With current suggestions to revise the episode frequency criterion for bulimia nervosa from at least twice a week for 3 months down to at least once a week for 3 months, a similar threshold might be warranted in the definition of purging disorder. Binford and le Grange5 required purging at least once a week for a 6-month period (a duration similar to that recommended for binge-eating disorder in DSM-IV).
Results from their studies were generally consistent with those from studies using higher minimum frequency thresholds, which suggests that there may not be a clinically meaningful difference between a once-versus twice-weekly minimum frequency criterion for purging behavior. Because there is no official threshold, and data for evaluating the optimal frequency threshold are limited, it is important for clinicians and researchers who examine purging disorder to specify the frequency of purging behavior. In a clinical setting, this assessment is crucial for tracking progress in treatment. In a research setting, this information is crucial for communicating possible sources of differences (or the lack thereof) between patients described in one article and patients described in another article.
Absence of objectively large binge-eating episodes
The central difference between a diagnosis of bulimia nervosa and purging disorder is the presence versus the absence of objectively large binge-eating episodes—episodes that involve the consumption of a large amount of food and the loss of control over eating during the episode. However, some investigators have diagnosed purging disorder in those with binge-eating episodes at subthreshold frequency and purging episodes at threshold frequency, while others have excluded those with objectively large binge-eating episodes.1-3,5-7,12 It seems likely that patients with subthreshold binge-eating frequency and threshold purging frequency might fit on a continuum with those patients with threshold binge-eating and purge frequency, potentially reflecting a less severe symptom presentation.7 It does not seem that such individuals would reflect a new or distinct eating disorder different from bulimia nervosa.
In contrast, there is some evidence that defining purging disorder by the absence of objectively large binge-eating episodes identifies a syndrome associated with a distinct subjective and physiological response to food intake compared with bulimia nervosa.2 Specifically, women with purging disorder who have no objectively large binge-eating episodes report significantly greater postprandial fullness and demonstrate a significantly greater postprandial cholecystokinin response to food intake compared with women who have bulimia nervosa.2 Such biological differences may confer differences in response to pharmacological treatments. However, there are no current data to test this hypothesis.
Normal weight range
The central difference between a case of purging disorder and a case of anorexia nervosa with purging is the presence of low weight in anorexia nervosa and its absence in purging disorder. Low weight in combination with binge-purge behaviors in anorexia nervosa has been associated with several distinctive features compared with the presence of binge-purge behaviors among normal-weight persons, including differences in cross-cultural representation,13 longitudinal course and outcome,14 and mortality.15 Thus, it seems likely that low weight also would be associated with clinically meaningful differences between anorexia nervosa with purging and purging disorder. However, there have been no studies that have examined distinctions between anorexia nervosa and purging disorder to my knowledge. Instead, previous studies have used bulimia nervosa as the comparison group in examinations of purging disorder.9 Purging disorder may reside on a continuum with anorexia nervosa-binge eating/purging type with a degree of weight loss/weight below normal that reflects the severity of illness.
Lifetime history of other eating disorders
Some studies have restricted the diagnosis of purging disorder to those with no history of anorexia nervosa or bulimia nervosa, while others have only excluded a history of bulimia nervosa.1,6 Both cases represent attempts to define syndromes that would not be captured by current eating disorder definitions. This approach further separates purging disorder from a residual or partially remitted phase in bulimia nervosa.
Although a diagnosis of bulimia nervosa excludes individuals who concurrently meet criteria for anorexia nervosa, and a diagnosis of eating disorder excludes individuals who concurrently meet criteria for anorexia nervosa or bulimia nervosa, there is no precedent for using lifetime history of eating disorder diagnoses as exclusion criteria for a current eating disorder diagnosis. This has led to criticism of the potential proliferation of distinct eating disorder diagnoses when an individual transitions from meeting criteria for anorexia nervosa to meeting criteria for bulimia nervosa to meeting criteria for binge-eating disorder.16
Indeed, the longitudinal instability of symptom presentation has been one of the greatest criticisms of current eating disorder diagnoses. However, this potential seems to be overstated in many cases because prospective longitudinal studies suggest that individuals are much more likely to retain the diagnosis they received at presentation than they are to cross over to another eating disorder diagnosis.17 The highest reported rates of crossover often come from studies that rely on retrospective reports of past histories of other eating disorder diagnoses.18 Such methods introduce errors and bias because participants may agree with symptom thresholds in the absence of a specific recall of their lowest weight or frequency of binge-eating episodes.19 Thus, it is unclear whether past eating disorder diagnoses should be used as exclusion criteria in the evaluation of current eating disorder symptoms for purging disorder or any other eating disorder.