Although the PHQ-9 is widely used in primary care, little is known about its performance
in quantifying improvement. The original validation study of the PHQ-9 defined clinically
significant change as a post-treatment score of ≤9 combined with improvement of 50%,
but it is unclear how this relates to other theoretically informed methods of defining
successful outcome. We compared a range of definitions of clinically significant change
(original definition, asymptomatic criterion, reliable and clinically significant
change criteria a, b and c) in a clinical trial of a community-level depression intervention.
Randomised Control Trial of collaborative care for depression. Levels of agreement
were calculated between the standard definition, other definitions, and gold-standard
The standard definition showed good agreement (kappa>0.60) with the other definitions
and had moderate, though acceptable, agreement with the diagnostic interview (kappa=0.58).
The standard definition corresponded closely to reliable and clinically significant
change criterion c, the recommended method of quantifying improvement when clinical
and non-clinical distributions overlap.
The absence of follow-up data meant that an asymptomatic criterion rather than remission
or recovery criteria were used.
The close agreement between the standard definition and reliable and clinically significant
change criterion c provides some support for the standard definition of improvement.
However, it may be preferable to use a reliable change index rather than 50% improvement.
Remission status, based on the asymptomatic range and a lower PHQ-9 score, may provide
a useful additional category of clinical change.
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