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Chemistry

Special Precautions

Accurate carbohydrate testing hinges on proper technique. It starts from the moment of specimen collection and continues through to troubleshooting any issues that might pop up

Special Precautions

  • Patient Preparation
    • Fasting: For accurate glucose and lipid measurements, patients typically need to fast for 8-12 hours before specimen collection. This ensures that postprandial effects don’t skew the results
    • Medications: Some drugs can influence carbohydrate metabolism. It’s important to know if the patient is taking corticosteroids, diuretics, or other meds that could impact glucose levels
    • Infection/Stress: Illness and stress can elevate glucose. If the patient is acutely ill, note this on the request form, as it can influence interpretation
  • Specimen Collection
    • Timing: Strict adherence to collection times is essential, especially for glucose tolerance tests. Meticulously document the time each sample is drawn
    • Order of Draw: Follow the correct order of draw when collecting multiple tubes. This minimizes the risk of cross-contamination from tube additives
    • Anticoagulants: Use the right anticoagulant for the test. For glucose, fluoride oxalate tubes are often used because they inhibit glycolysis
  • Specimen Handling
    • Prompt Processing: Glucose levels can change quickly in unseparated samples as cells continue to metabolize glucose. Process the sample as soon as possible
    • Temperature Control: Keep samples refrigerated (2-8°C) if testing is delayed, but avoid freezing whole blood
  • Reagent Handling
    • Storage: Store reagents according to the manufacturer’s instructions. Incorrect storage can degrade reagents, leading to inaccurate results
    • Expiration Dates: Always check expiration dates and discard expired reagents
    • Contamination: Prevent contamination of reagents. Use clean technique when handling, and avoid introducing bacteria or other substances that could interfere
  • Instrumentation
    • Calibration: Regularly calibrate instruments using appropriate standards. This ensures that readings are accurate and consistent over time
    • Maintenance: Follow the instrument’s maintenance schedule. Routine maintenance can prevent breakdowns and maintain performance
    • Quality Control: Run quality control samples at regular intervals to monitor the accuracy and precision of the instrument

Specimen Collection and Processing

  • Glucose
    • Specimen: Plasma (fluoride oxalate tube), serum
    • Collection: Fasting sample preferred. Collect in appropriate tube, invert gently to mix with anticoagulant
    • Processing: Centrifuge promptly (within 30 minutes of collection) to separate plasma or serum. Analyze immediately or refrigerate
    • Storage: Stable for several days refrigerated
  • Glucose Tolerance Test (GTT)
    • Specimen: Plasma (fluoride oxalate tube)
    • Collection
      • Patient fasts for 8-12 hours
      • Baseline sample taken
      • Patient drinks a standard glucose solution
      • Samples collected at specific intervals (e.g., 30 min, 1 hr, 2 hr, 3 hr)
    • Processing: Centrifuge promptly, analyze immediately or refrigerate
  • Glycated Hemoglobin (HbA1c)
    • Specimen: Whole blood (EDTA)
    • Collection: Routine venipuncture, no fasting required
    • Processing: Analyze directly or store refrigerated
    • Storage: Stable for up to a week refrigerated
  • Urine Glucose
    • Specimen: Random or timed urine sample
    • Collection: Collect in a clean container
    • Processing: Analyze immediately, as glucose can be consumed by bacteria
    • Storage: If delay is unavoidable, refrigerate, but prompt analysis is best
  • Ketones
    • Specimen: Serum, plasma, or urine
    • Collection: Collect in appropriate container
    • Processing: Analyze immediately or refrigerate

Troubleshooting

  • High Glucose Results
    • Pre-Analytical
      • Patient not fasting
      • Delayed processing
      • Contamination of specimen
    • Analytical
      • Incorrect calibration
      • Reagent deterioration
      • Interfering substances
  • Low Glucose Results
    • Pre-Analytical
      • Prolonged storage without proper anticoagulant
      • Glycolysis by cells
    • Analytical
      • Incorrect calibration
      • Reagent deterioration
  • HbA1c Discrepancies
    • Pre-Analytical
      • Improper collection tube
      • Storage issues
    • Analytical
      • Interfering hemoglobin variants
      • Method-specific issues
  • GTT Curve Abnormalities
    • Pre-Analytical
      • Incorrect glucose dose
      • Missed collection times
      • Patient vomiting
  • Urine Glucose Discrepancies
    • Pre-Analytical
      • Contamination
      • Improper storage
    • Analytical
      • Interfering substances affecting dipstick results

Interfering Substances

  • Ascorbic Acid (Vitamin C)
    • Interference: Can cause falsely low glucose results with some glucose oxidase methods
    • Mechanism: Ascorbic acid is a reducing agent that interferes with the chromogenic reaction
    • Mitigation: Use glucose oxidase methods that minimize ascorbic acid interference, or hexokinase methods
  • Bilirubin
    • Interference: High bilirubin levels can interfere with spectrophotometric assays
    • Mechanism: Bilirubin absorbs light at similar wavelengths, affecting absorbance readings
    • Mitigation: Use methods less susceptible to bilirubin interference, or employ bilirubin correction techniques
  • Lipemia
    • Interference: Turbidity from high lipid concentrations can affect spectrophotometric readings
    • Mechanism: Lipids scatter light, leading to inaccurate absorbance measurements
    • Mitigation: Use lipemia clearing techniques (ultracentrifugation, lipid clearing reagents), or consider methods less affected by turbidity
  • Hemoglobin
    • Interference: Hemolysis releases hemoglobin, which can interfere with enzymatic reactions
    • Mechanism: Hemoglobin can absorb light and interfere with enzyme activity
    • Mitigation: Avoid hemolysis during collection and processing
  • Urea
    • Interference: High urea concentrations can interfere with some enzymatic reactions
    • Mechanism: Urea can alter enzyme activity
    • Mitigation: Use methods less susceptible to urea interference
  • Drugs
    • Interference: Certain drugs can directly affect glucose levels
    • Examples
      • Corticosteroids: Increase glucose
      • Insulin: Decreases glucose
      • Diuretics: Can increase glucose
    • Mitigation: Be aware of patient’s medication list
  • Hemoglobin Variants
    • Interference: Can affect HbA1c measurements
    • Mechanism: Some variants don’t glycate normally or interfere with antibody binding
    • Mitigation: Use HbA1c methods that are not affected by common variants
  • Salicylates
    • Interference: High doses can cause hypoglycemia
    • Mechanism: Increased insulin secretion
    • Mitigation: Consider patient history

Key Terms

  • Pre-analytical: Processes that occur before the sample is analyzed
  • Analytical: Processes involved in analyzing the sample
  • Interfering Substance: A substance that affects the accuracy of a test
  • Glycolysis: The breakdown of glucose by cells
  • Calibration: Adjusting an instrument to ensure accurate readings
  • Quality Control: Samples used to monitor the accuracy and precision of a test
  • Lipemia: The presence of excess lipids in the blood
  • Hemolysis: The breakdown of red blood cells
  • Fluoride Oxalate: An anticoagulant that inhibits glycolysis