APACHE II Score: Acute Physiology and Chronic Health Evaluation II

The APACHE II (Acute Physiology and Chronic Health Evaluation II) score is one of the most widely used and validated tools for assessing disease severity and predicting mortality risk in adult intensive care unit (ICU) patients. Developed in 1985 by Knaus and colleagues, APACHE II has become a cornerstone of critical care medicine, providing clinicians with an objective, standardized method to evaluate patient prognosis, guide resource allocation, and support clinical decision-making.

The APACHE II scoring system integrates three key components: acute physiology abnormalities measured during the first 24 hours of ICU admission, patient age, and chronic health status. By quantifying these factors into a single numerical score ranging from 0 to 71, APACHE II provides a comprehensive assessment of disease severity that correlates strongly with in-hospital mortality risk. Higher scores indicate more severe illness and higher mortality risk, while lower scores suggest better prognosis.

APACHE II has been extensively validated across diverse ICU populations and remains one of the most reliable tools for mortality prediction in critical care settings. While newer scoring systems have been developed, APACHE II continues to be widely used due to its simplicity, reliability, and extensive validation in clinical practice.

What is APACHE II?

APACHE II is a severity-of-disease classification system designed to measure the intensity of therapeutic intervention required and predict hospital mortality risk for patients admitted to intensive care units. The score is calculated using the worst values of 12 physiological variables recorded during the first 24 hours of ICU admission, combined with points for age and chronic health conditions.

The acronym APACHE stands for Acute Physiology and Chronic Health Evaluation, with the "II" designation indicating this is the second version of the scoring system, following the original APACHE system. APACHE II was specifically designed to be more practical and easier to use than its predecessor while maintaining strong predictive accuracy.

Historical Development

The development of APACHE II represented a significant advancement in critical care medicine. Prior to its introduction, there was no standardized method for assessing disease severity in ICU patients, making it difficult to compare outcomes across different institutions, evaluate the effectiveness of treatments, or make informed decisions about resource allocation.

APACHE II was developed using data from 5,815 ICU admissions across 13 hospitals, making it one of the most extensively validated scoring systems in critical care. The system was designed to be:

• Simple enough for routine clinical use
• Based on readily available clinical data
• Applicable to a wide range of ICU patients
• Strongly predictive of mortality risk

Core Components

APACHE II consists of three main components that are combined to create the total score:

• Acute Physiology Score (APS): Points assigned based on the worst values of 12 physiological variables during the first 24 hours of ICU admission. Each variable is scored from 0 to 4 points, with higher points indicating more severe abnormalities. The maximum APS is 60 points.
• Age Points: Additional points based on the patient's age, ranging from 0 points for patients under 45 years to 6 points for patients 75 years and older.
• Chronic Health Points: Additional points for patients with severe organ system insufficiency or immunocompromised states, particularly if they are non-operative or emergency postoperative cases. This component adds 0 to 5 points.

Clinical Significance

APACHE II serves multiple critical functions in intensive care medicine, making it an indispensable tool for clinicians managing critically ill patients.

Mortality Risk Prediction

The primary function of APACHE II is to predict in-hospital mortality risk. The score correlates strongly with mortality, with higher scores indicating higher risk of death. This predictive capability is valuable for:

• Informing patients and families about prognosis
• Guiding decisions about intensity of care
• Supporting discussions about goals of care
• Helping clinicians understand the severity of a patient's condition

While APACHE II provides valuable prognostic information, it is important to remember that it is a statistical tool and cannot predict individual patient outcomes with certainty. Many factors beyond the APACHE II score influence patient survival, including the underlying disease process, response to treatment, and patient-specific factors.

Disease Severity Assessment

APACHE II provides an objective measure of disease severity that allows clinicians to:

• Quantify the severity of a patient's illness
• Compare disease severity across different patients
• Track changes in disease severity over time
• Assess the impact of interventions on disease severity

This objective assessment is particularly valuable in critical care, where subjective impressions can be influenced by various factors and may not accurately reflect the true severity of illness.

