Skip to main content

Chemistry

Precautions

Accurate blood gas analysis requires careful technique and attention to detail to ensure reliable results. This includes proper handling from collection to troubleshooting

  • Specimen Collection
  • Specimen Handling and Processing
  • Calibration and Quality Control
  • Interfering Substances
  • Troubleshooting

Specimen Collection

  • Patient Preparation
    • Steady State: Ensure the patient is in a steady state for at least 20-30 minutes before collection. Avoid recent changes in oxygen therapy or ventilation settings
    • Supplemental Oxygen: Document the amount of supplemental oxygen the patient is receiving (e.g., Fi\(O_2\), flow rate)
    • Anticoagulant Therapy: Be aware of patients on anticoagulant therapy, as this increases the risk of bleeding
  • Arterial vs. Venous Samples
    • Arterial samples are preferred for acid-base assessment and oxygenation status
    • Venous samples can be used for pH and carbon dioxide measurements, but oxygen levels will not be accurate
  • Site Selection
    • Radial Artery: Most common site due to accessibility and collateral circulation
    • Brachial Artery: Larger and easier to access, but higher risk of complications
    • Femoral Artery: Used in emergency situations, but highest risk of complications
  • Technique
    • Modified Allen Test: Perform a modified Allen test before radial artery puncture to assess collateral circulation
    • Proper Angle: Insert the needle at a 30-45 degree angle for radial artery puncture
    • Avoid Air Bubbles: Prevent air bubbles from entering the syringe, as this can affect gas measurements
  • Syringe Type
    • Use a pre-heparinized syringe designed for blood gas collection
    • Avoid using excessive heparin, as this can alter pH and electrolyte measurements
    • If using a dry heparin syringe, ensure the heparin is evenly distributed
  • Mixing
    • Immediately after collection, gently mix the sample by rolling the syringe to ensure adequate anticoagulation

Specimen Handling and Processing

  • Timing
    • Analyze the sample as soon as possible after collection (ideally within 15-30 minutes) to minimize changes in pH and gas levels due to cellular metabolism
  • Storage
    • If analysis is delayed, store the sample on ice to slow down cellular metabolism
    • Samples should be analyzed within 1-2 hours of collection, even when stored on ice
  • Air Exposure
    • Prevent air exposure, as this can affect Pa\(O_2\) and Pa\(CO_2\) measurements
    • If air bubbles are present, remove them immediately and mix the sample again
  • Clotting
    • Inspect the sample for clots. If clots are present, reject the sample
  • Transportation
    • Transport the sample to the laboratory in a timely manner, using appropriate temperature control (ice)

Interfering Substances

  • Air Contamination
    • Falsely elevates Pa\(O_2\) and decreases Pa\(CO_2\)
    • Prevent by carefully expelling air bubbles from the syringe
  • Excess Heparin
    • Can lower pH
    • Use pre-heparinized syringes or minimize the amount of heparin used
  • Metabolic Activity
    • Continued metabolism in the sample consumes oxygen and produces carbon dioxide, leading to decreased Pa\(O_2\) and increased Pa\(CO_2\)
    • Analyze samples promptly or store on ice to slow metabolism
  • Temperature
    • Temperature affects gas solubility and electrode function
    • Ensure the blood gas analyzer is temperature controlled
  • High Bilirubin or Lipids
    • Can interfere with some electrode measurements
    • Use methods less susceptible to interference or employ techniques to remove the interfering substance
  • Calibration Errors
    • Improper calibration can lead to inaccurate results
    • Follow the manufacturer’s calibration procedures and use certified reference materials

Troubleshooting

  • Unexpected Results
    • Check patient preparation
    • Review collection and processing procedures
    • Evaluate reagent quality and calibration
    • Consider interfering substances
    • Repeat the test using a new sample
  • Out-of-Control QC
    • Check the integrity of the QC materials
    • Prepare fresh QC materials
    • Re-calibrate the instrument
    • Review reagent quality
    • Repeat the QC testing
    • If the problem persists, contact the instrument manufacturer for assistance
  • Inconsistent Results
    • Check electrode performance, replace it as needed
    • Perform maintenance as instructed by the manufacturer
    • Verify proper instrument configuration

Key Terms

  • Pre-analytical: Processes that occur before the sample is analyzed
  • Analytical: Processes involved in analyzing the sample
  • Post-analytical: Processes that occur after the sample is analyzed
  • Arterial Blood Gas (ABG): A test that measures the levels of oxygen and carbon dioxide in arterial blood
  • Venous Blood Gas (VBG): A test that measures the levels of oxygen and carbon dioxide in venous blood
  • Calibration: Adjusting an instrument to ensure accurate readings
  • Quality Control: Samples used to monitor the accuracy and precision of a test
  • Interfering Substance: A substance that affects the accuracy of a test
  • Modified Allen Test: A test to assess collateral circulation in the hand
  • Collateral Circulation: Circulation in an area supplied by more than one artery or vein
  • Hypoventilation: The state in which a reduced amount of air enters the lungs
  • Hyperventilation: The state of breathing too fast or too deeply
  • Pa\(O_2\): Partial pressure of oxygen in arterial blood
  • Pa\(CO_2\): Partial pressure of carbon dioxide in arterial blood
  • Anticoagulation: Slows clotting of blood