ICD-10 Code F05: Delirium due to known physiological condition

Key Takeaways

ICD-10 Code F05 is a billable, specific ICD-10-CM diagnosis code for delirium due to a known physiological condition, valid for FY2026 (effective October 1, 2025).

F05 covers eight included terms: acute confusional state, sundowning, delirium superimposed on dementia, infective psychosis, and more – but excludes delirium NOS (R41.0) and substance-withdrawal delirium.

Always sequence F05 as a secondary code after the underlying physiological cause; documentation must explicitly link the two conditions.

Practice management software like Pabau helps clinicians capture the physiological linkage, onset details, and DSM-5 criteria needed to support F05 coding at audit.

ICD-10 Code F05 describes delirium due to a known physiological condition, and it is one of the most frequently undercoded diagnoses in inpatient and post-acute settings. Delirium goes unrecognized in a majority of cases, and when it is documented, incorrect code selection leads to claim denials and audit risk.

Dementia codes like F02.80 share similar documentation traps, but F05 carries its own sequencing rules that catch coders off guard.

This reference covers ICD-10 Code F05’s official definition, every included term, Excludes1 restrictions, coding and sequencing rules, documentation requirements, and a related-code crosswalk – everything coders and clinicians need for accurate FY2026 billing.

ICD-10 Code F05: Definition and 2026 edition details

ICD-10 Code F05 describes Delirium due to known physiological condition. It sits within the F01-F09 block (“Mental disorders due to known physiological conditions”) of Chapter 5 in ICD-10-CM.

The code is billable and specific – it can be submitted directly for reimbursement without a more detailed subcode. Per the CMS ICD-10 codes page, the FY2026 edition of F05 became effective on October 1, 2025.

What ICD-10 Code F05 includes: approximate synonyms and included terms

F05 has a broad scope. The CMS tabular list includes eight official terms under this code, all representing delirium arising from a physiological cause. Coders who miss these synonyms may query whether a separate code exists when F05 already covers the presentation.

• Acute or subacute brain syndrome
• Acute or subacute confusional state
• Acute or subacute infective psychosis
• Acute or subacute organic reaction
• Acute or subacute psycho-organic syndrome
• Delirium of mixed etiology
• Delirium superimposed on dementia
• Sundowning

The “acute or subacute” qualifier in the first five terms matters clinically. Delirium can persist for days to weeks.

When the onset is gradual or the course is prolonged, it may be tempting to reach for a dementia or encephalopathy code – but F05 remains correct as long as an underlying physiological cause drives the presentation. The CDC/NCHS ICD-10-CM lookup tool confirms all eight terms under a single F05 entry for FY2026.

Excludes1 and Excludes2 notes for F05

Excludes1 means the excluded code and F05 cannot be reported together for the same encounter. Excludes2 means the excluded condition is not part of F05, but both codes can coexist if both are present and documented.

The R41.0 exclusion catches the most coding errors. When the admitting diagnosis is “altered mental status” or “confusion” and the workup is still in progress, R41.0 may be appropriate at admission. Once the physiological cause is identified – sepsis, post-operative state, metabolic derangement – the coder should update the record to F05 with the cause coded first.

Coding and sequencing rules for ICD-10 Code F05

F05 is a manifestation code. Per the ICD-10-CM Official Guidelines for Coding and Reporting, the underlying physiological condition drives the principal diagnosis – F05 follows as an additional code.

Clinicians who document “delirium” without linking it to a cause give coders no basis to sequence correctly, which creates both undercoding and audit exposure. Compliance management workflows that flag incomplete diagnosis linkage can catch this before the claim is submitted.

1. Identify the underlying cause – sepsis (A41.xx), pneumonia (Jxx.x), hepatic encephalopathy (K72.x), postoperative state (T81.xx), or other physiological condition.
2. Sequence the cause first – the physiological condition is the principal/first-listed diagnosis.
3. Add F05 as an additional code – confirm the documentation explicitly states the delirium is due to (or a manifestation of) the underlying condition.
4. Add the dementia code if applicable – for delirium superimposed on dementia, code the dementia first (F01.xx-F03.xx as appropriate), then F05.
5. Assign the POA indicator – for inpatient claims, document whether the delirium was present on admission or hospital-acquired. Hospital-acquired delirium (POA = N) affects HAC payment adjustments.

