Which of the following methods is NOT used to determine blood alcohol content?

Field observation is not a method used to determine blood alcohol content (BAC) directly. Instead, it involves the officer's assessment of a person's behavior and physical coordination to identify potential signs of impairment. Techniques such as breath tests, blood tests, and urine tests are chemical analyses that provide a quantitative measurement of BAC.

Breath tests utilize a device to estimate the concentration of alcohol in the breath, which correlates with BAC levels. Blood tests involve taking a sample of blood to measure the exact amount of alcohol present. Urine tests also collect bodily fluids to assess alcohol levels, although they may be less accurate than blood tests due to the time lapse between drinking and testing.

Therefore, while field observation can help an officer make a judgment about impairment and can lead to the request for a chemical test, it does not provide a numerical BAC value and thus is not a method for determining blood alcohol content.