Resource Allocation and Triage

In settings with limited ICU resources, APACHE II can help guide decisions about:

• ICU admission priorities
• Allocation of specialized resources
• Transfer decisions between facilities
• Utilization of advanced life support measures

However, APACHE II should never be used as the sole criterion for resource allocation decisions. Clinical judgment, patient preferences, and other factors must always be considered.

Quality Improvement and Research

APACHE II is widely used in:

• Comparing outcomes across different ICUs
• Evaluating the effectiveness of treatments and interventions
• Conducting clinical research on critical care outcomes
• Benchmarking ICU performance
• Identifying areas for quality improvement

The standardized nature of APACHE II makes it valuable for comparing outcomes across different institutions and time periods, accounting for differences in case mix and disease severity.

Formula and Calculation

The APACHE II score is calculated using the following formula:

APACHE II = Age Points + Acute Physiology Score (APS) + Chronic Health Points

Acute Physiology Score (APS)

The APS is calculated by summing the points assigned to each of 12 physiological variables based on their worst values during the first 24 hours of ICU admission. Each variable is scored from 0 to 4 points (except Glasgow Coma Scale, which uses a modified scoring system), with higher points indicating more severe abnormalities.

Physiological Variables and Scoring

1. Temperature (°C): Body temperature is scored based on deviations from normal (36-38.4°C = 0 points). Severe hypothermia or hyperthermia receives higher scores.

• ≥41°C or ≤29.9°C: 4 points
• 39-40.9°C or 30-31.9°C: 3 points
• 32-33.9°C: 2 points
• 38.5-38.9°C or 34-35.9°C: 1 point
• 36-38.4°C: 0 points

2. Mean Arterial Pressure (MAP, mmHg): MAP reflects cardiovascular status and organ perfusion. Normal MAP (70-109 mmHg) receives 0 points.

• ≥160 mmHg or ≤49 mmHg: 4 points
• 130-159 mmHg: 3 points
• 110-129 mmHg or 50-69 mmHg: 2 points
• 70-109 mmHg: 0 points

3. Heart Rate (beats per minute): Heart rate reflects cardiovascular and autonomic function. Normal heart rate (70-109 bpm) receives 0 points.

• ≥180 bpm or ≤39 bpm: 4 points
• 140-179 bpm or 40-54 bpm: 3 points
• 110-139 bpm or 55-69 bpm: 2 points
• 70-109 bpm: 0 points

4. Respiratory Rate (breaths per minute): Respiratory rate reflects respiratory function and acid-base status. Normal respiratory rate (12-24 breaths/min) receives 0 points.

• ≥50 breaths/min or ≤5 breaths/min: 4 points
• 35-49 breaths/min or 6-9 breaths/min: 3 points
• 25-34 breaths/min or 10-11 breaths/min: 1 point
• 12-24 breaths/min: 0 points

5. Oxygenation (PaO₂ or A-a gradient): Oxygenation is assessed using PaO₂ if FiO₂ is less than 0.5, or A-a gradient if FiO₂ is 0.5 or greater. This reflects the efficiency of oxygen transfer and respiratory function.

• PaO₂ < 200 mmHg (or A-a gradient criteria): 4 points
• PaO₂ 200-349 mmHg: 2 points
• PaO₂ 350-499 mmHg: 1 point
• PaO₂ ≥ 500 mmHg: 0 points

6. Arterial pH: Arterial pH reflects acid-base balance. Normal pH (7.33-7.49) receives 0 points.

• ≥7.7 or < 7.15: 4 points
• 7.6-7.69 or 7.15-7.24: 3 points
• 7.5-7.59: 1 point
• 7.25-7.32: 2 points
• 7.33-7.49: 0 points

7. Serum Sodium (mEq/L): Serum sodium reflects fluid and electrolyte balance. Normal sodium (130-149 mEq/L) receives 0 points.

• ≥180 mEq/L or < 110 mEq/L: 4 points
• 160-179 mEq/L or 110-119 mEq/L: 3 points
• 155-159 mEq/L or 120-129 mEq/L: 2 points
• 150-154 mEq/L: 1 point
• 130-149 mEq/L: 0 points

8. Serum Potassium (mEq/L): Serum potassium reflects electrolyte balance and can affect cardiac function. Normal potassium (3.5-5.4 mEq/L) receives 0 points.

