When Are Blood Tests Used in Drink Driving Cases?
Blood tests are used in drink driving cases in several circumstances under the Road Traffic Act 1988. The most common is where a medical practitioner advises that a medical reason prevents the suspect from providing a breath specimen. Blood may also be taken where the police station breath testing device is unavailable, unreliable or has produced readings the officer has reason to believe are unreliable.
A blood specimen may also be taken when the suspect exercises the statutory option under section 8(2) where the lower breath reading is 50 micrograms or below. In cases involving suspected drug driving, or where the suspect is a patient at a hospital, blood is the primary specimen type. The decision to take blood rather than urine is made by the police officer, not the suspect.
Blood samples must be taken by a registered medical practitioner or a registered healthcare professional. The procedure must comply with strict legal requirements, and any failure to follow the correct process can form the basis of a defence challenge.
The Blood Test Procedure
At the Police Station
When blood is to be taken at the police station, a forensic medical examiner or registered healthcare professional attends the custody suite. The suspect must give consent to the taking of blood. The healthcare professional must use an approved blood sampling kit, which contains two vials treated with preservative and anti-coagulant to prevent fermentation and clotting.
The blood is drawn and divided between the two vials. One vial is sent to a Home Office approved forensic laboratory for analysis. The suspect is entitled to the second vial for independent analysis. The healthcare professional must also check for any medical reasons that would make it inadvisable to take blood.
Chain of Custody
The chain of custody refers to the documented trail showing who handled the blood sample from the point of collection to the point of analysis. Every transfer of the sample must be recorded, including the names of individuals who handled it, the times and dates of transfer, and the storage conditions maintained throughout.
Any break in the chain of custody can compromise the integrity of the sample. If the prosecution cannot demonstrate an unbroken chain from collection to laboratory analysis, a defence solicitor can argue that the result is unreliable and should be excluded from evidence.
Your Right to a Sample
Under section 15(5) of the Road Traffic Offenders Act 1988, you are entitled to be provided with one of the two blood samples for your own independent analysis. This is a fundamental right. If the police fail to offer you your portion of the sample, the blood test evidence may be rendered inadmissible.
If you receive your sample, you can instruct an independent forensic laboratory to analyse it. A discrepancy between the prosecution laboratory result and your independent result can provide powerful evidence to challenge the prosecution case.
Challenging Blood Test Evidence
Blood test results can be challenged on multiple grounds. Storage conditions are critical because blood samples must be kept refrigerated to prevent fermentation, which can artificially increase the alcohol content. If the sample was stored at room temperature or there were delays in transporting it to the laboratory, the result may be unreliable.
Contamination is another ground for challenge. The use of alcohol-based swabs to clean the skin before drawing blood can contaminate the sample. The approved procedure requires a non-alcohol based swab. Additionally, if the blood sampling kit was damaged, expired or improperly sealed, the integrity of the sample is compromised.
Laboratory analysis methodology can also be scrutinised. Defence experts can examine whether the laboratory followed accredited procedures, whether the equipment was properly calibrated, and whether the analyst was appropriately qualified. Any deficiency in the analytical process may undermine the prosecution evidence.
Blood Alcohol Back Calculation
Back calculation is a forensic toxicology technique used to estimate what a person's blood alcohol concentration would have been at the time of driving, rather than at the later time when the sample was taken. This is significant because the offence under section 5 of the Road Traffic Act 1988 requires the prosecution to prove the defendant was over the limit at the time of driving, not at the time of testing.
The average rate of alcohol elimination from the blood is approximately 15 to 18 milligrams per 100 millilitres per hour, though this varies between individuals. A forensic toxicologist can use the known blood alcohol result, the time of the last drink, the time of driving and the time of the blood sample to calculate a range of possible blood alcohol levels at the time of driving.
Back calculation evidence can work in favour of the defence or the prosecution. If a significant period elapsed between driving and the blood test, the defendant's blood alcohol level at the time of driving may have been lower than the sample result, potentially below the legal limit. Conversely, if the defendant was still absorbing alcohol at the time of driving, the level may have been rising.
Blood Tests vs Breath Tests
Blood tests and breath tests measure different things using different methods. Breath tests measure the concentration of alcohol in exhaled breath using infrared spectroscopy. Blood tests directly measure the concentration of alcohol in the blood using gas chromatography, which is generally considered more accurate and less susceptible to interference from external factors.
The prescribed limits reflect this difference. The breath limit is 35 micrograms per 100 millilitres of breath, the blood limit is 80 milligrams per 100 millilitres of blood, and the urine limit is 107 milligrams per 100 millilitres of urine. A blood test result carries significant evidential weight, but it is not immune from challenge on procedural, storage or analytical grounds.