An Arterial Blood Gas (ABG) test measures the levels of oxygen and carbon dioxide in the blood, along with the blood’s acidity (pH balance). Unlike most routine blood draws, the ABG test requires a sample taken directly from an artery, which carries oxygenated blood away from the heart and lungs. This rapid assessment is used in time-sensitive situations to evaluate a patient’s gas exchange efficiency and overall chemical balance.
Why an ABG Test is Performed and How Blood is Collected
The ABG test is frequently ordered to assess how effectively the lungs transfer oxygen into the bloodstream and remove carbon dioxide. It is an invaluable tool for monitoring patients in critical care settings, such as the Intensive Care Unit (ICU), or those experiencing acute respiratory failure. Healthcare providers use the results to diagnose or manage various conditions, including severe lung diseases, shock, or metabolic disorders like diabetic ketoacidosis.
The procedure is distinct because the sample must come from an artery, not a vein, to measure oxygen before it has been consumed by the body’s tissues. The radial artery in the wrist is the most common site for collection due to its easy accessibility. Before drawing blood from the wrist, a healthcare professional may perform a simple check called the Allen test to confirm that blood flow to the hand is adequate through a secondary artery.
The collection process involves inserting a small needle directly into the artery, which can be more uncomfortable than a standard venous blood draw. Because the blood is under higher pressure in the artery, the technician must apply firm pressure to the puncture site for at least five minutes after the needle is withdrawn. The collected arterial blood sample must be analyzed quickly to ensure the accuracy of the gas measurements.
Defining the Key Measurements
The ABG test reports several specific values. The first and most fundamental value is the blood’s pH, which indicates the level of acidity or alkalinity. The human body tightly regulates this number, with a normal range falling between 7.35 and 7.45. A lower number means the blood is too acidic, and a higher number means it is too alkaline.
The PaO2, or partial pressure of oxygen, measures the amount of oxygen gas dissolved in the arterial blood. This value directly reflects the efficiency of oxygen uptake by the lungs and typically ranges from 75 to 100 millimeters of mercury (mmHg) in a healthy person. PaCO2, the partial pressure of carbon dioxide, is a measurement of the carbon dioxide gas dissolved in the blood. Since the lungs eliminate carbon dioxide, this value primarily serves as an indicator of respiratory function, with a normal range of 35 to 45 mmHg.
Bicarbonate (HCO3) is an electrically charged compound that acts as a buffer to maintain the body’s pH balance. Unlike the other values, bicarbonate is primarily regulated by the kidneys, so it represents the metabolic contribution to the acid-base system. This value is generally calculated from the pH and PaCO2 results, and a normal range is between 22 and 26 milliequivalents per liter (mEq/L).
Interpreting Acid-Base Balance
The combined results of the pH, PaCO2, and HCO3 allow doctors to determine the patient’s acid-base status. A pH below 7.35 signifies acidemia (excessive acid), while a pH above 7.45 indicates alkalemia (excessive base). The PaCO2 and HCO3 values are then assessed to identify the source of the imbalance.
If the PaCO2 is abnormal, the problem is considered respiratory in origin, meaning the lungs are not managing carbon dioxide effectively. An abnormal HCO3 points to a metabolic issue, often involving the kidneys or other chemical processes. For instance, a low pH coupled with a high PaCO2 is diagnostic of a respiratory acidosis, where the lungs are retaining too much acid-forming carbon dioxide. When both the PaCO2 and HCO3 are abnormal, the body may be attempting to compensate for the primary problem to restore the pH to a normal level.