If you act for clients facing a drink-driving allegation, you know that the number on the report is only the beginning. What matters is how the blood was taken, how it was stored, how the laboratory measured ethanol, how uncertainty was applied, and whether the result has been interpreted correctly against the legal limits. This guide sets out what happens from collection to courtroom, the realistic timelines, common myths, and when to request raw data for an independent review that stands up in court.
When is a blood test used in drink-driving cases?
Police ordinarily rely on breath alcohol concentration (BrAC) via a station Breathalyzer. A blood test follows when a breath test is not possible or appropriate, for example due to medical reasons, equipment issues, or no reliable evidential breath sample being obtained. Yes, police do use a blood test for drink driving, but the evidential route should be recorded and justified in the case file. This record matters during review and in court.
You can refuse a blood test for drink driving, but refusal without a reasonable excuse is an offence in itself. Medical grounds are assessed by a healthcare professional at the time. Legal advice early on is sensible.
What is an evidential blood test measuring?
The evidential test quantifies ethanol in whole blood, typically reported in milligrams of alcohol per 100 millilitres of blood (mg/100 mL). Laboratory analysis uses gas chromatography with flame ionisation detection (GC-FID), usually by headspace sampling, with internal standards and duplicate measurement. Properly run methods include:
Two independent measurements per sample, with acceptance criteria for agreement.
Separate calibration sets that bracket expected concentrations.
Use of preserved, anticoagulated collection tubes, usually sodium fluoride and potassium oxalate, to inhibit fermentation and clotting.
Laboratories apply an uncertainty of measurement, then report a final value with wording that aligns with the legal threshold. Interpretation must be explicit about whether uncertainty has moved the value below or above the prescribed limit.
Timelines: how long do results take?
Two timelines matter. First, police casework. From blood collection to a laboratory ethanol result, most cases complete in 2 to 6 weeks once the sample reaches the lab. Delays occur if the chain of custody is incomplete, the tube appears underfilled or clotted, the laboratory has a backlog, or a retest is needed because duplicates did not agree. Complexities such as suspected post-sampling fermentation or container faults can extend the timeline.
Collection, storage, and chain of custody
Reliability begins at the point of collection. Key points for defence review include:
– Correct kit, with intact preservatives and anticoagulants.
– Adequate fill volume to maintain the preservative to blood ratio.
– Thorough mixing to distribute preservative and anticoagulant.
– Proper sealing, labelling, and immediate documentation.
– Refrigerated storage and prompt transport to the laboratory.
Any deviation increases risk of fermentation or degradation. For example, underfilled tubes may reduce effective fluoride concentration. Warm storage increases microbial activity. Clear chain of custody should show where the sample was at each step.
Laboratory methods and common pitfalls
The core method is headspace GC-FID with dual columns or duplicate runs. An independent review often focuses on:
Calibration records. Were standards within date, traceable, and correctly bracketed?
Quality controls. Did low and high controls pass acceptance criteria on the day?
Duplicate agreement. Did duplicates meet method limits, or was a retest required?
Carryover checks. Was there a blank after a high sample to exclude contamination?
Internal standard integrity. Was the internal standard response stable?
Unit conversions. Any conversion to mg/100 mL must be correct and consistent.
Reporting language. Was uncertainty of measurement applied and stated, with the guard band defined?
Calibration drift, mislabelled vials, incorrect conversions between units, or overconfident wording around uncertainty are common sources of error that can alter the stated legal position.
Uncertainty, cut offs, and how reports should read
Every measurement carries uncertainty. Sound reports provide:
– The measured value.
– The expanded uncertainty at a stated confidence level.
– Clear interpretation versus the legal limit, with conservative wording.
Where a result is close to the limit, it is essential to see how uncertainty has been applied. Some reports present a lower bound, others provide a numeric range. Either approach must be transparent and defensible.
Fermentation risk and preservative issues
Endogenous ethanol can form after collection if microbes ferment residual sugars. Risk increases when:
– Tubes lack sodium fluoride, or it is insufficient due to underfilling.
– Samples are stored warm or for prolonged periods before analysis.
– There is visible haemolysis or clotting, suggesting anticoagulant failure.
– Headspace vials were not promptly sealed during preparation.
Independent toxicology review looks for signals of fermentation, such as discordant duplicates, presence of n‑propanol or other volatiles in chromatograms, and case circumstances that favour microbial growth. These indicators are not always conclusive on their own, so they must be considered alongside storage and handling records.
What to request for an independent review
If the outcome turns on the ethanol value, request:
– Full chromatograms for all injections, including blanks and controls.
– Raw data files, peak tables, and integration reports.
– Calibration curves, standards certificates, and daily QC results.
– Method validation and current method Standard Operating Procedure .
– Sample receipt, storage, and chain of custody records.
Instrument maintenance logs and any non‑conformance reports.
An experienced forensic toxicologist can then assess whether the number is robust, whether uncertainty and interpretation are appropriate, and whether alternative explanations like fermentation or contamination are plausible. Where breath testing is part of the case, an expert can also review breathalyzer and breath alcohol results for internal consistency and device performance.
When to involve an independent expert
Engage an expert when:
The result is near the legal limit or the wording around uncertainty is pivotal.
There are delays, storage concerns, or kit issues.
The client’s drinking history conflicts with the reported value.
Independent, unbiased review provides clarity, identifies procedural or interpretative errors, and supports court‑admissible outcomes. If needed, seek a formal toxicology report with expert witness testimony.
Bericon Forensics provides independent forensic toxicology, including review of blood test drink driving cases and breath alcohol data, with clear, court‑ready reporting.
