How to diagnose depression and what are the criteria for major depression

Five or more of the following symptoms can happen during two week.

Period representing a change from normal functioning. 
At least one of the symptoms is either depressed mood or decreased interest or pleasure. Do not include symptoms due to medical conditions, delusion or hallucination. 

1. Depressed mood*.
2. Substantial weight loss or weight Gain.
3. Insomnia/lack of sleep or hypersomnia.
4. Feelings of worthlessness or inappropriate guilt.
5. Recurrent thoughts of death or suicide or suicide attempt.
6. Decreased interest or pleasure*.
7. Psychomotor retardation or agitation.
8. Fatigue or loss of energy
9. Diminished ability to think or concentrate

*From Diagnostic and Statistical Manual of Mental Disorders, fourth edition

*One of these symptoms must be present 

• Symptom should not meet the criteria for mixed episode.
• There  is  clinically significant distress in social, occupational, or other important areas of functioning.
• These symptoms are not due to the direct physiologic effects of a substance (e.g. drug of abuse, medication) or a general medical condition (e.g. hypothyroidism)
• The symptoms are not better accounted for by bereavement (i.e. after the loss of a loved one).

The episodes are diagnosed as mild, moderate and severe depending on the number of symptoms and intensity. Minimum duration of the whole episode is expected to be about two weeks.

Screening questions for depression

1. How have you been feeling recently?
2. Have you been low in spirits?
3. Have you been able to enjoy the things you usually enjoy?
4. Have you had your usual level of energy, or have you been feeling tired?
5. How has your sleep been?
6. Have you been able to concentrate on newspaper articles or your favourite television or radio programmers? 

Following are the difficulties for a non psychiatrist in diagnosing depression

1. Making sense of the emotional experience of the patient.
2. Pinning down the predominant mood state.
3. Assumptions /attributions to a physical or external factor.
4. Difficult questions to explore deeply.
5. Either normalising or interpreting any sadness as depression.