In a recent issue in the journal, Humpston, Bebbington, and Marwaha (2021) reported the results of the first large epidemiological study of bipolar disorder in England. The prevalence estimate of bipolar disorder was based on the Mood Disorders Questionnaire (MDQ), a self-administered screening scale for a lifetime history of mania/hypomania. Humpston and colleagues found that the prevalence of bipolar disorder in England was similar to the rates in other parts of the world, and they stated that most individuals with bipolar disorder in England did not receive treatment for the disorder in the past year. A better understanding of the statistics of screening and the performance of the MDQ would indicate that the conclusions drawn from the epidemiological study are not justified.
I review the principles and statistics of diagnostic screening and how screening is distinguished from case-finding. I then review the performance of the MDQ in the general population and calculate the positive predictive value of the MDQ in the study by Humpston and colleagues.
The developers of the MDQ reported a sensitivity of 28.1% and specificity of 97.2% in the general population based on a cutoff score of 7. Using this same cutoff, Humpston and colleagues reported the prevalence of “probable” bipolar disorder was 1.7%. Based on these data, the positive predictive value of the MDQ would be only 14.8%.
Humpson et al. did not compare the MDQ to a diagnostic interview in their study; thus, the performance of the MDQ was estimated from another general population study.
When studying a disorder with a relatively low prevalence, it is near impossible for a screening test to have sufficient positive predictive value to be used to validly compare the individuals who do and do not screen positive. Most of the individuals whom Humpston et al. considered to have “probable” bipolar disorder would not have received the diagnosis had they been directly interviewed. Thus, the results of the Humpston et al. study are not a valid indicator of the correlates of bipolar disorder in the general population of England.

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