What Is the Difference Between Acute Lung Injury and ARDS?

If one doctor tells you your loved one has “acute lung injury” (ALI) and another calls it “acute respiratory distress syndrome” (ARDS), it is completely normal to feel confused and worried. The most important thing to know is that these terms describe the same pattern of severe lung inflammation, not two completely different diseases. ARDS is a syndrome—a collection of symptoms—that can be caused by various issues like severe infection, trauma, or sepsis.

To understand why both terms are still used, it helps to know how the medical guidelines have changed over time.

The History of the Terms

In 1994, medical experts created a classification system to help diagnose and treat inflammatory lung damage ^[1]. Under this older system, acute lung injury (ALI) was a broad category for this type of lung damage. ARDS was considered a more severe subset of ALI ^[1].

Because the categories overlapped, having two different names for the same process was confusing. In 2012, an updated set of guidelines called the Berlin Definition was introduced to improve how the condition is diagnosed ^[2].

The Berlin Definition: From ALI to ARDS Severity Levels

The 2012 Berlin Definition officially retired the term “acute lung injury” as a formal diagnosis ^[2]. Today, doctors classify the condition entirely under the umbrella of ARDS, divided into three levels of severity based on how well the lungs can move oxygen into the blood:

• Mild ARDS: This roughly corresponds to patients who previously would have been diagnosed with ALI but not ARDS under the older criteria ^[1][2].
• Moderate ARDS: The lungs are having a harder time transferring oxygen ^[2].
• Severe ARDS: The lungs are struggling significantly ^[2].

Doctors determine these levels by looking at a measurement called the PaO2/FiO2 ratio. This ratio compares the oxygen level in the arterial blood (PaO2) to the concentration of oxygen the patient is breathing in (FiO2) ^[2]. Under the Berlin Definition, these measurements are taken while the patient receives a specific amount of breathing support, such as positive end-expiratory pressure (PEEP) ^[2].

The thresholds are generally defined as:

• Mild ARDS: A ratio between 201 and 300 mm Hg ^[2].
• Moderate ARDS: A ratio between 101 and 200 mm Hg ^[2].
• Severe ARDS: A ratio of 100 mm Hg or less ^[2].

Why You Might Still Hear “Acute Lung Injury”

Even though the official medical terminology was updated in 2012 to use only “ARDS,” habits take time to change. Many doctors, nurses, and medical textbooks still use the term acute lung injury informally. They might use it to describe a milder case of ARDS, or simply as a general way to describe that the lungs are acutely injured and inflamed.

If a doctor uses the term ALI, it does not automatically mean a specific treatment plan is in place. Treatment always depends on the person’s current oxygenation, the underlying cause of the lung injury, and what level of breathing support they need. The care team is focused on supporting your loved one’s lungs while treating the underlying cause, regardless of which term they use in conversation.