Diagnosis

It is hard for people who have not experienced clinical depression, either personally or by regular exposure to people suffering it, to understand its emotional impact and severity, interpreting it instead as being similar to "having the blues" or "feeling down." As the list of symptoms below indicates, clinical depression is a serious, potentially lethal systemic disorder characterized by the psychiatric profession as interlocking physical, affective, and cognitive symptoms that have consequences for function and survival well beyond sad or painful feelings.

DSM-IV-TR criteria

According to the[26]DSM-IV-TR criteria for diagnosing a major depressive disorder (cautionary statement) one of the following two elements must be present for a period of at least two weeks:
Depressed mood, or
Anhedonia

It is sufficient to have either of these symptoms in conjunction with five of a list of other symptoms over a two-week period. These include:
Feelings of overwhelming sadness and/or fear, or the seeming inability to feel emotion (emptiness).
A decrease in the amount of interest or pleasure in all, or almost all, daily activities.
Changing appetite and marked weight gain or loss.
Disturbed sleep patterns, such as insomnia, loss of REM sleep, or excessive sleep (hypersomnia).
Psychomotor agitation or retardation nearly every day.
Fatigue, mental or physical, also loss of energy.
Intense feelings of guilt, nervousness, helplessness, hopelessness, worthlessness, isolation/loneliness and/or anxiety.
Trouble concentrating, keeping focus or making decisions or a generalized slowing and obtunding of cognition, including memory.
Recurrent thoughts of death (not just fear of dying), desire to just "lie down and die" or "stop breathing", recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide.
Feeling and/or fear of being abandoned by those close to one.

Mnemonics commonly used to remember the DSM-IV criteria are SIGECAPS[27] (sleep, interest (anhedonia), guilt, energy, concentration, appetite, psychomotor, suicidality), DEAD SWAMP[28] (depressed mood, energy, anhedonia, death (thoughts of), sleep, worthlessness/guilt, appetite, mentation, psychomotor) and DIG SPACES (depressed mood, interest (lack of), guilt/worthlessness, suicidal ideation, psychomotor agitation/retardation, anorexia/weight loss, concentration difficulties, energy loss/fatigue, sleep disturbances).

Patient Health Questionnaire

The Patient Health Questionnaire (PHQ2) is a faster, two question questionnaire that may be as sensitive as the DSM-IV[29]: "During the past month, have you often been bothered by:"
"little interest or pleasure in doing things?"
"feeling down, depressed, or hopeless?"

If either question is positive, then the SALSA questionnaire should be used for more certainty[30]. A positive test is one of the above answers positive and two of the answers below positive:
Sleep disturbance nearly every day for the last 2 weeks?
Have you experienced little interest or pleasure in doing things nearly every day for the last 2 weeks (Anhedonia)?
Have you experienced Low Self esteem nearly every day for the last 2 weeks?
Have you experienced decreased Appetite nearly every day for the last 2 weeks?"

Beck Depression Inventory

One of the most widely used instruments for measuring depression severity is the Beck Depression Inventory, a 21-question multiple choice survey.

Schedules for Clinical Assessment in Neuropsychiatry

Another tool, created by WHO, that can be useful in diagnosing a variety of mental disorders, including depression, is the SCAN interview (Schedules for Clinical Assessment in Neuropsychiatry).

Other symptoms

Other symptoms often reported but not usually taken into account in diagnosis include:
Self-loathing.
A decrease in self-esteem.
Inattention to personal hygiene.
Sensitivity to noise.
Physical aches and pains, and the belief these may be signs of serious illness.
Fear of 'going mad'.
Change in perception of time.
Periods of sobbing.
Possible behavioral changes, such as aggression and/or irritability.

An additional indicator could be the excessive use of drugs or alcohol. Depressed adolescents are at particular risk of further destructive behaviours, such as eating disorders and self-harm.

A recent study in Journal of Nervous and Mental Disease showed that alternative symptoms of depression including diminished drive, hopelessness and helplessness, lack of reactivity, anger, psychic and somatic anxiety can be as effective as current DSM-IV criteria in diagnosis. According to this study, diminished drive has a higher diagnostic criteria than all others except for depressed mood with sensitivity of 88.2 of specificity of 69.9 [31].

Depression in children is not as obvious as it is in adults. Children may show symptoms such as:
Loss of appetite.
Irritability.
Sleep problems, such as recurrent nightmares.
Learning or memory problems where none existed before.
Significant behavioral changes; such as withdrawal, social isolation, and aggression.