It is important to distinguish obsessive compulsive disorder (OCD) from other psychiatric disorders in which obsessive symptoms are present.

The important differentials are:

• primary depressive disorder:
  • depressed patients may present with obsessional symptoms
  • as many as 30% of severe depressives develop obsessional symptoms
  • diagnosis is complicated because the course of OCD is often punctuated with periods of depression
    
    
• schizophrenia:
  • patients may present with obsessional symptoms
  • schizophrenia should be suspected if the thoughts or rituals are particularly bizarre
    
    
• Tourette's syndrome:
  • the incidence of OCD in families with Tourette's syndrome is increased
    
    
• organic psychiatric syndromes:
  • these are rare, an example is encephalitis lethargica
    
    
    
    
    
    
• anankastic personality disorder