This guideline makes recommendations on the identification, treatment and management of depression in adults aged 18 years and older who also have a chronic physical health problem (such as cancer, heart disease, diabetes, or a musculoskeletal, respiratory or neurological disorder).

Depression is a broad and heterogeneous diagnosis. Central to it is depressed mood and/or loss of pleasure in most activities. A chronic physical health problem can both cause and exacerbate depression: pain, functional impairment and disability associated with chronic physical health problems can greatly increase the risk of depression in people with physical illness, and depression can also exacerbate the pain and distress associated with physical illnesses and adversely affect outcomes, including shortening life expectancy. Furthermore, depression can be a risk factor in the development of a range of physical illnesses, such as cardiovascular disease. When a person has both depression and a chronic physical health problem, functional impairment is likely to be greater than if a person has depression or the physical health problem alone.

Depression is approximately 2 to 3 times more common in patients with a chronic physical health problem than in people who have good physical health and occurs in about 20% of people with a chronic physical health problem.

Severity of depression is determined by both the number and severity of symptoms, as well as the degree of functional impairment. A formal diagnosis using the ICD 10 classification system requires at least 4 out of 10 depressive symptoms, whereas the DSM IV system requires at least 5 out of 9 for a diagnosis of major depression (referred to in this guideline as 'depression'). Symptoms should be present for at least 2 weeks and each symptom should be present at sufficient severity for most of every day. Both diagnostic systems require at least 1 (DSM IV) or 2 (ICD 10) key symptoms (low mood [in both ICD 10 and DSM IV], loss of interest and pleasure [in both ICD 10 and DSM IV], or loss of energy [in ICD 10 only]) to be present.

Increasingly, it is recognised that depressive symptoms below the DSM IV and ICD 10 threshold criteria can be distressing and disabling if persistent. Therefore this guideline covers 'subthreshold depressive symptoms', which fall below the criteria for a diagnosis of major depression, and are defined as at least one key symptom of depression but with insufficient other symptoms and/or functional impairment to meet the criteria for full diagnosis. Symptoms are considered persistent if they continue despite active monitoring and/or low-intensity intervention, or have been present for a considerable time, typically several months. For a diagnosis of dysthymia, symptoms should be present for at least 2 years. Both DSM IV and ICD 10 have the category of dysthymia, which consists of depressive symptoms that are subthreshold for major depression but that persist (by definition for more than 2 years). There appears to be no empirical evidence that dysthymia is distinct from subthreshold depressive symptoms apart from duration of symptoms, and the term 'persistent subthreshold depressive symptoms' is preferred in this guideline.

The presence of a physical illness can complicate the assessment of depression and some symptoms, such as fatigue, are common to both mental and physical disorders.

It should be noted that classificatory systems are agreed conventions that seek to define different severities of depression in order to guide diagnosis and treatment, and their value is determined by how useful they are in practice. After careful review of the diagnostic criteria and the evidence, the Guideline Development Group decided to adopt DSM IV rather than ICD 10, which was used in the previous depression guideline (the NICE guideline on depression: management of depression in primary and secondary care). This is because DSM IV is used in nearly all the evidence reviewed and it provides definitions for atypical symptoms and seasonal depression. Its definition of severity also makes it less likely that a diagnosis of depression will be based solely on symptom counting. In practical terms, clinicians are not expected to switch to DSM IV but should be aware that the threshold for mild depression is higher than with ICD 10 (5 symptoms not 4) and that degree of functional impairment should be routinely assessed before making a diagnosis. Using DSM IV enables the guideline to better target the use of specific interventions, such as antidepressants, for more severe degrees of depression.

In addition to physical illness, a wide range of psychological and social factors, which are not captured well by current diagnostic systems, have a significant impact on the course of depression and the response to treatment. Therefore it is also important to consider both personal past history and family history of depression when undertaking a diagnostic assessment (see the appendix on assessing depression and its severity for further details).

Treating depression in people with a chronic physical health problem has the potential to increase their quality of life and life expectancy. Depression often has a remitting and relapsing course, and symptoms may persist between episodes. Where possible, the key goal of an intervention for depression should be complete relief of symptoms (remission) – this is associated with better functioning and a lower likelihood of relapse than lesser degrees of response, as well as potentially better physical health outcomes.

The guideline will assume that prescribers will use a drug's summary of product characteristics (SPC) and the BNF to inform decisions made with individual patients.