Guilt is a feature of Major clinical Depression. My doubt is, can guilt be considered delusional for making a diagnosis of mood-congruent psychotic depression? Can there be a delusional guilt? Because, wouldn't a person with depression if guilty, be always firm in his belief (which is characteristic of a delusion)?
According to DSM-5 (American Psychiatric Association, 2013, p. 819) a delusion is a "false belief based on incorrect inference about external reality that is firmly held despite what almost everyone else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary. ... When a false belief involves a value judgement it is regarded as a delusion only when the judgment is so extreme as to defy credibility."
Similarly, Østergaard, et al. (2012), citing Maj, et al. (2007), state: "To avoid false-positive diagnoses of PD [psychotic depression], only beliefs that have ‘delusional proportions’, i.e. defy credibility, and are held with ‘delusional intensity’, i.e. not changed by rational counterarguments, are classified as delusions." (emphasis added)
An intriguing possibility exists that a specific, measurable cognitive bias, the Jumping to Conclusions (JTC) bias, might serve as a marker for delusions (McLean, Mattiske, & Balzan, 2017). The JTC bias is characterized by making interpretations or judgments early (quickly) and basing such interpretations or judgments on inadequate evidence. Regarding measurement:
JTC is most frequently measured by the beads task. Applying this task with people with schizophrenia, Huq et al showed participants 2 jars of colored beads. Each jar contained pink and green beads in an 85:15 ratio, with one jar containing mostly pink beads and the other mostly green beads. The jars were hidden from view and beads were drawn from one of the jars in a purportedly random but in fact pre-determined sequence. With each draw, participants were invited to indicate whether they had decided which jar (mostly pink or mostly green) beads were being drawn from. Huq et al9 found that participants with current delusions required fewer draws-to-decision (DTD) than those without current delusions, demonstrating a JTC bias. (McLean, Mattiske, & Balzan, 2017, p. 345)
Further research is needed before we will know if measuring JTC improves diagnostic accuracy.
Stephens & Graham (20014) describe four criteria that define beliefs generally:
(1) beliefs have a representational content;
(2) the person holding the believe has a high degree of confidence that the content of his or her belief, e.g., "I am a horrible person", is an accurately represents reality;
(3) beliefs form the basis for both reasoning and action, such that individuals draw conclusions based on the belief ("because I am a horrible person I am doomed to Hell") and might take actions based on the belief (a Catholic person visits a priest asking, "How do I prepare for an eternity in Hell?"); and finally
(4) beliefs are associated with an emotional response, e.g., a belief that one is a horrible person may engender or exacerbate feelings of sadness, shame, guilt, despair, or hopelessness.
This four-component model of beliefs can serve as a heuristic to probe the "delusional proportionality" and "delusional intensity" of a patient's beliefs, e.g., by asking questions designed to assess the patients degree of confidence in the belief; the extent to which the patient has drawn conclusions and taken actions based on the belief; and emotions experienced when discussing the belief.
Like many symptoms of mental disorders, if a clinician can observe and interact with a patient over time, and if the doctor can interview family members or friends who know the patient well, then determining if a belief qualifies as a delusional becomes somewhat easier.
American Psychiatric Association, 2013. Diagnostic and statistical manual of mental disorders (DSM-5®). American Psychiatric Pub. (ISBN 9780890425558).
Maj, M., Pirozzi, R., Magliano, L., Fiorillo, A. and Bartoli, L., 2007. Phenomenology and prognostic significance of delusions in major depressive disorder: a 10-year prospective follow-up study. The Journal of clinical psychiatry, 68(9), pp.1411-1417.
McLean, B.F., Mattiske, J.K. and Balzan, R.P., 2017. Association of the jumping to conclusions and evidence integration biases with delusions in psychosis: a detailed meta-analysis. Schizophrenia bulletin, 43(2), pp.344-354.
Østergaard, S.D., Rothschild, A.J., Uggerby, P., Munk-Jørgensen, P., Bech, P. and Mors, O., 2012. Considerations on the ICD-11 classification of psychotic depression. Psychotherapy and psychosomatics, 81(3), pp.135-144.
Stephens, G.L. and Graham, G., 2004. Reconceiving delusion. International Review of Psychiatry, 16(3), pp.236-241.