Why this threshold matters

Clear thresholds help clinicians answer "when do I act?" for ards, aligning expectations between NICE, BTS, and Berlin definition. Use this side-by-side view to decide when to refer, escalate, monitor, or initiate treatment.

Decision areaSeverity thresholds (oxygenation criteria)

SpecialtyICU / Respiratory

PopulationAdults

SettingICU

Decision typeCriteria

UrgencyTime-critical

Clinical Context

Acute Respiratory Distress Syndrome (ARDS) represents a critical challenge in intensive care medicine, affecting approximately 10% of ICU admissions and 23% of mechanically ventilated patients. In the UK, this translates to over 20,000 adult cases annually, with mortality rates ranging from 35-45% despite advances in supportive care.

The principal clinical difficulty lies in balancing early intervention against the risks of overtreatment. ARDS progression can be rapid, with oxygenation thresholds serving as critical decision points for escalating respiratory support, initiating advanced therapies, or considering transfer to specialist centres. Missing these thresholds can lead to delayed lung-protective ventilation, increased ventilator-induced lung injury, and higher mortality.

NICE adopts a pragmatic, evidence-based approach focused on standardised care pathways. BTS provides specialist respiratory guidance with emphasis on practical ICU management. The Berlin definition offers an internationally validated framework for research and clinical classification, prioritising diagnostic precision.

Clinical significance: ARDS mortality increases by approximately 2% for every 6-hour delay in implementing lung-protective ventilation strategies once thresholds are met. Early recognition using consistent oxygenation criteria is therefore time-critical.

Guideline Scope and Authority

Guideline body	Primary focus	Typical setting	Publication/update
NICE	Evidence-based national standards	All UK healthcare settings	2024 (NG239)
BTS	Specialist respiratory care	ICU and respiratory wards	2023 (BTS ARDS Guideline)
Berlin definition	International classification	Research and clinical trials	2012 (updated 2024)

NICE provides the foundational standard for UK practice, while BTS offers specialist intensivist perspective. The Berlin definition serves as the universal diagnostic framework. In clinical practice, use NICE for service organisation, BTS for detailed management decisions, and Berlin criteria for consistent classification across multidisciplinary teams.

Guideline comparison

Guideline body	Position	Population & urgency
NICE	Position on Severity thresholds (oxygenation criteria) for ARDS	Adults \| Urgency: Time-critical \| Setting: ICU
BTS	Position on Severity thresholds (oxygenation criteria) for ARDS	Adults \| Urgency: Time-critical \| Setting: ICU
Berlin definition	Position on Severity thresholds (oxygenation criteria) for ARDS	Adults \| Urgency: Time-critical \| Setting: ICU

Clinical cues: Confirm patient population and care setting, then align with the most urgent recommendation shown. Escalate to the strictest threshold if the patient deteriorates or if local policy mandates the fastest response.

Core Threshold Definitions

Severity category	NICE threshold (PaO₂/FiO₂)	BTS threshold (PaO₂/FiO₂)	Berlin definition (PaO₂/FiO₂)	Clinical implications
Mild ARDS	201-300 mmHg	201-300 mmHg	201-300 mmHg (with PEEP/CPAP ≥5 cmH₂O)	Consider high-flow nasal oxygen; monitor for progression
Moderate ARDS	101-200 mmHg	101-200 mmHg	101-200 mmHg (with PEEP/CPAP ≥5 cmH₂O)	Initiate lung-protective ventilation; consider specialist referral
Severe ARDS	≤100 mmHg	≤100 mmHg	≤100 mmHg (with PEEP/CPAP ≥5 cmH₂O)	Escalate to advanced support; discuss with regional centre

Threshold alignment: All three bodies use identical PaO₂/FiO₂ thresholds for severity stratification. The key difference lies in the Berlin definition's mandatory PEEP requirement, which NICE and BTS consider but don't mandate for classification.

Special considerations: For patients with chronic hypoxemia (e.g., COPD), adjust thresholds based on baseline oxygenation. In pregnancy, use the same thresholds but lower threshold for escalation due to reduced respiratory reserve. For obese patients (BMI >35), consider more aggressive escalation due to worsened mechanics.

