This post, which mostly provides useful citations, was originally written as an answer to the following Quora question:
It really depends on what you mean by “objective”, but the answer is “probably not”.
Since we do not understand the underlying causes of ADHD — or any major psychiatric disorder — we diagnose them based on clusters of symptoms.
In the United States and several other countries, a large number of psychiatrists use a book called Diagnostic and Statistical Manual of Mental Disorders (DSM).
DSM (now in it’s 5th, revised edition, DSM-5) essentially uses a system of checklists to enable a clinician to assess if a person has a given disorder. This is a controversial book for various reasons, but for now it is what most psychiatrists use.
Instead of the complex philosophical question of ‘objectivity’, the usefulness of DSM can be assessed using statistical measures of ‘reliability’.
Given that one clinician uses DSM-5 to give the diagnosis of ADHD, how likely is another clinician to do so using the DSM-5? Measures of “test-retest reliability” capture this probability.
Here is a paper that explains the statistical measurement of reliability in some detail:
There are conflicting reports on the reliability of DSM-5, but here is one paper that reports statistical assessments:
“There were a total of 15 adult and eight child/adolescent diagnoses for which adequate sample sizes were obtained to report adequately precise estimates of the intraclass kappa. Overall, five diagnoses were in the very good range(kappa=0.60–0.79), nine in the good range(kappa=0.40–0.59), six in the questionable range (kappa = 0.20–0.39), and three in the unacceptable range (kappa values,0.20). Eight diagnoses had insufficient sample sizes to generate precise kappa estimates at any site.”
“Two were in the very good (kappa=0.60–0.79) range: autism spectrum disorder and ADHD.”
For more on the quantity reported here, kappa, see this paper:
The quantity kappa ranges from 0 to 1. Zero means that there was no agreement between raters (clinicians in this case), and 1 means there was perfect agreement.
As I said before, the DSM is controversial — and not just because of reliability issues. Here is a sampling of papers and popular articles on the general topic:
“However, the standards for evaluating κ-statistics have relaxed substantially over time. In the early days of systematic reliability research, Spitzer and Fleiss  suggested that in psychiatric research κ-values ≥0.90 are excellent; values between 0.70 and 0.90 are good, while values ≤0.70 are unacceptable. In 1977, Landis and Koch  proposed the frequently used thresholds: values ≥0.75 are excellent; values between 0.40 and 0.75 indicate fair to good reliability, and values ≤0.40 indicate poor reliability. More recently, Baer and Blais  suggested that κ-values >0.70 are excellent; values between 0.60 and 0.70 are good; values between 0.41 and 0.59 are questionable, and values ≤0.40 are poor. Considering these standards, the norms used in the DSM-5 field trial are unacceptably generous.”
“Today, 26 years later, did the DSM system succeed in improving the reliability of psychiatric diagnoses? Two answers exist. The DSM did improve the reliability of psychiatric diagnoses at the research level. If a researcher or a clinician can afford to spend 2 to 3 hours per patient using the DSM criteria and a structured interview or a rating scale, the reliability would improve.  For psychiatrists and clinicians, who live in a world without hours to spare, the reliability of psychiatric diagnoses is still poor. [2,3]”
“The fifth revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) was the most controversial in the manual’s history. This review selectively surveys some of the most important changes in DSM-5, including structural/organizational changes, modifications of diagnostic criteria, and newly introduced categories. It analyzes why these changes led to such heated controversies, which included objections to the revision’s process, its goals, and the content of altered criteria and new categories. The central focus is on disputes concerning the false positives problem of setting a valid boundary between disorder and normal variation. Finally, this review highlights key problems and issues that currently remain unresolved and need to be addressed in the future, including systematically identifying false positive weaknesses in criteria, distinguishing risk from disorder, including context in diagnostic criteria, clarifying how to handle fuzzy boundaries, and improving the guidelines for “other specified” diagnosis.”
“You will need to display fewer and fewer symptoms to get labeled with certain disorders, for example Attention Deficit Disorder and Generalized Anxiety Disorder. Children will have more and more mental disorder labels available to pin on them. These are clearly boons to the mental health industry but are they legitimate additions to the manual that mental health professionals use to diagnose their clients?”
“This is the saddest moment in my 45 year career of studying, practicing, and teaching psychiatry. The Board of Trustees of the American Psychiatric Association has given its final approval to a deeply flawed DSM 5 containing many changes that seem clearly unsafe and scientifically unsound. My best advice to clinicians, to the press, and to the general public – be skeptical and don’t follow DSM 5 blindly down a road likely to lead to massive over-diagnosis and harmful over-medication. Just ignore the ten changes that make no sense.”
“Among the flashpoints: Asperger’s disorder will be folded into autism spectrum disorder; grief will no longer exempt someone from a diagnosis of depression; irritable children who throw frequent temper tantrums can be diagnosed with disruptive mood dysregulation disorder. [Hypersex to Hoarding: 7 New Psychological Disorders]
“One prominent critic has been Allen Frances, a professor emeritus of psychiatry at Duke University who chaired the DSM-IV task force.
“Frances charges that through a combination of new disorders and lowered thresholds, the DSM-5 is expanding the boundaries of psychiatry to encompass many whom he describes as the “worried well.””