Can Fingertip Blood Give a Lower Alcohol Result Than Venous Blood?

Yes. In a small paired-sampling study, fingertip capillary blood generally contained less ethanol than venous blood collected one hour after drinking ended. The reported average venous-minus-capillary difference was 3.38 mg/100 mL with 100-microlitre analytical aliquots and 4.13 mg/100 mL with 10-microlitre aliquots. Those averages are study findings, not fixed corrections that can be added to any driver’s fingertip result.

Taylor and colleagues’ 2021 paper addresses two separate questions: whether very small blood volumes can be analyzed, and whether blood from two collection sites gives equivalent ethanol concentrations. Successful microsample analysis does not establish equivalence between sampling sites.

Thirteen volunteers provided repeated paired specimens

Thirteen healthy social drinkers provided 40 paired sampling occasions, with some returning on separate days. They consumed beer or premixed gin and tonic, finished within 40 minutes and underwent sampling one hour after drinking ceased. Food intake before the study was not standardized.

The researchers collected antecubital venous blood and fingertip blood from the same arm, reversing collection order partway through the series. Venous specimens were collected in fluoride/oxalate containers; capillary specimens used EDTA microcontainers. Analysis occurred within 24 hours.

The distinction between volunteers and observations matters: 40 pairs do not represent 40 independent people. Repeated specimens improve the amount of paired data but do not provide the population diversity of a larger independent participant sample.

Both measurements were whole-blood ethanol

The analytical method was dual-column, dual-detector headspace gas chromatography with flame-ionization detection. Each specimen was tested using conventional 100-microlitre and smaller 10-microlitre aliquots, with duplicate analyses.

These were capillary and venous whole-blood results. The comparison is different from interpreting hospital serum alcohol, where the tested specimen lacks the whole-blood cellular composition. Breath, urine and exposure biomarkers also require their own interpretation.

An aliquot is the quantity placed into the analytical preparation. It is not necessarily the total volume originally collected. The investigators collected considerably more blood than 10 microlitres to support their repeated analyses.

The sampling-site difference was statistically significant

The paper’s main summary table reports 39 usable pairs at 100 microlitres and 38 at 10 microlitres. The mean differences were 3.38 mg/100 mL and 4.13 mg/100 mL respectively, both with p < .001. The corresponding reported confidence intervals for the mean difference were 2.74–4.03 and 3.34–4.93 mg/100 mL.

These are intervals around an average difference, not prediction intervals identifying the difference expected for a new person. The observed variation and occasional reversed site ordering preclude treating the mean as an exact personal conversion.

The paper gives somewhat different summary numbers in portions of its Discussion and Conclusion. Its main table and abstract support the figures above. That internal inconsistency is another reason to avoid overstating the numerical precision of a universal correction.

The small-volume findings were not uniformly null

For venous blood, the population comparison between 100- and 10-microlitre aliquots was not significant (p = .209). For capillary blood, the corresponding comparison was significant (p = .001), with a reported mean difference of 1.21 mg/100 mL.

The authors emphasize the feasibility of microsampling and many nonsignificant individual specimen comparisons. Nevertheless, it would be inaccurate to say the two preparation volumes were indistinguishable in every analysis. A nonsignificant comparison also does not, by itself, demonstrate formal analytical equivalence.

Timing and collection conditions limit application

Only one time point after drinking cessation was studied. The experiment therefore did not directly map the full absorption and elimination curves for each sampling site. It cannot establish that capillary ethanol is always lower, especially during active absorption.

Other limitations include the small participant sample, repeated observations, different collection containers and preservatives, uncontrolled prior meals and beverage differences. Higher ethanol concentrations were inadequately represented; the authors specifically identify the absence of values above 80 mg/100 mL as a limitation.

Different collection order helps examine delay but does not make every aspect of capillary and venous collection identical. The study does not isolate one physiological mechanism as the sole explanation for the measured difference.

What to ask when reviewing a DUI blood result

Identify whether the sample was venous or capillary, whether it was whole blood or another fraction, the actual collection time, container, analytical aliquot and method validation. A report simply labeled “blood alcohol” can conceal distinctions relevant to comparison.

The paper supports investigating sampling-site differences. It does not authorize adding a fixed amount to a fingertip result, treating a retest from a different site as interchangeable, reconstructing an earlier BAC without additional evidence or inferring behavioral impairment from detection alone. Current collection authority and legal consequences require separate legal analysis; this laboratory study does not establish them.

Primary source

Taylor L, Remeškevičius V, Saskoy L, et al. Determination of ethanol in micro-volumes of blood by headspace gas chromatography: Statistical comparison between capillary and venous sampling sites. Medicine, Science and the Law. 2021;61(2):86–96. https://doi.org/10.1177/0025802420928632.