From Your Virtual Librarian: How Accurate is that Breath Test?

Breath test are based on assumptions. Assumptions that there is a ratio between the alcohol in the blood and the alcohol in the breath. These assumptions are based on Henry’s law. A law that states in a closed container, at a constant temperature and a constant pressure, you can calculate a ratio between the liquid and the gas. However, the body is not a closed container, and when the body is absorbing alcohol, breath can overestimate the blood by up to 230%. That’s a number that cannot be ignored.

While alcohol is being absorbed, a breath test will overestimate a blood test. This absorptive time can last up to 195 minutes with a range of 60-120 minutes in a fasting person. Long after a person has consumed alcohol, the breath test can improperly calculate the blood alcohol content. As Simpson pointed out in Accuracy and Precision of Breath Alcohol Measurements for Subjects in the Absorptive State, Clin. Chm. 33/6, 753-756 (1987), “Breath testing is not a reliable means of estimating a subject’s BAC during absorption.” You can find the article in the VFL here: Accuracy and Precision of Breath Alcohol Measurments for Subjects in the Absorptive State 

When a driver is stopped and then tested within two hours of drinking, the test is most likely an overestimation. If it’s likely to be from 30% to 230% high, that may bring your client below the limit. This is especially helpful if your client’s appearance is inconsistent with her Brac result. The next time this happens, you can point this out to the government’s “expert” to demonstrate your client was below the limit.