F00 vs F05
F05 — Delirium
Distinguish a chronic, stable dementia from an acute, fluctuating delirium — the single most important and most frequently missed boundary in this block, since delirium can be superimposed on dementia.
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Show overlap — what both share
Both can present with disorientation, memory difficulty, and behavioural change, and both are common in older adults — delirium can also be superimposed on an existing dementia, which is the hardest version of this boundary to judge.
Show decisive evidence
- F00 has a gradual onset over months to years; F05 has an acute onset over hours to days.
- In F00 consciousness and attention remain clear; in F05 consciousness and attention are disturbed and characteristically fluctuate over the course of a day.
- F00's course is stable or slowly progressive day to day; F05 characteristically fluctuates within the same day, often worse in the evening or at night.
- F05 usually has an identifiable acute physical trigger (infection, metabolic disturbance, medication, withdrawal); F00 does not require one.
What weakens F00: A sudden change in level of alertness or attention, fluctuating over hours, especially with a plausible acute medical trigger, weakens a straightforward F00 classification and should prompt urgent consideration of delirium — including delirium superimposed on an existing dementia.
Show duration / course
F00: gradual, months to years, continuously progressive. F05: acute, hours to days, fluctuating markedly within a single day.
Show exclusions / context
A known dementia diagnosis does not exclude a new, superimposed delirium; any new acute fluctuation should be actively assessed rather than attributed automatically to 'worsening dementia'.
Show information still needed
Whether the change is acute or gradual, whether consciousness/attention fluctuates within a day, and whether an acute physical trigger is present.
Show the common trap
Assuming an acutely confused older adult, especially one with known memory problems, is simply experiencing a normal part of ageing or a worsening of their dementia, without checking for acuity, fluctuation, and a treatable underlying cause.
Mini challenge — practise this distinction
Apply the discriminating evidence above to a fresh case before checking the full feedback.