The K6 was developed with the goal of being sensitive to the upper 90th
percentile range of the population distribution of mental distress (Kessler et al., 2002
). It is estimated that about 6% of the US population meets criteria for serious mental illness, defined as meeting DSM-IV diagnostic criteria for at least one psychiatric disorder in the past 12 months, that has resulted in serious impairment, not counting a substance use disorder (Kessler et al., 1996
). In this California sample, the prevalence of serious mental illness as detected with a K6 scale score ≥ 13 was 8.5%.
The current study examined the utility of the K6 scale for identifying mental distress at a moderate level that nevertheless impacts functioning and necessitates treatment. ROC curve analysis identified a K6 scale score ≥ 5 as optimal in identifying respondents with mental health treatment needs. The cut-point provided a balance between sensitivity (0.76) and specificity (0.75) with an overall classification accuracy of 0.74 and little variance by ethnic/racial group. Further, the AUC value of 0.82 was comparable to that reported for the K6 cut-point of ≥ 13 when predicting serious mental illness (AUC=0.865)(Kessler et al., 2003
Using the identified cut-point of 5 ≤ K6 <13, an additional 27.9% of respondents identified themselves as experiencing mental distress at a moderate level that impacted functioning across a number of impairment domains (work, household, social, family/friends, disability) and was associated with increased utilization of mental health treatment. Correlates of mental distress were similar at both serious and moderate levels with a greater likelihood among younger adults, women, those below the poverty level, and those who identified their race/ethnicity as other. These ethnic, gender, and socioeconomic patterns with mental distress parallel those reported nationally in the 1993–2001 Behavioral Risk Factor Surveillance System Surveys (Zahran et al., 2004
). The current study also found that mental distress was more prevalent among the less educated, those who were unemployed and looking for work, those who were not married, binge drinkers, current and former smokers, those who were not regularly physically active, and those who were obese.
Prior research has demonstrated that individuals with mental illness are at greater risk for engagement in multiple risk behaviors such as smoking, alcohol misuse, sedentary behavior, and obesity (Lasser et al., 2000
; Miles et al., 2003
; Sanchez-Villegas et al., 2008
). In the current study, adults with moderate mental distress had profiles comparable to adults with serious mental distress in increased risk for substance use and additional risk behaviors. For current tobacco use, sedentary behavior, and obesity, a linear relationship was demonstrated with adults with serious mental distress being at the highest risk and adults with no or low mental distress being at the lowest risk.
The K6 scale has demonstrated utility for providing aggregate estimates of serious mental illness prevalence and correlates (Kessler et al., 2010
). To date, it has largely been used by the nation and states to identify the most at-risk individuals to demonstrate the need for community intervention. The K6 also has been suggested as a useful screening scale in health risk appraisal surveys and primary care screening batteries (Kessler et al., 2002
). The findings from the current study demonstrate the utility of examining a fuller range of K6 scale scores to identify populations experiencing mental distress at a moderate or sub-threshold level that still may warrant clinical attention and health policy interventions.
Strengths of the current study include analysis of data from a large, population-based survey and examination of the sub-threshold cut-point across major ethnic/racial groups. Consistent with prior investigations demonstrating little bias in the K6 scale with regard to gender and education (Baillie, 2005
), the current study found high consistency by race/ethnicity in selection of the optimal cut-point on the K6 for moderate mental illness. The study data were limited to the state of California, and it is unknown how findings may generalize. A limitation of the data collection was that several mental health utilization and impairment items were not asked of respondents scoring K6 ≤ 5. The decision rule was intended to capture 15% to 20% of the California adult population with the highest level of mental health symptoms based on examination of past-30-day K6 data from the 2005 CHIS. The current findings suggest that future CHIS surveys should assess mental health impairments and treatment utilization with individuals who score as low as 5 on the K6 scale.
A limitation of the data collection was that several mental health utilization and impairment items were not asked of respondents scoring K6 ≤ 5. The decision rule was intended to capture 15% to 20% of the California adult population with the highest level of mental health symptoms based on examination of past-30-day K6 data from the 2005 CHIS. The current findings suggest that future CHIS surveys should assess mental health impairments and treatment utilization with individuals who score as low as 5 on the K6 scale.
This study concurrently determined a sub-threshold cutoff criteria on the K6 scale indicative of moderate mental distress and assessed the validity of this criterion by comparing participants meeting the sub-threshold cutoff to serious mental distress participants meeting the traditional K6 cutoff on mental health care utilization, mental health impairment, substance use as well as additional risk behaviors. The choice of criterion for establishment of the sub-threshold cut-point was based on the inference that individuals in treatment or perceiving the need for treatment are experiencing clinically relevant mental health symptoms. Future studies should examine the new cut-point in relation to other established mental health symptomatology or diagnostic measures.
The current findings, including the consistency in the optimal cut-point results from the ROC curve analysis across diverse ethnic/racial groups, 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. Further, the elevated risks for tobacco and heavy alcohol use, sedentary behavior, and obesity among respondents with identified serious and moderate mental distress underscore the need for comprehensive interventions that address the multiple risks with which these groups present.