How is blood alcohol concentration measured?

Alcohol is the most commonly measured substance in forensic toxicology, above all in driving cases. Blood alcohol concentration (BAC) can be measured two ways, with very different roles.

Breath testing

At the roadside and in the station, alcohol is measured in breath. Modern instruments use infrared absorption or fuel-cell sensors to estimate BAC from a breath sample, relying on the known relationship between alcohol in the breath and in the blood. Breath testing is fast and non-invasive, which is why it's the front line.

Blood testing

For a definitive result, a blood sample is analysed in the laboratory by headspace gas chromatography (typically GC-FID). The blood is warmed, and the alcohol vapour above it is separated and measured with high accuracy. This is the gold-standard method and the one usually relied on in court.

Why timing and interpretation matter

BAC is a moving target. Alcohol is absorbed, peaks, then eliminated over time, so the level at the moment of testing differs from the level at the time of the offence. Analysts sometimes use retrograde extrapolation to estimate the earlier value, but it depends on assumptions about absorption and elimination rate. Individual variation, the breath-to-blood partition ratio, and sample handling all introduce uncertainty — which is why proper procedure and careful interpretation matter as much as the instrument.