Diagnosis
In terms of differential diagnosis it is important to differentiate the avoidant personality disorder from Schizoid Personality Disorder, although that differential may be difficult. Essentially schizoids claim not to want any relationships and avoidant people claim to want them but to be frightened of them. Another important differential is between avoidant personality disorder and phobias of all kinds, because people who are agoraphobic, people who have simple phobias, people with social phobias, will have the same avoidant mechanisms. However, often these mechanisms are quite isolated. These are people who don’t need the same kind of uncritical acceptance. These are not people who are as sensitive to perceived criticism as people with APD.

ICD-10 Criteria (Europe)
The ICD-10 Classification of Mental and Behavioural Disorders World Health Organization, Geneva, 1992 , describes F60.6 Anxious (Avoidant) Personality Disorder as:
A personality disorder characterized by at least 3 of the following:

1. persistent and pervasive feelings of tension and apprehension
2. belief that one is socially inept, personally unappealing, or inferior to others
3. excessive preoccupation with being criticized or rejected in social situations
4. unwillingness to become involved with people unless certain of being liked
5. restrictions in lifestyle because of need to have physical security
6. avoidance of social or occupational activities that involve significant interpersonal contact because of fear of criticism, disapproval, or rejection
 
Associated features may include hypersensitivity to rejection and criticism

DSM Criteria (United states)
The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), describes APD as:
A pervasive pattern of social inhibition, feelings of inadequacy & hypersensitivity to negative evaluation, beginning by early adulthood & present in a variety of contexts, as indicated by 4 (or more) of the following:

1. avoids occupational activities that involve significant interpersonal contact because of fears of criticism, disapproval or rejection
2. is unwilling to get involved with people unless certain of being liked
3. shows restraint within intimate relationships because of the fear of being shamed or ridiculed
4. is preoccupied with being criticized or rejected in social situations
5. is inhibited in new interpersonal situations because of feelings of inadequacy
6. views self as socially inept, personally unappealing or inferior to others
7. is unusually reluctant to take personal risks or to engage in any new activities because they may prove embarrassing