Sequencing rules can vary by payer. Some Medicare Advantage plans and state Medicaid programs have supplemental guidelines. Verify against the applicable payer’s LCD/LCA before finalizing the claim.

Documentation requirements for F05

Without adequate documentation, even the correct code selection becomes indefensible at audit. Clinicians using safer clinical note practices build the evidentiary chain reviewers look for. A structured psychiatric evaluation template speeds up this process considerably.

The record must capture four elements to fully support F05:

• Named physiological cause – state it explicitly (e.g. “delirium due to urinary tract infection”). Vague language like “altered mental status in the context of infection” is insufficient for precise F05 coding.
• Onset and fluctuation – DSM-5 Criterion B requires acute onset and fluctuating course. Document when symptoms first appeared and any variation over the day.
• Attention and awareness disturbance – DSM-5 Criterion A. Formal assessments (CAM, CAM-ICU, 4AT) provide objective evidence. Reference the tool and the score.
• Causal linkage – the note must state the delirium is “due to,” “caused by,” or “a result of” the physiological condition. Temporal coincidence alone is not sufficient.

Using digital intake forms pre-configured with CAM or 4AT screening fields ensures the assessment data is captured at the bedside and flows directly into the clinical record, rather than being reconstructed from memory later.

This is especially useful in high-volume environments like post-surgical units – including after anesthesia-heavy procedures such as 00192 – or geriatric wards where F05 is frequently applicable.

Delirium vs dementia: ICD-10 Code F05 coding differences

Delirium and dementia overlap in presentation, and the distinction changes both the diagnosis code and sequencing. Getting this wrong at coding risks DRG miscalculation and clinical record inaccuracy.

The “both present” scenario is the most clinically common and the most frequently miscoded. Dementia patients are disproportionately affected by delirium – sundowning is its clearest expression. When both are documented, two codes are required. The dementia code goes first; F05 follows. Omitting F05 understates case complexity and can affect DRG assignment.

Clinical subtypes: hyperactive, hypoactive, and mixed delirium

All three motor subtypes of delirium are coded as F05 – there is no ICD-10-CM subcode for hyperactive versus hypoactive presentation. The distinction matters clinically and for documentation, but not for code selection.

Hypoactive delirium is the most clinically dangerous from a documentation standpoint. Its quiet presentation means it’s often attributed to fatigue, post-sedation, or depression.

Clinicians unfamiliar with the subtype may not use delirium language in the note – meaning coders have no basis for F05. Resources on healthcare burnout highlight how cognitive load affects documentation accuracy across specialties. Formal screening tools (CAM, 4AT) catch hypoactive delirium before it disappears from the record.

Common clinical scenarios requiring F05

F05 applies across a wide range of settings. The unifying requirement is an identified physiological cause – not the care environment or the patient’s age.

• Postoperative delirium: Code F05 as additional diagnosis; principal is the surgical procedure or postoperative complication – whether a joint procedure such as 27130 or a device implant such as L8680. Document that delirium emerged in the post-anesthesia or early recovery period.
• ICU delirium: Common in mechanically ventilated patients. Use CAM-ICU for screening. Code F05 with sepsis (A41.xx), respiratory failure (J96.xx), or other ICU driver as principal.
• Sundowning in dementia patients: Code the dementia type first, then F05. The term “sundowning” alone in the record is sufficient – it is an official included term.
• Infection-related delirium: Pneumonia, UTI, and sepsis are the most common causes in elderly patients. The infection code is sequenced first.
• Metabolic delirium: Hyponatremia, hypercalcemia, hepatic encephalopathy, thyroid storm. Code the metabolic condition first; add F05 as secondary.
• Delirium on substance withdrawal background (physiological, not substance-induced): If an alcohol-dependent patient develops delirium from hepatic failure rather than withdrawal, F05 applies – not F10.231. The distinction is in the documented cause, not the substance history.