• ≥7 mEq/L or < 2.5 mEq/L: 4 points
• 6-6.9 mEq/L: 3 points
• 2.5-2.9 mEq/L: 2 points
• 5.5-5.9 mEq/L or 3-3.4 mEq/L: 1 point
• 3.5-5.4 mEq/L: 0 points

9. Serum Creatinine (mg/dL): Serum creatinine reflects renal function. Scoring depends on whether acute renal failure is present. Normal creatinine (0.6-1.4 mg/dL) receives 0 points in the absence of acute renal failure.

• ≥3.5 mg/dL: 4 points (8 points if acute renal failure)
• 2-3.4 mg/dL: 3 points (6 points if acute renal failure)
• 1.5-1.9 mg/dL or < 0.6 mg/dL: 2 points (4 points if acute renal failure)
• 0.6-1.4 mg/dL: 0 points (2 points if acute renal failure)

10. Hematocrit (%): Hematocrit reflects oxygen-carrying capacity and volume status. Normal hematocrit (30-49.9%) receives 0 points.

• ≥60% or < 20%: 4 points
• 50-59.9% or 20-29.9%: 2 points
• 30-49.9%: 0 points

11. White Blood Cell Count (×10³/mm³): White blood cell count reflects immune function and can indicate infection or inflammation. Normal WBC (3-14.9 ×10³/mm³) receives 0 points.

• ≥40 ×10³/mm³ or < 1 ×10³/mm³: 4 points
• 20-39.9 ×10³/mm³ or 1-2.9 ×10³/mm³: 2 points
• 15-19.9 ×10³/mm³: 1 point
• 3-14.9 ×10³/mm³: 0 points

12. Glasgow Coma Scale (GCS): GCS reflects neurological function. APACHE II uses a modified scoring system based on (15 - GCS).

• GCS 3-5 (15-GCS = 10-12): 5 points
• GCS 6-9 (15-GCS = 6-9): 4 points
• GCS 10-12 (15-GCS = 3-5): 3 points
• GCS 13-14 (15-GCS = 1-2): 2 points
• GCS 15 (15-GCS = 0): 0 points

Age Points

Age is an important predictor of mortality in critical illness. APACHE II assigns points based on age ranges:

• < 45 years: 0 points
• 45-54 years: 2 points
• 55-64 years: 3 points
• 65-74 years: 5 points
• ≥ 75 years: 6 points

Older patients receive more points because age is independently associated with increased mortality risk in critical illness, even after accounting for acute physiology and chronic health conditions.

Chronic Health Points

Chronic health points are assigned to patients with severe organ system insufficiency or immunocompromised states. The points depend on whether the patient is non-operative, emergency postoperative, or elective postoperative:

• No chronic health issues: 0 points
• Non-operative or Emergency Postoperative with organ insufficiency/immunocompromised: 5 points
• Elective Postoperative with organ insufficiency/immunocompromised: 2 points

Chronic health conditions that qualify include:

• Severe cardiovascular disease (New York Heart Association class IV)
• Severe respiratory disease (chronic restriction, obstruction, or vascular disease)
• Chronic liver disease (biopsy-proven cirrhosis, portal hypertension, or hepatic encephalopathy)
• Chronic renal disease (requiring chronic dialysis)
• Immunocompromised states (immunosuppression, chemotherapy, radiation, high-dose steroids, AIDS)

Interpretation of APACHE II Scores

APACHE II scores range from 0 to 71, with higher scores indicating more severe illness and higher mortality risk. However, interpretation must consider the clinical context and other factors beyond the numerical score.

