P2.PS.2Characteristics of clinical depression (unipolar) and the ICD criteria (low mood, loss of interest, sleep changes etc.)

Clinical (unipolar) depression is more than feeling sad. It is a diagnosable mental disorder with persistent low mood and significant functional impairment. The ICD-11 (International Classification of Diseases, used by NHS) lists clear diagnostic criteria.

Two cardinal symptoms

For a depressive episode the person must show, most of the day, nearly every day, for at least two weeks, at least one of:

1. Persistently low or sad mood.
2. Anhedonia — markedly diminished interest or pleasure in activities previously enjoyed.

Additional symptoms

Alongside cardinal symptoms, the person typically shows several of:

• Significant change in appetite or weight (loss or gain).
• Sleep disturbance — insomnia or hypersomnia, often early-morning waking.
• Psychomotor changes — agitation (restlessness) or retardation (slowed movement and speech).
• Fatigue or loss of energy.
• Feelings of worthlessness or excessive guilt.
• Reduced concentration and indecisiveness.
• Recurrent thoughts of death, suicidal ideation, plans or attempts.

ICD-11 grades severity (mild / moderate / severe) by how many symptoms are present and how much they impair functioning.

Functional impairment

A crucial criterion: the symptoms must cause significant distress or impairment in social, occupational or educational functioning. Sadness alone — even severe — is not depression unless it disrupts daily life.

Distinguishing depression from sadness and grief

• Sadness is a transient mood; depression persists for weeks/months.
• Grief following bereavement shares symptoms but is normally bound to the loss, fluctuates with reminders, and improves with time. Diagnoses can be revised if grief becomes prolonged or develops the full depressive syndrome.
• Adjustment disorder describes lower-grade response to a specific stressor that does not meet full depression criteria.

Subtypes

• Mild depression — fewer additional symptoms; mild functional impairment; person continues most activities with effort.
• Moderate depression — clearer symptom load and impairment; usually requires treatment.
• Severe depression — many symptoms, marked impairment, possible psychotic features (delusions of guilt, hallucinations).
• Recurrent depressive disorder — repeated episodes separated by recovery periods.

Prevalence and risk

• About 1 in 6 adults in the UK experiences depression in a given week (NHS data; combined common-mental-health figure).
• Twice as common in women as men, in most populations.
• Strong age pattern — peak in late teens to mid-life.
• Suicide risk is markedly elevated; clinical follow-up is essential.

Why diagnostic criteria matter

• Standardise diagnosis across clinicians (reliability).
• Distinguish depression from normal sadness for treatment decisions.
• Allow comparable epidemiological data.
• Support evidence-based treatment matching (P2.PS.4).