Validity study of the K6 scale as a measure of moderate mental distress based on mental health treatment need and utilization

The widely-used Kessler K6 non-specific distress scale screens for severe mental illness defined as a K6 score ≥ 13, estimated to afflict about 6% of US adults. The K6, as currently used, fails to capture individuals struggling with more moderate mental distress that nonetheless warrants mental health intervention. The current study determined a cutoff criterion on the K6 scale indicative of moderate mental distress based on mental health treatment need and assessed the validity of this criterion by comparing participants with identified moderate and severe mental distress on relevant clinical, impairment, and risk behavior measures. Data were analyzed from 50,880 adult participants in the 2007 California Health Interview Survey. Receiver operating characteristic curve analysis identified K6 ≥ 5 as the optimal lower threshold cut-point indicative of moderate mental distress. Based on the K6, 8.6% of California adults had serious mental distress and another 27.9% had moderate mental distress. Correlates of moderate and serious mental distress were similar. Respondents with moderate mental distress had rates of mental health care utilization, impairment, substance use and other risks lower than respondents with serious mental distress and greater than respondents with none/low mental distress. The findings support expanded use and analysis of the K6 scale in quantifying and examining correlates of mental distress at a moderate, yet still clinically relevant, level.