the Cmax in the fasting state was significantly greater than after a meal, 39.9 mg/dL vs 21.3 mg/dL. The same applied to the mean AUC, 54.8 mg/dL h vs 33.6 mg/dL h. Ethanol is distributed into the total body water and hence differences in age, sex and bodyweight will affect the concentration-time profile of ethanol [7]. The enzymes mainly responsible for the metabolism of ethanol are under genetic control. The alcohol dehydrogenase and aldehyde dehydrogenase responsible for converting ethanol to acetaldehyde and acetaldehyde to acetate, respectively shows polymorphism, which contributes to differences in enzyme activity and ethnic differences have been reported. Another ethanol metabolising enzyme, microsomal CYP2E1 is inducible and increases the clearance of ethanol in heavy drinkers. Feeding increases the rate of ethanol metabolism by enhancing enzyme activity. Women have shown to display a higher clearance of ethanol per unit of lean body mass [4].

Forensic toxicologists are often called as expert witnesses in drinking and driving cases where the suspect has claimed the hip-flask defence, to assess the credibility of the explanation. Several approaches to help the expert have been introduced [8]. Two consecutive blood samples have been used as well as congener analysis and recently the determination of ethyl glucuronide and ethyl sulphate concentrations in blood [9].

There is a number of controlled drinking experiments where peak BAC and times of reaching the peak after drinking known amounts of alcohol have been studied. In many of them the highest BAC was reached within 30 min after drinking ended. On the other hand there are other studies where individuals reached the highest concentration after 60 min or even 90 min after drinking was ended. Those variations explain why two blood samples taken 30–60 min apart are not enough to assess whether there has been a new intake of alcohol based on a BAC increase or decrease between samples [8].

In Sweden the preferred method is to take one blood sample and two urine samples within 60 min and then relate the blood concentrations to the urine concentrations. Approximately 0.5–2.5% of the total amount of alcohol ingested is excreted unchanged with the urine. Urine is produced at a rate of about 1 mL per minute and the expected UAC/BAC ratio for primary or ureter urine is 1.25:1 as expected from differences in water content (100/80 = 1.25). The BAC might show a rapid rise during absorption, depending on the dose, the speed of drinking and gastric emptying. By contrast, the UAC changes much more slowly and thus the UAC/BAC ratio for the first void is a lot lower than 1:25:1 [10], [11]. After finishing drinking the concentrations of ethanol in urine starts to exceed the concentrations in blood. During the entire post-absorptive phase of ethanol kinetics the urine alcohol concentration is always higher than in blood. If the ratio between urine alcohol and blood alcohol reaches 1.25 or more this indicates that the person is in the post-absorptive phase and that drinking ended at least 1–2 h earlier. If the second urine sample has a higher ethanol concentration than the first sample and a ratio between the concentration in the first urine sample and the concomitant blood sample close to unity, it indicates a recent intake (i.e. less than 2 h ago), supporting the hip-flask defence [12].

In 2017, the National Board of Forensic Medicine in Sweden received 4091 DUI cases. Expert opinion reports on the hip-flask defence were written in 237 cases in 2017. Hence, the hip-flask defence is a common claim from people suspected of driving under influence of alcohol. The scientific data used to support or challenge this is solely based on data from controlled single doses of ethanol administered during a short time and in abstinent subjects. In reality, we believe that even in drinking after driving cases, the subject many times has alcohol in the body at time of the hip-flask drink. This questions the usefulness of the two major parameters used to investigate the hip-flask defence; the relationship between two urinary voids and the UAC/BAC ratio. Therefore we aimed to investigate how these parameters vary after an initial drinking session of beer and then a subsequent hip-flask drink of three different doses of whiskey.