Respiratory failure laboratory findings

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


Laboratory findings consistent with the diagnosis of respiratory failure include abnormal bicarbonate, oxygen, phosphate, and magnesium levels.

Tests of Oxygenation

PaO2/FiO2 ratio (PF ratio)
<math>{P/F\ ratio} = \left (\frac{PaO_2}{Fi0_2}\right) \times 100</math>
  • Normal is 500
  • ARDS is < 200

This measure is easier to calculate. Comparative studies suggest it correlates better with pulmonary shunts than does the A-a gradient.[1][2][3]

Alveolar-arterial oxygen (A-a) gradient (alveolar-arterial oxygen difference - AVO2D)
<math>\mbox{A-a gradient} = {PAO_2}\ -\ {PaO_2}</math>
<math>{PAO_2} = {Fi0_2} *\left ({760 - 47}\right) \ -\ \frac{PaCO_2}{0.8}</math>
  • Normal is < 10 mm Hg

The A-a gradient is harder to calculate, but accounts for changes in respiration as measured by the partial pressure of carbon dioxide. However, this calculation relies on the respiratory quotient being constant in the prediction of alveolar CO2 When compared to the PF ratio, the A-a gradient is found to correlate less well with pulmonary shunting.[1][2][3]

Among outpatients with possible pulmonary embolism, the A-a gradient may be a better test.[4]

An online calculator for the A-a gradient is at

Other Laboratory Findings


  1. 1.0 1.1 Covelli HD, Nessan VJ, Tuttle WK (1983). "Oxygen derived variables in acute respiratory failure". Crit. Care Med. 11 (8): 646–9. PMID 6409506.
  2. 2.0 2.1 El-Khatib MF, Jamaleddine GW (2004). "A new oxygenation index for reflecting intrapulmonary shunting in patients undergoing open-heart surgery". Chest. 125 (2): 592–6. PMID 14769743.
  3. 3.0 3.1 Cane RD, Shapiro BA, Templin R, Walther K (1988). "Unreliability of oxygen tension-based indices in reflecting intrapulmonary shunting in critically ill patients". Crit. Care Med. 16 (12): 1243–5. PMID 3191742.
  4. McFarlane MJ, Imperiale TF (1994). "Use of the alveolar-arterial oxygen gradient in the diagnosis of pulmonary embolism". Am. J. Med. 96 (1): 57–62. PMID 8304364.
  5. O'Driscoll BR, Howard LS, Earis J, Mak V (June 2017). "BTS guideline for oxygen use in adults in healthcare and emergency settings". Thorax. 72 (Suppl 1): ii1–ii90. doi:10.1136/thoraxjnl-2016-209729. PMID 28507176.
  6. Celli BR, MacNee W (June 2004). "Standards for the diagnosis and treatment of patients with COPD: a summary of the ATS/ERS position paper". Eur. Respir. J. 23 (6): 932–46. PMID 15219010.

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