Score Ranges and Mortality Risk

While specific mortality predictions require additional calculations and consideration of the primary diagnosis, general patterns emerge:

• APACHE II 0-10: Low mortality risk. Patients in this range typically have relatively mild critical illness or are recovering from acute events. Mortality risk is generally less than 10-15%.
• APACHE II 11-20: Moderate mortality risk. Patients have significant critical illness requiring intensive care. Mortality risk is generally 15-30%.
• APACHE II 21-30: High mortality risk. Patients have severe critical illness with multiple organ system involvement. Mortality risk is generally 30-50%.
• APACHE II > 30: Very high mortality risk. Patients have extremely severe critical illness. Mortality risk is generally greater than 50% and may exceed 80% in the highest ranges.

It is important to note that these are general guidelines and actual mortality risk varies significantly based on:

• Primary diagnosis and underlying disease
• Response to treatment
• Patient-specific factors
• Quality of care provided
• Timing of score calculation

Factors Influencing Interpretation

Several factors must be considered when interpreting APACHE II scores:

• Primary Diagnosis: The same APACHE II score may have different prognostic implications depending on the underlying disease. For example, a score of 25 may have different meaning in a patient with acute respiratory distress syndrome versus a patient with severe sepsis.
• Timing: APACHE II is calculated using the worst values during the first 24 hours. Scores calculated later in the ICU stay may have different prognostic value.
• Reversibility: Some conditions are more reversible than others. A high score due to reversible causes may have better prognosis than a high score due to irreversible conditions.
• Response to Treatment: Patients who respond well to treatment may have better outcomes than predicted by the initial APACHE II score.
• Patient-Specific Factors: Factors such as functional status before illness, patient preferences, and family support can influence outcomes independently of the APACHE II score.

Clinical Applications

ICU Admission Assessment

APACHE II is commonly calculated during or shortly after ICU admission to:

• Assess initial disease severity
• Provide baseline prognostic information
• Guide initial treatment intensity
• Inform discussions with patients and families
• Support triage and resource allocation decisions

The score provides an objective measure that complements clinical judgment and helps ensure consistent assessment across different clinicians and time periods.

Prognostic Counseling

APACHE II scores can support discussions with patients and families about:

• Expected outcomes and prognosis
• Likelihood of survival
• Potential for recovery
• Goals of care
• Treatment options and intensity

However, it is crucial to present APACHE II information appropriately:

• Emphasize that it is a statistical tool, not a prediction for individual patients
• Explain that many factors beyond the score influence outcomes
• Use the score as one piece of information in a comprehensive discussion
• Avoid using the score to make definitive statements about individual prognosis
• Consider patient and family preferences and values

Research and Quality Improvement

APACHE II is extensively used in critical care research to:

• Standardize disease severity assessment across studies
• Compare outcomes between different treatment groups
• Evaluate the effectiveness of interventions
• Benchmark ICU performance
• Identify areas for quality improvement

The standardized nature of APACHE II makes it valuable for comparing outcomes across different institutions, time periods, and patient populations.

Resource Allocation

In settings with limited ICU resources, APACHE II can inform decisions about:

• ICU bed allocation
• Use of specialized equipment and therapies
• Transfer to higher-level facilities
• Allocation of nursing and physician resources

However, APACHE II should never be used as the sole criterion for resource allocation. Ethical considerations, patient preferences, clinical judgment, and other factors must always be considered.

Limitations and Considerations

Population Limitations

APACHE II has important population limitations:

• Age Restriction: APACHE II is validated for adult patients (≥16 years) only. It should not be used for pediatric patients, as children have different normal physiological values and different patterns of critical illness.
• ICU-Specific: APACHE II is designed for use in ICU patients. Its predictive value may be different in other settings, such as emergency departments or general wards.
• First 24 Hours: APACHE II is calculated using values from the first 24 hours of ICU admission. Scores calculated at other times may have different prognostic value.

