Epidemiological studies have shown that total scores in depression screening scales change with age, but the mechanism underlying these age-related changes remains unclear. Previous research has indicated that item responses in depression screening scales exhibit characteristic distributions in the general population. We analyzed Patient Health Questionnaire-9 (PHQ-9) data from a representative survey conducted in the USA, to determine how the response pattern for each item changed with age and whether the pattern of responses contributed to age-related changes in total scores.
We analyzed PHQ-9 data for 17,274 participants in the 2011–2016 National Health and Nutrition Examination Survey. The PHQ-9 allows respondents to self-rate the frequency of depressive symptoms using a four-point scale ranging from “not at all” to “nearly every day”.
The lines for all nine item responses followed the same characteristic pattern across all age groups, which was marked by intersection at a single point between “not at all” and “several days” and parallel patterns between “several days” and “nearly every day” on a logarithmic scale. The probability of “nearly every day” showed a reverse U-shaped pattern, in that it was low from 12–29 years, increased during 30–50 years, and then decreased at ≥60 years. The age-related change in the probability of a response of “nearly every day” coincided with the trajectory of the PHQ-9 total scores.
This study demonstrated that item responses for the PHQ-9 followed a similar mathematical pattern across the adult lifespan. Moreover, our findings suggested that the probability of a response of “nearly every day” played an important role in age-related changes in PHQ-9 total scores across adulthood.