(CNN) There's an alarming disparity in the treatment of depression, and experts are calling for a stronger effort to help those with the mood disorder.
Fewer than one-third of American adults who screen positive for depression are actually treated, according to a new paper published in the journal JAMA International Medicine on Monday. Meanwhile, most adults who do receive treatment may not have screened positive.
Major depression is one of the most common mental disorders in the United States, affecting about 14.8 million American adults in a given year.
"I was actually somewhat surprised. There has been a substantial increase in prescribing antidepressants over the last several years, and so I think it is tempting to assume that under-treatment of depression is no longer a big deal," said Dr. Mark Olfson, professor of psychiatry at Columbia University Medical Center and lead author of the paper.
"But our findings suggest that a great majority of people who screen positive for depression don't receive any treatment," he added. "There were some groups -- men, adults with less education, ethnic and racial minorities -- who are especially unlikely to receive treatment for their symptoms."
Olfson and his colleagues analyzed self-reported mental health data on 46,417 people. The data were collected in 2012 and 2013 as part of the Medical Expenditure Panel Surveys.
The researchers took a close look at which participants reported that they had been screened for depression and which reported that they had received treatment.
The researchers discovered that about 8.4% of all adults surveyed had screened positive for depression, but among them, only 28.7% received any depression treatment during the yearlong period of the survey.
Meanwhile, among the 8.1% of all adults surveyed who had been receiving some form of depression treatment, only 29.9% had screened positive for depression. The researchers did not receive any information concerning the treatment outcomes.
"At the same time, we find that, among those who are treated, those with more serious psychological distress are less likely to receive antidepressants than those with lesser distress," Olfson said. "So not only are there problems with depressed individuals not receiving any treatment, there are also problems with matching people who receive treatment to the right treatment."
Antidepressants were the most common treatment received by the study participants, followed by psychotherapy, anxiolytics, antipsychotics and mood stabilizers, according to the research findings.
"I think the paper highlights a significant concern that both patients and health care professionals have recognized for quite a while, in that many people are not receiving the care they need, and that is particularly true among minorities and those with lower income, lack of insurance and less education," said Jonathan Becker, assistant professor of clinical psychiatry and director of the neuromodulation service at Vanderbilt University Medical Center, who was not involved in the new paper.
"The methodology does a good job of getting an overview of the larger problem. It is limited in that it is not possible to draw specific conclusions as to what exactly leads to the lack of treatment or why the differences in treatment exist," he said of the paper.
Olfson said he has some ideas about why certain patients are not receiving treatment, and there are many reasons. Some might not realize that they could benefit from treatment, or they are facing stigma and shame, he said. Other people might have medical problems that are overshadowing their depression symptoms.
The disorder is often accompanied by feelings of sadness and fatigue, loss of interest in normal activities, sleep disturbances, anxiety, changes in appetite, difficulty with concentration and suicidal thoughts, among other symptoms.
What's needed to close this gap in depression treatment? Olfson said more screening tools and mental health services, such as psychologists, psychiatrists and other mental health professionals, should be used in primary care.
"Most adults, at some point during the course of the year, make a primary care visit," he said. "There are brief screening tools that exist for primary care and just take a couple minutes of the patient's time that can give a pretty good read on whether the patient has depressive symptoms at that time."
Some health care systems, such as the Group Health Cooperative in Seattle, have adopted this approach by offering mental health screenings and services to their patients, Olfson said.
"Large health care systems, like Group Health and Kaiser, have implemented systematic programs to screen for depression and, even more important, promote continued engagement in treatment and monitor treatment outcomes. Screening for depression is important, but it is certainly not enough," said Dr. Gregory Simon, a psychiatrist at Group Health and chairman of the Scientific Advisory Board of the Depression and Bipolar Support Alliance.
"While the proportion of people who receive depression treatment in the United States has increased significantly over the last 20 years, we still find that many people who would benefit from depression treatment are not receiving it. Or they are not receiving effective treatment: the right kind of treatment continued long enough to be effective," added Simon, who was not involved in the new paper. "We could say that depression treatment is a mile wide and growing wider but still only an inch deep."