Not a Substitute for Clinical Judgment

APACHE II is a tool to support clinical decision-making, not a replacement for comprehensive clinical evaluation. Important considerations include:

• The score provides statistical information about groups of patients, not individual predictions
• Many factors beyond the score influence patient outcomes
• Clinical judgment, patient preferences, and other factors must always be considered
• The score should be interpreted in the context of the complete clinical picture

Technical Limitations

Several technical factors can affect APACHE II calculation and interpretation:

• Missing Data: If physiological values are not available, they are assigned 0 points, which may underestimate disease severity
• Timing: The score uses worst values during the first 24 hours. Values obtained at different times may not accurately reflect the worst physiological state
• Laboratory Variability: Different laboratories may use different reference ranges or methodologies, potentially affecting some components of the score
• Chronic Health Assessment: Accurate assessment of chronic health conditions requires detailed medical history, which may not always be available

Disease-Specific Considerations

APACHE II may have different predictive value for different diseases:

• Some conditions may be better predicted by disease-specific scoring systems
• The same score may have different implications depending on the primary diagnosis
• Some conditions may not be well-captured by the physiological variables included in APACHE II

Evolution of Disease

APACHE II is calculated at a single point in time (first 24 hours). However, critical illness is dynamic, and:

• Disease severity may change rapidly
• Response to treatment may alter prognosis
• Serial assessments may provide more meaningful information than a single score
• The initial score may not reflect the ultimate disease course

Clinical Pearls

When to Calculate APACHE II

Consider calculating APACHE II when:

• Assessing disease severity in newly admitted ICU patients
• Providing prognostic information to patients and families
• Comparing outcomes across different patient groups or time periods
• Conducting research on critical care outcomes
• Evaluating ICU performance and quality
• Supporting resource allocation decisions

How to Use APACHE II Effectively

• Use Worst Values: Always use the worst values during the first 24 hours, not values at a single point in time
• Complete Data: Strive to obtain all physiological measurements to ensure accurate scoring
• Clinical Context: Always interpret the score in the context of the complete clinical picture
• Patient-Specific Factors: Consider factors beyond the score that may influence outcomes
• Communication: Present APACHE II information appropriately to patients and families, emphasizing its limitations

Common Pitfalls

Avoid these common mistakes when using APACHE II:

• Using Single Time Point Values: Always use worst values during the first 24 hours
• Ignoring Missing Data: Missing values are assigned 0 points, which may underestimate severity
• Overreliance on Score: The score is a tool, not a definitive prediction
• Ignoring Clinical Context: Always consider the complete clinical picture
• Using for Non-ICU Patients: APACHE II is validated for ICU patients only
• Using for Pediatric Patients: APACHE II is for adults (≥16 years) only

Case Examples

Case 1: Low APACHE II Score

Presentation: A 35-year-old previously healthy man is admitted to the ICU after an uncomplicated appendectomy. He develops postoperative ileus and requires close monitoring.

Physiological Values (worst during first 24 hours):

• Temperature: 37.2°C (0 points)
• MAP: 85 mmHg (0 points)
• Heart Rate: 88 bpm (0 points)
• Respiratory Rate: 18 breaths/min (0 points)
• PaO₂: 95 mmHg on room air (0 points)
• Arterial pH: 7.40 (0 points)
• Sodium: 138 mEq/L (0 points)
• Potassium: 4.2 mEq/L (0 points)
• Creatinine: 0.9 mg/dL (0 points)
• Hematocrit: 42% (0 points)
• WBC: 8.5 ×10³/mm³ (0 points)
• GCS: 15 (0 points)

Age: 35 years (0 points)

Chronic Health: None (0 points)

APACHE II Score: 0 + 0 + 0 = 0

Interpretation: APACHE II score of 0 indicates minimal disease severity. This patient has normal physiological parameters and no chronic health conditions. The low score reflects the patient's stable condition and suggests excellent prognosis. This patient likely requires ICU monitoring for reasons other than severe critical illness, such as postoperative observation or close monitoring of a reversible condition.

Case 2: Moderate APACHE II Score

Presentation: A 60-year-old woman with a history of diabetes and hypertension is admitted to the ICU with community-acquired pneumonia and respiratory failure requiring mechanical ventilation.

