Acute respiratory distress syndrome laboratory tests

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1], Associate Editor(s)-in-Chief: Brian Shaller, M.D. [2]

Overview

Laboratory findings consistent with the diagnosis of ARDS include an arterial partial pressure of oxygen (PaO₂) that is inappropriately low relative to the fraction of inspired oxygen (FIO₂) that is being breathed by the patient. This is referred to as the PaO₂/FIO₂ ratio (sometimes abbreviated as P/F ratio) and is calculated by dividing the PaO₂ (in mmHg) by the FIO₂ (as a decimal rather than a percentage).

Laboratory Findings

Laboratory findings consistent with the diagnosis of ARDS include an arterial partial pressure of oxygen (PaO₂) that is inappropriately low relative to the fraction of inspired oxygen (FIO₂) that is being breathed by the patient. This is referred to as the PaO₂/FIO₂ ratio (sometimes abbreviated as P/F ratio) and is calculated by dividing the PaO₂ (in mmHg) by the FIO₂ (as a decimal rather than a percentage). Below are three examples of P/F ratio calculations that correspond to the three different degrees of ARDS severity:

A patient with a PaO₂ of 80 mm Hg who is mechanically ventilated with an FIO₂ of 35% and a PEEP of 5 cm H₂O has a PaO₂/FIO₂ ratio of 80/0.35 = 229 (mild ARDS)
A patient with a PaO₂ of 80 mm Hg who is mechanically ventilated with an FIO₂ of 50% and a PEEP of 5 cm H₂O has a PaO₂/FIO₂ ratio of 80/0.40 = 160 (moderate ARDS)
A patient with a PaO₂ of 80 mm Hg who is mechanically ventilated with an FIO₂ of 80% and a PEEP of 5 cm H₂O has a PaO₂/FIO₂ ratio of 80/0.80 = 100 (severe ARDS)

Additional Laboratory Tests

Other notable diagnostic findings are those that support an underlying diagnosis that may have led to the development of ARDS (e.g., positive blood cultures and an elevated white blood cell count in a septic patient or an elevated lipase in a patient with acute pancreatitis). Additional laboratory tests that may be helpful in diagnosing the underlying cause of ARDS include: