Doctors get nearly a decade of higher education under their belts with the hopes of honing their expertise, to the point where diagnoses are obvious. But physicians are still humans, and humans are error-prone. Not all diagnoses are correct, and those that aren’t come with great risk and, too often, heavy cost.

You might think that physical ailments are easier to observe than mental disorders. Diagnosing a stab wound comes with visible proof; depression, not so much. But even physical conditions aren’t created equal, as both psychiatric disorders and infectious diseases can look, to the most highly trained of eyes, like something else entirely.

1. ADHD and OCD

Attention deficit hyperactivity disorder and obsessive compulsive disorder are, in many ways, two sides of the same coin. But misdiagnosing one for the other could end in disaster for kids, whose fragile, malleable brains are still developing. While both are attentional disorders, ADHD manifests extreme difficulty with concentration, and OCD shows intense concentration levels, often to a damaging degree.

A 2012 study found that ADHD patients are, indeed, more impulsive than people with OCD — even if neither group is able to inhibit their kneejerk responses. The danger comes with medication. ADHD sufferers are given Ritalin to calm their excitability. Giving the same drug, one that intensifies focus, to OCD patients would only make matters worse.

"It's more likely that a young student will be diagnosed with ADHD instead of OCD because teachers see so many people with attention problems and not many with OCD," said co-researcher Professor Reuven Dar. "If you don't look carefully enough, you could make a mistake."

2. Depression and Diabetes Distress

People with diabetes have good cause to feel major stress, what with the constant checking, medicating, and dieting. More and more, researchers are finding this stress looks a lot like depression. But treating an underlying case of depression may be unnecessary — not to mention dangerous if antidepressants are prescribed — as managing the symptoms of diabetes distress often “cures” the depression doctors once saw.

"Because depression is measured with scales that are symptom-based and not tied to cause, in many cases these symptoms may actually reflect the distress that people are having about their diabetes, and not a clinical diagnosis of depression," said Dr. Lawrence Fisher, lead author of a recent study presented at the American Diabetes Association 74th Scientific Sessions conference.

Clinicians should treat symptoms of stress as a manifestation of the patient’s diabetes, Fisher says; they “don’t have to be considered psychopathology.” If one goes, the other will probably follow.

3. Bipolar Disorder and Personality Disorders

For all the field’s advances, medical science’s best tool for diagnosing bipolar disorder is still a face-to-face interview with the patient. This, of course, is not ideal. Major disorders aren’t isolated incidents that patients can point to and say, This is why I’m bipolar. They’re extended patterns of behavior, broad in nature and long in duration.

Clinical psychologist Dr. Russ Federman says three diagnoses often take its place: unipolar depression, ADHD, and the group of personality disorders, with borderline and/or narcissistic personality traits arising most often. Together, each misdiagnosis offers a component of bipolar disorder that a physician would expect to find. Depression sufferers vary in their temperaments; ADHD involves quickly moving between mental states; borderline patients are prone to bursts of rage.

Federman says this ambiguity makes correct diagnosis all the more critical. “If the diagnostic conclusion of your mental health professional doesn’t ring true for you, if you do not get a thorough and detailed explanation as to why the bipolar diagnosis is likely,” he writes, “it is absolutely appropriate to pursue a second opinion.”

4. HIV and the Flu

The human immunodeficiency virus (HIV) can take up to three months to register on ordinary tests. Before then lies a risky period of uncertainty, as the common early symptoms of infection — fever, nausea, vomiting, weight loss, oral ulcers, and swollen lymph glands — are easily mistaken as signs of the flu.

A new study from the British Columbia Centre for Excellence in HIV/AIDS provides clinicians with a set of guidelines on how to tell the difference between the two and how to treat them differently, either with retroviral drugs or a simple prescription for water and rest. At any rate, the study authors say, the research isn’t mean to criticize physicians. Rather, they want to highlight the importance of early testing and preventative efforts.

"A simple blood test in conjunction with clinical evaluation,” said B.C. Centre Director Dr. Julio Montaner, “enables us to detect this disease early and provide life-saving treatment.”