A new blood test could be a first step towards personalised treatment for depression, say scientists.
The test accurately predicts whether or not patients will respond to common antidepressant drugs.
Around half of people with depression are not helped by "first-line" antidepressants and a third are resistant to all available drug treatments.
Until now there has been no way of knowing in advance which patients will need more aggressive treatment, which might involve a combination of different drugs.
Instead, most treatment for depression is conducted on a trial and error basis.
The new test measures two inflammation biomarkers in the blood shown in previous studies to be linked to a poor response to antidepressants.
Scientists found that patients with levels of both biomarkers above a certain threshold were 100% certain not to respond to conventional, commonly prescribed drugs.
Those with inflammation markers below the threshold could be expected to respond to first-line antidepressants.
Study author Professor Carmine Pariante from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King's College London, said: "The identification of biomarkers that predict treatment response is crucial in reducing the social and economic burden of depression, and improving quality of life of patients.
"This study provides a clinically suitable approach for personalising antidepressant therapy - patients who have blood inflammation above a certain threshold could be directed toward earlier access to more assertive antidepressant strategies, including the addition of other antidepressants or anti-inflammatory drugs."
The research is reported in The International Journal of Neuropsychopharmacology.
Brian Dow, from the charity Rethink Mental Illness, said: "This new approach to antidepressants has the potential to be a real game changer, and these findings are a promising start.
"Anti-depressants can be a lifeline if you have a mental illness like depression, OCD (obsessive compulsive disorder) or anxiety, but it's not much of a lifeline if it takes years of trial and error to get to the right kind.
"In the meantime, changing from one kind of medication to the next can mean having to deal with different side effects every time, not to mention withdrawal symptoms from coming off one to go on another.
"We hope this new research creates a much needed short cut to a future where it's no longer luck of the draw when it comes to vital medication."