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Immunology & Autoimmune
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The P O2 Capillary test measures the partial pressure of oxygen dissolved in a small sample of capillary blood, usually collected from a warmed fingertip or earlobe. It is part of blood gas testing and reflects the amount of oxygen available to tissues at the moment of sampling. Results are reported in millimeters of mercury (mmHg).
This test helps estimate how well oxygen moves from your lungs into your blood. When the skin is properly warmed to improve blood flow, capillary values can approximate arterial results, offering a less invasive option than an arterial puncture, especially in children or when frequent checks are needed.
Clinicians use capillary pO2 to assess breathing problems, monitor chronic lung or heart conditions, and guide oxygen therapy. It can help evaluate changes during acute illness, recovery from surgery, or response to treatments such as inhaled medications or ventilatory support. In pediatrics, it can reduce the need for arterial blood draws while still providing useful information.
The result offers insight into oxygen transfer in the lungs, but it is interpreted together with other blood gas values and your symptoms. A simple fingerstick carries small risks such as brief pain, bruising, or local infection, and the sample can be less reliable if blood flow to the skin is poor.
Your provider will interpret your capillary pO2 in the context of how you feel, your physical exam, and other blood gas measurements such as carbon dioxide, pH, and oxygen saturation. Capillary values can differ from arterial values, especially if the puncture site is not well warmed or if circulation to the skin is reduced.
If your result does not match your symptoms or pulse oximetry, your team may repeat the test, check an arterial blood gas, or adjust oxygen therapy. Factors like altitude, recent activity, medications, and underlying lung or heart disease are considered. Follow up may include monitoring, treating the cause of low oxygen levels, or changing respiratory support under medical guidance.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
Breathing oxygen during or shortly before sampling can raise the measured pO2. Your care team needs to know the flow rate and delivery method.
Adequate warming arterializes the capillary bed and reduces error. Air bubbles, prolonged delays to analysis, or squeezing the site can skew results.
Cold extremities, shock, severe vasoconstriction, or poor circulation can cause capillary values to deviate from arterial values and reduce reliability.
Higher altitude and low ambient oxygen lower the inspired oxygen pressure, which can influence measured pO2 in otherwise healthy people.
Opioids, sedatives, and some anesthetics can depress breathing and reduce oxygen levels. Inhaled bronchodilators may improve oxygenation.
CO exposure can cause tissue hypoxia without lowering pO2, so clinicians may order co-oximetry if exposure is suspected or symptoms do not fit.
Expected values differ in infants and young children. Providers use age-appropriate ranges and may prefer capillary sampling in pediatrics.
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