Acute respiratory distress syndrome laboratory tests

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Brian Shaller, M.D. [2] Abiodun Akanmode,M.D.[3]

Overview

Laboratory findings consistent with the diagnosis of ARDS include severe hypoxemia (low PaO₂ with decreased P/F ratio) and hypercapnia (increased PaCO₂) secondary to an increase in pulmonary dead space.

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). 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)

In addition to severe hypoxemia, hypercapnia (increased PaCO₂) secondary to an increased pulmonary dead space may also be present.^[1]

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:

Arterial blood gas analysis
Complete blood count (CBC) with differential
Comprehensive metabolic panel (serum electrolytes, blood urea nitrogen and creatinine, and tests of liver function)
Coagulation studies (partial thromboplastin time and prothrombin time with international normalized ratio)
Blood, sputum, and urine cultures
Serum lactate level

Further testing should be guided by clinical suspicion and the patient’s medical history. These laboratory tests include:

Serum lipase level
Urine or blood toxicology tests
Blood alcohol level
Human immunodeficiency virus (HIV) test
Respiratory virus screen (direct fluorescent antibody [DFA] or polymerase chain reaction [PCR] assay)
Influenza virus testing
Fungal cultures
Tests for atypical pathogens that may cause pneumonia:^[2]

Legionella pneumophila culture and urine antigen testing
Mycoplasma pneumoniae culture and antibody titers
Pneumocystis jirovecii sputum silver stain and culture
Mycobacterium tuberculosis sputum smear for acid-fast bacilli (AFB) and culture

References

↑ Kasper, Dennis (2015). Harrison’s principles of internal medicine. New York: McGraw Hill Education. ISBN 978-0071802154.
↑ Matthay MA, Zemans RL, Zimmerman GA, Arabi YM, Beitler JR, Mercat A; et al. (2019). “Acute respiratory distress syndrome”. Nat Rev Dis Primers. 5 (1): 18. doi:10.1038/s41572-019-0069-0. PMC 6709677 Check |pmc= value (help). PMID 30872586 PMID: 30872586 Check |pmid= value (help).

[1] Kasper, Dennis (2015). Harrison’s principles of internal medicine. New York: McGraw Hill Education. ISBN 978-0071802154.

[pmidPMID:_30872586-2] Matthay MA, Zemans RL, Zimmerman GA, Arabi YM, Beitler JR, Mercat A; et al. (2019). “Acute respiratory distress syndrome”. Nat Rev Dis Primers. 5 (1): 18. doi:10.1038/s41572-019-0069-0. PMC 6709677 Check |pmc= value (help). PMID 30872586 PMID: 30872586 Check |pmid= value (help).

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