F20 vs F22

Both can involve a firmly held false belief. Both can persist for a long period. Functioning may be relatively preserved in both, especially early on.

• F22 is defined by one delusion or a set of related delusions (often non-bizarre, e.g. persecution, jealousy, litigation) that dominate the picture, with little else going on around it.
• F20 involves a broader disturbance: characteristic hallucinations (especially third-person commentary voices), thought disorder, negative symptoms, or catatonic features are typically present alongside, or instead of, a delusional system.
• Hallucinations in F22, if present at all, are typically not the persistent, prominent, characteristically schizophrenic type described under F20's key features.
• Bizarre, culturally implausible delusional content (e.g. thought broadcast) fits F20 more readily than the often more plausible-sounding delusions of F22.

What weakens F20: A single, internally consistent, non-bizarre delusion (for example, a conviction of being watched by a specific neighbour) with normal affect, no hallucinations, and preserved functioning elsewhere weakens F20 and favours F22.

Both can be persistent; the distinguishing factor here is breadth of disturbance rather than duration.

A grief-related or culturally sanctioned belief should not automatically be treated as a persistent delusional disorder; context and impairment both matter.

Is there one delusional theme or several unrelated disturbances? Are there hallucinations of the characteristic schizophrenic type? Is functioning affected broadly or only in relation to the delusional theme?

Applying F20 to any fixed false belief, without checking whether the disturbance is actually broader than a single delusional theme.