Physiological Values (worst during first 24 hours):

• Temperature: 38.8°C (1 point)
• MAP: 95 mmHg (0 points)
• Heart Rate: 115 bpm (2 points)
• Respiratory Rate: 28 breaths/min (1 point)
• PaO₂: 75 mmHg on FiO₂ 0.5 (4 points - using A-a gradient)
• Arterial pH: 7.32 (2 points)
• Sodium: 142 mEq/L (0 points)
• Potassium: 4.5 mEq/L (0 points)
• Creatinine: 1.8 mg/dL (2 points)
• Hematocrit: 38% (0 points)
• WBC: 18 ×10³/mm³ (1 point)
• GCS: 13 (2 points - sedated on ventilator)

Age: 60 years (3 points)

Chronic Health: Diabetes and hypertension, but no severe organ insufficiency (0 points)

APACHE II Score: 3 + 15 + 0 = 18

Interpretation: APACHE II score of 18 indicates moderate disease severity. The patient has significant respiratory failure requiring mechanical ventilation, as reflected in the oxygenation and pH abnormalities. The moderate score suggests moderate mortality risk, likely in the range of 20-30%. This patient requires intensive care and aggressive treatment, but has a reasonable chance of recovery with appropriate management. The score helps inform discussions with the family about prognosis and supports decisions about treatment intensity.

Case 3: High APACHE II Score

Presentation: A 72-year-old man with a history of cirrhosis and chronic kidney disease is admitted to the ICU with severe sepsis and multi-organ failure.

Physiological Values (worst during first 24 hours):

• Temperature: 39.5°C (3 points)
• MAP: 55 mmHg (4 points)
• Heart Rate: 145 bpm (3 points)
• Respiratory Rate: 38 breaths/min (3 points)
• PaO₂: 60 mmHg on FiO₂ 0.8 (4 points)
• Arterial pH: 7.18 (3 points)
• Sodium: 152 mEq/L (1 point)
• Potassium: 5.8 mEq/L (3 points)
• Creatinine: 4.2 mg/dL (4 points)
• Hematocrit: 28% (2 points)
• WBC: 25 ×10³/mm³ (2 points)
• GCS: 8 (4 points)

Age: 72 years (5 points)

Chronic Health: Cirrhosis and chronic kidney disease (5 points - non-operative with organ insufficiency)

APACHE II Score: 5 + 36 + 5 = 46

Interpretation: APACHE II score of 46 indicates extremely high disease severity and very high mortality risk, likely exceeding 70-80%. The patient has severe multi-organ failure affecting cardiovascular, respiratory, renal, and neurological systems. The combination of severe acute physiology abnormalities, advanced age, and significant chronic health conditions results in a very high score. This score supports discussions with the family about the grave prognosis and helps guide decisions about goals of care, including consideration of comfort measures and limitation of life-sustaining treatments. However, even with this high score, some patients may survive, and the score should not be used as the sole determinant of treatment decisions.

Comparison with Other Scoring Systems

APACHE III and IV

Newer versions of the APACHE system have been developed:

• APACHE III: Includes more variables and uses different weighting. More complex but potentially more accurate.
• APACHE IV: Further refined with additional variables and improved calibration. Most accurate but most complex.

However, APACHE II remains widely used because:

• It is simpler and easier to calculate
• It has extensive validation
• It requires fewer variables
• It is well-established in clinical practice

Simplified Acute Physiology Score (SAPS)

SAPS is another widely used ICU scoring system that:

• Uses fewer variables than APACHE II
• May be simpler to calculate
• Has been validated in multiple populations
• Provides similar predictive accuracy

Sequential Organ Failure Assessment (SOFA)

SOFA is designed to assess organ dysfunction over time:

• Can be calculated daily
• Focuses on organ-specific dysfunction
• Useful for tracking disease progression
• Less focused on mortality prediction than APACHE II

APACHE II and SOFA serve complementary purposes and may be used together in clinical practice.

Future Directions

Critical care scoring continues to evolve, with ongoing research focused on:

• Improving predictive accuracy
• Incorporating new biomarkers and technologies
• Developing disease-specific scoring systems
• Integrating machine learning and artificial intelligence
• Personalizing risk prediction
• Real-time scoring and monitoring

Despite these advances, APACHE II remains a valuable and widely used tool in critical care medicine, providing clinicians with an objective, standardized method for assessing disease severity and predicting mortality risk in ICU patients.