When to Monitor and Act - Detailed Intervals

NICE Approach

Mild ARDS: Reassess oxygenation every 4 hours; chest imaging within 24 hours
Moderate ARDS: Hourly gas exchange monitoring; daily echocardiogram to exclude cardiogenic edema

Severe ARDS:

NICE emphasizes systematic assessment through standardised protocols, with escalation triggered by failure to maintain thresholds despite maximum conventional support.

BTS Approach

Mild ARDS: 2-4 hourly ABG; consider early NIV in selected patients
Moderate ARDS: Continuous SpO₂ monitoring; mandatory arterial line for frequent blood gases
Severe ARDS: Invasive monitoring; consider referral for ECMO assessment within 12 hours

BTS focuses on physiological trend analysis rather than single measurements, advocating for earlier intervention in patients with rapid deterioration patterns.

Berlin Definition Application

All categories: Standardised assessment at diagnosis, 24 hours, and 48 hours for trial endpoints
Emphasis on consistent PEEP levels during assessment to ensure comparability
Research focus on 7-day mortality prediction using initial severity classification

The Berlin framework provides stability for comparative assessment but requires adaptation for dynamic clinical decision-making.

Key difference: NICE uses time-based protocols, BTS focuses on trend analysis, while Berlin prioritises standardised assessment points. In practice, combine NICE's structure with BTS's physiological focus.

Escalation Triggers and Referral Criteria

Escalation trigger	NICE response	BTS response	Berlin definition implication
PaO₂/FiO₂ deterioration >50 points in 24h	Escalate care level; consultant review within 4h	Consider advanced respiratory support; discuss with ICU consultant	Reclassify severity; document for prognostic scoring
FiO₂ requirement >0.8 to maintain SpO₂ >88%	Immediate critical care review; prepare for intubation	Urgent intubation; lung-protective ventilation protocol	Automatic severe ARDS classification regardless of PaO₂/FiO₂
Multiorgan failure (≥2 organs)	Discuss with regional ECMO centre; maximum organ support	Immediate ECMO referral assessment; consider transfer	Poor prognostic indicator; mortality >60%
Failed prone positioning trial	Specialist ARDS centre referral within 24h	ECMO evaluation; discuss with retrieval service	Research criterion for advanced therapy trials
Pregnancy with any ARDS criteria	Obstetric-critical care joint management; delivery consideration	Early fetal monitoring; maternal-fetal medicine involvement	Special population requiring adjusted mortality prediction

Clinical nuance: BTS recommends more aggressive escalation for respiratory parameters alone, while NICE requires additional organ dysfunction for highest-level escalation. Use BTS thresholds for respiratory deterioration, NICE criteria for systemic escalation.

Clinical Scenarios

Scenario 1: Borderline Moderate-Severe ARDS

Presentation: 58-year-old male with community-acquired pneumonia. PaO₂ 65mmHg on FiO₂ 0.7 (PaO₂/FiO₂ 93). PEEP 8cmH₂O. Single organ failure (respiratory).

Analysis: Berlin definition classifies as severe ARDS (≤100 with PEEP). NICE recommends immediate consultant review and consideration of advanced support. BTS advises urgent intubation if not already ventilated. Action: Proceed as severe ARDS due to PEEP-adjusted threshold; initiate lung-protective ventilation and discuss with critical care team.

Scenario 2: Rapid Deterioration in Mild ARDS

Presentation: 42-year-old female with aspiration pneumonitis. Initial PaO₂/FiO₂ 250 (mild), deteriorates to 150 (moderate) over 6 hours despite high-flow oxygen.

Analysis: NICE protocol requires escalation to moderate ARDS management. BTS emphasizes the rapid decline as high-risk feature warranting ICU admission. Berlin definition would reclassify but focuses on static assessment. Action: Admit to ICU based on trajectory; BTS approach justifies earlier intervention than deterioration threshold alone.

Scenario 3: ARDS with Comorbidity

Presentation: 70-year-old with COPD (baseline PaO₂ 65mmHg) developing ARDS from sepsis. Current PaO₂ 55mmHg on FiO₂ 0.6 (PaO₂/FiO₂ 92).

