Differential diagnosis is the bread and butter of our task as clinicians. Most patients do not come to the office saying, “I have major depressive disorder . . . give me an antidepressant” (although some do!). More typically, the patient consults us seeking some relief from particular symptoms such as depressed mood and fatigue (the “chief complaints” in the parlance of medicine) that are the source of clinically significant distress or impairment. When we are confronted with these presenting symptoms, our job is to cull from all of the myriad conditions included in DSM-5-TR those that could possibly account for them (e.g., for depressed mood and fatigue, the possibilities include Major Depressive Disorder, Persistent Depressive Disorder, Bipolar I Disorder, Bipolar II Disorder, Schizoaffective Disorder, Depressive Disorder Due to Another Medical Condition, Substance/Medication-Induced Depressive Disorder, Adjustment Disorders). Once we have determined a list of candidates, our next job is to collect additional information—from personal history, other informants, treatment records, mental status examination, and laboratory investigations—that will allow a winnowing down of this differential diagnosis list to a single most likely contender, which becomes the initial diagnosis leading to an initial treatment plan. We must still keep an open mind, however, for the possibility that additional information that becomes available after the initial assessment is completed might justify a change in the diagnosis and possibly the treatment plan. For example, an initial diagnosis of recurrent Major Depressive Disorder might be changed to Bipolar I Disorder after a requested copy of the medical record for a past hospitalization reveals that what was reported by a patient as a past Major Depressive Episode was in fact a Manic Episode With Mixed Features.

    This handbook should improve your skill in formulating a comprehensive differential diagnosis by presenting the problem from a number of different perspectives. Chapter 1, “Differential Diagnosis Step by Step,” explores the differential diagnostic issues that must be considered in each and every patient being evaluated by providing a six-step diagnostic framework. In Chapter 2, “Differential Diagnosis by the Trees,” the differential diagnosis is approached from the bottom up—that is, from a point of origin that begins with the patient’s presenting symptom(s) such as depressed mood, delusions, and insomnia. Each of the 30 decision trees indicates which DSM-5-TR diagnoses must be considered in the differential diagnosis of that particular symptom, and offers decision points reflecting the thinking process involved in choosing from among the possible contenders. In Chapter 3, “Differential Diagnosis by the Tables,” the differential diagnosis is approached from a later point in the diagnostic assessment process—that is, after you have reached a tentative diagnosis and want to ensure that all reasonable alternatives have received adequate consideration. The chapter contains 67 differential diagnosis tables, one for each of the most important DSM-5-TR disorders. To facilitate the linkage between the decision trees in Chapter 2 and the differential diagnosis tables in Chapter 3, each of the disorders included in the terminal branches of the decision trees indicates the corresponding differential diagnosis table. Additionally, appendixes to this handbook include the DSM-5-TR Classification, which has been included to facilitate coding and to provide an overview of all the DSM-5-TR diagnoses that must be considered in formulating a differential diagnosis, as well as alphabetical indexes of the decision trees and differential diagnosis tables, which provide an alternate way to locate a particular decision tree or differential diagnostic table that may be of interest.

    The information provided in the decision trees and the differential diagnosis tables is somewhat overlapping, but each format has its own strengths and may be more or less useful, depending on the situation. The decision trees highlight the overall algorithmic rules that govern the classification of a particular symptom. Differential diagnosis tables are provided for most of the disorders in DSM-5-TR and indicate those disorders that share important features and thus should be considered and ruled out. The tables have the advantage of providing a head-to-head comparison of each disorder, highlighting both the points of similarity and the points of differentiation. Various readers will have different purposes for and different methods of using this handbook. Some individuals will be interested in a comprehensive overview of the process of making DSM-5-TR diagnoses and will find it rewarding to review the handbook cover to cover. Others will use the handbook more as a reference guide to assist in the differential diagnosis of a particular patient.

    The art and science of psychiatric diagnosis is both impeded and blessed by the fact that individuals are so much more complex than the diagnostic rules laid out in any set of decision trees or tables. On the one hand, clinicians must always guard the temptation to apply the DSM-5-TR criteria or the decision trees and differential diagnosis tables in this handbook in a rote or cookbook fashion. The approaches outlined here are meant to enhance and not to replace the central role of clinical judgment and the wisdom of accumulated experience. On the other hand, clinicians who are not aware of the guidelines for differential diagnosis included in DSM-5-TR may become idiosyncratic in their diagnostic habits, undermining one of the central functions of DSM-5-TR, which is to facilitate communication of diagnostic information among clinicians and between clinicians and their patients and family members. It is useful to know and take advantage of the precision afforded by following the DSM-5-TR rules but not to be enslaved by them.