Sundowning and delirium superimposed on dementia: specific guidance

These two included terms under F05 generate more coding questions than all others combined. Both involve elderly patients with underlying cognitive conditions, and both are frequently undercoded because clinicians treat them as expected features of dementia rather than separately billable complications.

Sundowning

Sundowning refers to the late-afternoon and evening exacerbation of confusion, agitation, and disorientation seen in dementia patients. It is an official included term under F05, not a standalone code. To code it correctly:

1. Document “sundowning” explicitly – the term alone is sufficient to support F05.
2. Code the underlying dementia first (e.g., F03.90 for unspecified dementia without behavioral disturbance).
3. Add F05 as an additional code.
4. Do not use R41.3 (other amnesia) or R41.89 (other symptoms of cognitive functions) in place of F05 when sundowning is documented.

Delirium superimposed on dementia

When delirium develops in a patient with established dementia, both conditions are coded. This is not a situation where one replaces the other. The sequencing follows the same rule: dementia first (F01.xx-F03.xx), then F05. The documentation must state the delirium is acute and represents a change from the patient’s cognitive baseline – not just worsened chronic confusion.

Key documentation tip: include a sentence comparing current cognition to the patient’s known baseline. “Patient’s family reports she is significantly more confused than her usual dementia presentation over the past 48 hours” gives coders the acute-on-chronic distinction they need to apply F05 on top of the dementia code.

For clinicians seeking a reference framework, Pabau’s psychiatric evaluation template includes a baseline-versus-current cognitive comparison section.

Related ICD-10 codes to know alongside F05

Several codes are commonly confused with F05, used alongside it, or serve as the correct alternative when F05’s conditions are not met. The AAPC ICD-10-CM code search and the WHO ICD-10 browser are the primary references for navigating the full classification hierarchy.

For related psychiatric coding, see F32.9 and F10 for sequencing parallels.

How Pabau supports accurate ICD-10 Code F05 documentation

Delirium documentation fails most often not because clinicians don’t know the diagnosis, but because the note structure doesn’t capture what coders need. Pabau’s mental health EMR and psychiatry EMR software include structured assessment templates that can be configured to capture the four F05 documentation requirements at the point of care.

Pabau’s client record management system timestamps all clinical entries, providing an audit trail that supports the acute-onset documentation DSM-5 requires. Pabau Scribe, our AI scribe, transcribes consultations and structures the output into chart notes – including diagnosis linkage language – so the causal verb (“delirium due to”) is captured automatically rather than as an afterthought.

For geriatric and post-acute settings where sundowning and delirium superimposed on dementia are most common, pre-configured digital forms can include CAM screening fields and a baseline-versus-current cognition comparison prompt, ensuring every F05 encounter has the documentation to survive payer review.

Reduce coding errors and audit exposure

Pabau helps clinicians capture the diagnosis linkage, onset timeline, and screening data needed to support accurate ICD-10 Code F05 coding – all within a structured clinical record that's audit-ready from the moment the consultation ends.

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Conclusion

F05 is a deceptively simple code – one descriptor, eight included terms, and a clear sequencing rule. The failure points are documentation, not code selection: missing the causal linkage, skipping the underlying condition code, and leaving hypoactive delirium undocumented as depression. Each error affects DRG accuracy, case mix index, and payer audit exposure.

Pabau’s structured clinical notes, configurable screening forms, and AI-assisted documentation capture the four elements needed to support F05 at audit. To see how Pabau handles clinical documentation for behavioral health and neuropsychiatric diagnoses, explore Pabau’s psychiatry EMR software or speak with the team.

Continue your research

Need to code a negative dementia screening result? G9920 is the companion HCPCS code often billed alongside geriatric encounters where F05 also applies.

Not sure the record supports delirium yet? R41.82 covers altered mental status before a specific cause is confirmed.

Ruling out psychiatric causes before coding delirium? F20.9 is the code when symptoms point to schizophrenia rather than a physiological or substance-induced cause.

Frequently Asked Questions