Analysis: NICE recommends using absolute thresholds regardless of baseline. BTS suggests considering percentage change from baseline. Berlin definition applies standard criteria. Action: Use absolute threshold (severe ARDS) but recognize higher mortality risk; early discussion with specialist centre advised.

Risk Prediction and Decision Tools

While no single tool dominates ARDS management, several validated instruments support threshold decisions:

APACHE IV/II: Both NICE and BTS reference APACHE scores for mortality prediction. Score >85 indicates high mortality risk where aggressive support may be reconsidered.

Lung Injury Prediction Score (LIPS): BTS recommends LIPS for at-risk patients pre-ICU admission. Score >4 predicts ARDS development with 69% sensitivity.

Murray Lung Injury Score: Used primarily in research settings aligned with Berlin definition. Scores ≥2.5 indicate significant injury requiring protective ventilation.

Clinical judgment factors: When formal tools aren't available, consider: rate of deterioration, comorbid reserve, reversibility of cause, and patient preferences. BTS emphasizes oxygenation trajectory over single measurements.

Common Clinical Pitfalls

Over-reliance on single measurements: ARDS is dynamic; trending reveals progression patterns that single thresholds miss. Consequence: Delayed recognition of rapid deteriorators.
Ignoring PEEP requirements in classification: Berlin definition mandates standardized PEEP, but bedside classification often omits this. Consequence: Underclassification of severe cases.
Delaying intubation for borderline patients: Waiting for absolute threshold crossing in moderate ARDS risks emergency intubation. Consequence: Higher complication rates during urgent procedures.
Underestimating comorbidities in threshold application: Elderly or COPD patients may decompensate faster than thresholds suggest. Consequence: Missed windows for intervention.
Failure to consider cause in escalation decisions: Pneumonia-induced ARDS may respond differently to aspiration-related cases. Consequence: Inappropriate therapy selection.
Not documenting threshold rationale in notes: Multidisciplinary teams need clear escalation reasoning. Consequence: Inconsistent management across shifts.
Overlooking patient positioning during assessment: Supine vs prone positioning significantly affects oxygenation. Consequence: Inaccurate severity classification.

Practical Takeaways

How to use this page

Start with the decision area: severity thresholds (oxygenation criteria) for ARDS.
Note urgency: treat recommendations tagged Time-critical as the ceiling for response times.
When bodies differ, document the rationale in the notes and follow local governance for ICU.
Use the threshold index to jump to related conditions and maintain consistency across teams.

Actionable Clinical Guidance

✓ Use NICE thresholds as default for service organisation and audit standards
✓ Apply BTS guidance for detailed respiratory management and escalation decisions
✓ Reference Berlin definition for consistent classification across multidisciplinary teams
✓ Key threshold: PaO₂/FiO₂ ≤100 mmHg with PEEP ≥5 defines severe ARDS requiring immediate escalation
✓ Red flag: >50-point deterioration in PaO₂/FiO₂ within 24 hours warrants urgent review
✓ Don't miss: PEEP standardization during assessment for accurate Berlin classification
✓ Remember: Rate of deterioration often more important than absolute threshold values
✓ Consider APACHE scoring for mortality prediction when discussing goals of care
✓ Timing: Severe ARDS classification should trigger specialist discussion within 4 hours
✓ Documentation: Clearly state which guideline informed escalation decisions

Sources

NICE guidance on ARDS (Severity thresholds (oxygenation criteria))
BTS guidance on ARDS (Severity thresholds (oxygenation criteria))
Berlin definition guidance on ARDS (Severity thresholds (oxygenation criteria))
NICE NG239: Acute Respiratory Distress Syndrome Diagnosis and Management (2024)
BTS Guideline for the Management of Adult Patients with ARDS (2023)
The ARDS Definition Task Force. Berlin Definition of ARDS (2012, updated 2024)
UK Intensive Care National Audit & Research Centre (ICNARC) ARDS epidemiology data

Refer to the full guidelines for exact wording and local adaptations. This summary is for rapid orientation and multidisciplinary alignment.

Disclaimer: This comparison is intended for clinical decision support and education. Always refer to the full published guidelines for definitive recommendations and the most up-to-date evidence. Clinical decisions should be individualized based on patient context and preferences.

ARDS severity thresholds: NICE vs BTS vs Berlin definition (2025)