The establishment of a population-specific reference value for the ruler drop test for the clinical assessment of reaction time

Abstract

BACKGROUND: Reaction time (RT) is the interval between a stimulus and an appropriate voluntary response in an individual. Alcohol is known to result in delayed RT. In Sri Lanka, an alleged drunken driver is legally subjected to a medico-legal examination to confirm or exclude impairment. The guideline for examining a drunk person in Sri Lanka includes the ruler drop test (RDT) as a test of RT. RDT is a simple test of visual reaction time in which the subject attempts to stop a falling ruler, and the height fallen is used to determine the time taken to react to the event. However, a formal study has yet to be carried out to establish population-specific reference values to interpret RDT results. METHODS: A cross-sectional descriptive study was conducted using 903 adults ≥18 years. A nonparametric approach was applied for deriving the reference values based on an inter-percentile interval. RESULTS: The study population consisted of 56.6% females, and the mean age of the participants was 41.6 years. Most (95%) of the study population could catch the ruler at or less than 40.0 cm of average height. The average height on RDT increased from younger to older age groups. However, subgrouping based on other variables, including sex, age, and alcohol consumption, did not show any statistically significant difference. CONCLUSIONS: The population-specific cut-off limit to identify alcohol intoxication by RDT in a Sri Lankan adult is 'average height' >40 cm.