Investigations

1st investigations to order

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New onset of bilateral opacities that is not fully explained by effusions, lobar/lung collapse, or nodules is part of the clinical diagnostic criteria for ARDS.[1] Therefore, chest x-ray is 100% sensitive.

Specificity is poor because other conditions may cause bilateral pulmonary infiltrates, including cardiogenic pulmonary edema and diffuse alveolar hemorrhage.com.bmj.content.model.Caption@39535d94[Figure caption and citation for the preceding image starts]: Chest x-ray image of bilateral infiltrates in a patient with ARDSFrom the personal collection of Dr Lorraine Ware; used with permission [Citation ends].

Result

bilateral infiltrates

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A PaO₂/FiO₂ (inspired oxygen) ratio of ≤300 on positive end-expiratory pressure (PEEP) or continuous positive airway pressure (CPAP) ≥5 cm H₂O is part of the diagnostic criteria for ARDS.[1]

It is 100% sensitive, but specificity is poor because many other conditions can cause hypoxemia.

Result

low partial oxygen pressure

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Sputum cultures are recommended to test for any possible underlying infection (as sepsis is the most common cause of ARDS).

Result

positive if underlying infection

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Blood cultures are recommended to test for any possible underlying infection (as sepsis is the most common cause of ARDS).

Result

positive if underlying infection

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A urine culture is recommended to test for any possible underlying infection (as sepsis is the most common cause of ARDS).

Result

positive if underlying infection

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Serum amylase and lipase, in conjunction with clinical assessment, can be used to help establish whether the patient has acute pancreatitis, a common cause of ARDS.[48] Both tests have similar sensitivity and specificity but lipase levels remain elevated for longer (up to 14 days after symptom onset vs. 5 days for amylase).[46] Its prolonged elevation creates a wider diagnostic window than amylase.

Result

amylase and/or lipase 3 times the upper limit of the normal range in cases of acute pancreatitis

Investigations to consider

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BNP levels <100 picograms/mL make heart failure unlikely and thus ARDS more likely.

BNP levels >500 picograms/mL make heart failure likely and thus ARDS less likely.

BNP levels between 100 and 500 picograms/mL are indeterminate.

BNP levels may be difficult to interpret in patients with acute or chronic kidney failure. However, BNP levels should be <200 picograms/mL in patients without heart failure with an estimated glomerular filtration rate <60 mL/minute.

Result

BNP levels <100 picograms/mL

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Abnormal left ventricular systolic or diastolic function suggests cardiogenic pulmonary edema rather than ARDS.

Some patients may have both ARDS and cardiac dysfunction.

Result

usually normal

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PAOP ≤18 mmHg suggests ARDS.

Pulmonary artery catheterization should not be used routinely to manage patients with ARDS.

Can be used to determine whether pulmonary edema is cardiogenic if the diagnosis is still in doubt after measuring BNP levels and carrying out echocardiography.

Some patients can have an increased left ventricular end-diastolic pressure superimposed on ARDS. For this reason, PAOP measurements are no longer included in the definition of ARDS.[1]

In the ARDS Network FACTT trial, approximately 20% of patients had an initial PAOP >18 mmHg, although elevations >24 mmHg were unusual.[41]

Result

pulmonary artery occlusion pressure (PAOP) ≤18 mmHg

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Recommended in some patients with suspected pneumonia and patients without a defined predisposing condition, to exclude a noninfectious parenchymal lung disease.

Avoid in patients with suspected COVID-19-related ARDS.[43]

Result

identification of infectious pathogens; characteristic findings of alternative diagnoses

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CT scanning of the thorax is not routinely required to diagnose or manage ARDS. A CT scan provides more information than a plain chest x-ray and may be helpful in some cases for diagnosing pneumonia or another underlying lung disease.

Result

may be helpful in identifying pulmonary causes of ARDS such as pneumonia

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