Which of the following is NOT a method for measuring blood alcohol concentration?

The method described as a field assessment does not directly measure blood alcohol concentration (BAC). Rather, it often involves observational procedures or standardized tests conducted in the field to assess a person's level of impairment, which can be influenced by BAC but does not yield an actual blood alcohol measurement.

In contrast, breath tests and blood tests are established techniques for directly quantifying BAC. Breath tests, often conducted using devices like breathalyzers, estimate the alcohol in a person's breath, which is correlated with their blood alcohol level. Blood tests measure the precise concentration of alcohol present in the blood, providing an accurate determination of a person's BAC. The passive breath test serves as a preliminary method to ascertain whether a person has been drinking, detecting exhaled alcohol without requiring active participation from the individual.

Thus, the other options represent recognized methods for measuring blood alcohol concentration, while a field assessment is more about evaluating behavior and signs of impairment rather than measuring BAC directly.