Dr Fred Kavalier: Diagnosis can be the doctor's toughest task

Depression can be a serious and life-threatening illness. When doctors are faced with serious illness, the decision about how to treat it is usually simple.

Everyone agrees that the best way to treat bacterial meningitis, for example, is with powerful antibiotics. There may be some debate about which is the best antibiotic, or what is the right dose. But there is no debate about the best treatment.

The best way of treating depression is much less clear. One reason for this is that there is no definitive test to prove someone has depression. A doctor who is faced with a patient who seems to be depressed, first has to clarify the diagnosis. Not all sadness is depression. Bereavement, for example, can lead to a state of deep and prolonged grief, sadness and depression. But this is not necessarily an illness in need of treatment.

There are two internationally accepted diagnostic guidelines to help health professionals make a diagnosis of depression. The American Psychiatric Association's DSM (Diagnostic and Statistical Manual of Mental Disorders) and The World Health Organisation's ICD (International Classification of Diseases) give diagnostic criteria which are, in effect, definitions of depression. In the eyes of many psychiatrists, if you meet the DSM or ICD criteria for depression, you have depression. If you don't, you may be suffering, but it's not depression.

Once a diagnosis of depression is confirmed, the next step is to decide if any treatment is necessary. Many people with depression will get better with no specific treatment. Periods of depression can be part of the cycle of life. If the times of depression are short and not too severe, the best treatment may simply be a listening ear, compassionate understanding, and the passage of time.

But as sufferers of more serious depression will know, these simple measures may not be enough.

The other options fall into four main categories: lifestyle interventions, drug treatments, so-called "talking" therapies, and more aggressive treatments such as ECT (electro-convulsive therapy).

There is plenty of evidence to show that lifestyle interventions can help depression. Regular exercise, for example, improves mood and Nice (the National Institute for Health and Clinical Excellence), in its guideline on the treatment of depression, tells doctors to recommend 45-60 minutes of regular exercise three times a week as a first-line treatment for depression.

The next step up the treatment ladder is drug treatments and relatively simple psychological treatments. The most commonly used drugs for depression are the SSRIs (selective serotonin reuptake inhibitors), such as fluoxetine (Prozac), paroxetine (Seroxat) and citalopram (Cipramil).

A debate still rages over which are best. The GP's rule of thumb has to be that if it works, it's the right anti-depressant for that patient. But even this approach is complicated by the fact that depression may get better without treatment, and the placebo effect of all tablets is powerful.

Talking therapies – counselling, CBT (cognitive behavioural therapy, either face to face or with the help of a computer), psychotherapy, psychoanalysis – all have their supporters. The current favourite is CBT, with or without antidepressants. This appeals to the NHS and the Government because there is evidence that it works, and it may be the most cost effective form of treatment. Supporters of other forms of psychological treatments believe that CBT is a "quick fix" which does not get to the deep roots of the problem.

The various schools of psychotherapy and psychology will never agree on which form of talking therapy is best. Philosophical and practical differences between therapists have fuelled a turf war which has been actively fought since the time of Freud.

When all else fails, and depression is deep, intractable and life-threatening, ECT can sometimes help. But its popularity has waned and it is now reserved for a small number of severely depressed people.