Even though bipolar disorder is suggested to have a benign course with episodes and remissions, it has been shown that interepisode functioning of bipolar disorder is not so good. Subclinical residual mood symptoms are found to be in 24% of bipolar patients during remission. Furthermore, it appears that 30-60% of individuals with bipolar disorder fail to regain full functioning in occupational and social domains. In cognitive domain, euthymic bipolar patients demonstrate relatively marked impairment in aspects of executive function such as processing speed and verbal fluency, sustained attention, and verbal memory.

It is estimated that 30-50% of largely remitted patients fail to attain premorbid levels of psychosocial functioning due to their cognitive impairment. It is also suggested that the subsyndromal residual depressive symptoms have substantial effect on the persistence of cognitive and psychosocial impairment after remission. Thus, residual symptoms play an important role in the inter-episode functioning of the bipolar patients.

In the assessment of interepisode functioning, subjective QOL in bipolar patients may not accurately reflect objective functional outcome status. Especially, in the subjective assessment of cognitive functions, bipolar patients tend to over-estimate themselves. There is only weak to moderate correlation between subjective cognitive complains and objective cognitive tests. Therefore, it would be more accurate to assess the inter-episode functioning of the bipolar patients via objective tests and ratings.