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      Calc Function

    • Calcs that help predict probability of a diseaseDiagnosis
    • Subcategory of 'Diagnosis' designed to be very sensitiveRule Out
    • Disease is diagnosed: prognosticate to guide treatmentPrognosis
    • Numerical inputs and outputsFormula
    • Med treatment and moreTreatment
    • Suggested protocolsAlgorithm





    Chief Complaint


    Organ System


    Patent Pending

    A-a O₂ Gradient

    Assesses for degree of shunting and V/Q mismatch.


    We launched a COVID-19 Resource Center, including a critical review of recommended calcs.

    When to Use
    Why Use
    • Patients with unexplained hypoxia.
    • Patients with hypoxia exceeding the degree of their clinical illness.

    The A-a Gradient can help determine the cause of hypoxia; it pinpoints the location of the hypoxia as intra- or extra-pulmonary.

    mm Hg
    mm Hg
    mm Hg
    Confirmed positive
    Confirmed negative


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    Next Steps
    Creator Insights
    Dr. H. Fred Helmholz

    About the Creator

    H. Fred Helmholz, MD, (d. 2012) spent most of his career at the Mayo Graduate School of Medicine. He was active in most aspects of respiratory care which included research and clinical practice in high-altitude medicine, oxygen therapy, pulmonary function testing, and mechanical ventilation.

    To view Dr. H. Fred Helmholz's publications, visit PubMed

    Are you Dr. H. Fred Helmholz? Send us a message to review your photo and bio, and find out how to submit Creator Insights!
    MDCalc loves calculator creators – researchers who, through intelligent and often complex methods, discover tools that describe scientific facts that can then be applied in practice. These are real scientific discoveries about the nature of the human body, which can be invaluable to physicians taking care of patients.
    About the Creator
    Dr. H. Fred Helmholz
    Are you Dr. H. Fred Helmholz?