Which of the following is a critical factor in interpreting HGN results accurately?

The critical factor in interpreting HGN results accurately is the environmental conditions during testing. Various environmental factors can significantly impact the results of the Horizontal Gaze Nystagmus test. For instance, factors such as lighting conditions, weather elements, and distractions in the surrounding area can influence the subject's ability to focus and the clarity with which the tester can observe nystagmus. Proper environmental conditions are essential for ensuring that the test results are reliable and valid.

While subject testimonials, age, and the time of day can provide some context for the testing situation, they do not have the same level of direct influence on the physiological response being measured by the HGN. Therefore, it's crucial to control and take into account environmental conditions to ensure accurate interpretation of HGN test results.