How Can a Biochemist Expert Witness Help Interpret Scientific Evidence in Court?
A driver is told his blood alcohol level was over the limit. The sample was taken at two in the morning and tested nine days later, and the report says nothing about what happened in the meantime. Maybe nothing did. A forensic biochemist is the person who asks and can tell a court whether the number on the page can carry the weight being put on it.
Almost every case where chemistry meets the law has a version of that question. A result looks final, but it depends on how the sample was kept and how the test was run, and a number close to a legal limit can go either way. Courts do not need someone to repeat the result. They need someone who can say how much weight it will bear.
Why results are less certain than they look
Take a blood sample during a roadside arrest. If it was left in a tube without enough preservative, the bacteria and yeast would start to produce alcohol, and the cells would consume up the glucose, causing it to decrease. Both are absent from the report. A result that is just beyond the legal limit presents a new issue since there is always some degree of error in measurements, and when the margin is so small, the uncertainty may determine the outcome.
That question appears in nearly every instance when chemistry and the law collide. A result appears certain, but it relies on how the test was conducted and how the sample was maintained, and a figure around a legal limit can go either way. The outcome does not need to be repeated in court. Someone who can estimate the weight it will support is needed.
Screening tests have their own habits. Many react to more than one substance, which is why a positive has to be confirmed by a second, more specific method. And some substances turn up naturally. The body makes small amounts of GHB, so finding some does not show that anyone was given it. What matters is the level, the timing and how the sample was kept.
What a biochemist asks to see
The summary report is rarely enough. A biochemist will ask for the calibration records, the quality control results from the day, the chain of custody, and the raw output from the instrument. Then they set what the lab did against what its own method says it should have done.
Often there’s no dramatic error, but there’s a skipped check or a gap in the paperwork, and the question becomes whether it matters.
Where these cases turn up
Drink and drug driving prosecutions depend on narrow margins. In care proceedings, family courts hear hair and blood test evidence where a result can affect whether a child stays at home, and hair that’s been bleached or dyed can complicate the reading. Poisoning and drug-facilitated assault cases ask what was taken, how much and when. Clinical negligence claims sometimes hinge on a lab result that was misread or never followed up, and some product and injury claims involve exposure to a chemical.
What changed in 2026
The Forensic Science Regulator’s annual report for 2024 to 2025, published in July 2026, reported a compliance level of 79% for casework toxicology against its Code. That’s classed as high, though not the top band. All six organisations doing toxicology work in the period were commercial providers, with no police labs among them. For drug-driving analysis, the Regulator issued new guidance this year, and updated requirements for those tests took effect on 2 April 2026.
Standards are still moving, and a biochemist who follows them can see quickly where a lab fell behind.
Picking the right biochemistry expert witness
Biochemistry is a wide field. Clinical biochemists deal with hospital blood tests and metabolic conditions, toxicologists with drugs and alcohol, and molecular specialists with DNA and proteins, so the expert’s field should match the question. Time spent in a working lab counts for more than a long list of publications.
A few things are worth asking before you instruct:
Have you given evidence before, and under which rules? CrimPR Part 19, CPR Part 35 and FPR Part 25 apply in different courts.
What would you want to see before forming a view?
How do you explain measurement uncertainty to people without a science background?
Have you worked in, or audited an accredited laboratory?
If you are a biochemist thinking about expert work
Instead of the side giving you instructions, you answer to the court. The opposing side will enquire, so make notes about what you reviewed and the approach you took. You should prepare a response in your report for the question of why you trusted one number over another.
Solicitors can locate biochemistry expert witnesses whose history aligns with the evidence at the heart of the case by using Expert Witness Gateway, which lists expert witnesses by specialisation and experience.