Rates of depression have been rising in the credit crunch
GPs have difficulty spotting depression among their patients, a review of research suggests.
The overview of studies involving more than 50,000 patients found substantial numbers were missed or wrongly identified as having depression.
In fact, depression was more commonly misdiagnosed than correctly spotted following an initial consultation.
The University of Leicester study, featured in the Lancet, suggests closer patient assessment is essential.
The researchers, who examined a total of 41 trials, found GPs were able to recognise only about half of people who had clinical depression.
For a typical GP trying to spot depression in an urban practice and seeing 100 cases over two days, there would be 20 true cases of depression.
The GP would correctly diagnose 10 people as depressed but miss about the same number with depression.
Of the remaining 80 non-depressed patients, the GP would be likely to over-diagnose 15 people, and correctly reassure the other 65.
In a rural setting, false-positive diagnoses of depression would outnumber correct diagnoses by three to one.
The researchers calculated that in a typical practice, where 78% of patients see their GP during a 12 month period, about 12% would have clinical depression, and about half would be picked up.
Of the remaining 66% of the population who are not depressed and consult their GP, up to 12% would be at risk of being misdiagnosed as depressed if GPs relied upon a single clinical assessment.
Not enough time
The researchers said GPs were better at picking up more severe depression.
They said the fact that most consultations lasted only for a short time might be to blame, as patients may be reluctant to discuss their problems fully.
The researchers said that if GPs evaluated people who might have depression over two appointments instead of one their diagnostic accuracy rate would rise to 90%.
Writing in the journal, the researchers said: "Our results should not be interpreted as a criticism of GPs for failing to diagnose depression but rather a call for better understanding of the problems that non-specialists face."
Professor Peter Tyrer, an expert in depression at Imperial College London, said: "If the diagnosis of depression cannot be agreed satisfactorily by the best minds in psychiatry, why should we expect the general practitioner to be a reliable assessor of the condition?"
Professor Andre Tylee is an expert in the treatment of mental health in primary care at the Institute of Psychiatry at King's College London.
He said: "My experience is that psychiatrists who have tried to work in everyday ten minute GP consultations find it very difficult to assess patients despite their training because of all the factors that militate against a diagnosis - not least because there is still often a stigma attached to a diagnosis of depression for many people.
"Recognition rates appear to have remained similar for a decade or two and perhaps it will only improve when the attitudes of employers, insurers and society improve."
Hard to spot
Dr Andrew McCulloch, chief executive of the Mental Health Foundation, said: "Sadly, these results are not surprising.
"GPs have too little time and sometimes too little training to always diagnose mental illness accurately, despite the fact that at least a third of their caseload will be mental-health related.
"A proper process of clinical diagnosis will usually need longer than the few minutes available for a GP consultation.
"We need to develop better primary mental health care by giving GPs more support and resources to help them in this vital role."
Paul Farmer, chief executive of mental health charity Mind, said: "While a high temperature is recognisable pretty quickly, mental wellbeing and mental distress are much harder to judge in a one-off meeting.
"Spending longer with a patient, or seeing them over a number of appointments, could help improve diagnosing common mental health problems."
Mr Farmer said depression could be particularly difficult to spot in men, who often masked their symptoms through anger.