"Let's see how things go."

So psychiatrists often say to one another after a patient has been diagnosed with the first disorder—not because the diagnosis is not correct, but because psychiatrists know that psychiatric diagnoses have a tendency to change over the years.

In fact, 47% of psychiatric patients are diagnosed with a different diagnose within 10 years of receiving their first diagnosis.

"Ultimately, we hope it can help improve treatment and ensure evidence-based follow-up. The more you know about the probable course of illness, the better the treatment is likely to be," says Associate Professor Anders Jørgensen.

This is the result of a new study mapping the diagnostic development of more than 180,000 psychiatric patients in Denmark.

One of the researchers behind the study is a clinical research associate professor at the Department of Clinical Medicine, Anders Jørgensen. He is not surprised by the results of the study.

"Mental disorders are dynamic. They change over the course of a life. Therefore, I am not surprised by the relatively great diagnostic development in these patients," says Jørgensen.

Typical development for the three most common diagnoses analyzed in the study

• Depression: 60% of those diagnosed with a single depressive episode are diagnosed with a new disorder within 10 years. 20% develop periodic depression, 10% a stress disorder and 6% a personality disorder.
• Addiction: 52% are diagnosed with a new disorder within 10 years. Eight percent develop a stress disorder, 5% a personality disorder and 5% schizophrenia. The category includes all addiction diagnoses such as alcohol, opioid, cannabis etc. Some forms of addiction are more uncertain than others.
• Stress reaction disorder: 36% are diagnosed with a new disorder within 10 years. Eight percent develop a single depressive episode, 7% a personality disorder and 6% periodic depression.

The study shows which development is probable and which is improbable for the 20 most common mental diagnoses. The most uncertain, i.e., the ones that are most likely to change, include the diagnoses acute psychosis, addiction and depression.

The most certain, i.e., the ones that are least likely to change, include the diagnoses of functional disabilities, which are long-term physical disabilities with no physical cause, eating disorders and sexual disorders such as reduced sexual interest or erectile dysfunction with no physical cause.

The study is useful from the moment a patient is diagnosed with his or her first disorder, as it enables doctors to look up the 10-year diagnostic development of other patients.

"Doctors wanting to plan the right course of treatment and be able to tell patients what they can expect need these figures. Ultimately, we hope it can help improve treatment and ensure evidence-based follow-up. The more you know about the probable course of illness, the better the treatment is likely to be," says Jørgensen.

The study is limited to patients treated in the psychiatric health care system. This means that the people who go to their GP and are referred to a psychologist are not included in the study.

"We only look at people who have been diagnosed in psychiatric hospitals and who typically experience more severe courses of illness than those who make an appointment with their GP," says Anders Jørgensen.

Depression is one of the most uncertain diagnoses

Among the three most common diagnoses analyzed in the study, patients diagnosed with a single episode of depression have the highest risk of being diagnosed with a new disorder within 10 years.

"According to the study, patients with this diagnosis have a 60% chance of being diagnosed with a new disorder within 10 years," says Associate Professor Terese Sara Høj Jørgensen from the Section of Social Medicine at the Department of Public Health.

But numbers can be deceiving, as the majority (20%) of those diagnosed with a single episode of depression are subsequently diagnosed with periodic depression, which is the name for recurring depressions.

"It is not surprising that a single episode of depression can develop into recurring depression," says Anders Jørgensen.

Next to periodic depression, personality disorder and stress reaction disorders are the diagnoses most likely to follow a depression diagnosis. A stress reaction disorder is when a major incident such as divorce or death causes the patient to develop a mental disorder resembling stress or depression.

Anders Jørgensen hopes the new data can help improve treatment for people who suffer a depression.

"Unlike patients who experience their first psychosis, we currently have no uniform treatment option for patients who experience their first depression. We may look into developing such an option, and our figures can support the development of effective treatment," says Anders Jørgensen.

Using Danish register data, the researchers identified psychiatric patients aged 18 years or more diagnosed with one of the 20 most common mental disorders. This gave them a group of 184,949 individuals.

The researchers looked at how the patients' diagnoses have change since the first diagnosis was given. They used so-called sequence analysis to analyze the development.

The study, "Mapping diagnostic trajectories from the first hospital diagnosis of a psychiatric disorder: a Danish nationwide cohort study using sequence analysis," is published in The Lancet Psychiatry.