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Hematology

Hematocrit

Overview of Hematocrit (HCT)

  • Definition: The hematocrit is the percentage of blood volume occupied by red blood cells (RBCs). It’s a simple but valuable measurement in assessing a patient’s red blood cell status
  • Alternate Names: Packed Cell Volume (PCV)
  • Clinical Significance:
    • Detection and Monitoring of Anemia: Low HCT indicates anemia (decreased red blood cell mass)
    • Detection of Polycythemia/Erythrocytosis: High HCT indicates increased red blood cell mass
    • Assessment of Dehydration: Can be elevated in dehydration due to decreased plasma volume
    • Monitoring Fluid Balance: Helps assess changes in blood volume
  • Methods:
    • Manual Methods: Centrifugation of blood in a capillary tube (microhematocrit method)
    • Automated Methods: Calculated by automated hematology analyzers based on RBC count and MCV

Manual Microhematocrit Method

  • Principle:

    • Whole blood is collected in a capillary tube coated with an anticoagulant (usually heparin)
    • The tube is centrifuged to pack the red blood cells at the bottom of the tube
    • The height of the packed red blood cell column is measured and compared to the total height of the blood column to determine the hematocrit percentage

  • Materials:

    • Capillary Tubes:
      • Glass or plastic capillary tubes (typically 75 mm long with an internal diameter of 1-2 mm)
      • Coated with an anticoagulant (heparin)
    • Centrifuge:
      • A specialized centrifuge designed to hold and spin capillary tubes at high speeds
      • Should be capable of generating a relative centrifugal force (RCF) of at least 10,000 g
    • Reader:
      • A device for accurately measuring the height of the packed red blood cell column and the total blood column in the capillary tube
      • Can be a microhematocrit reader card or a digital reader
    • Sealing Clay:
      • Used to seal one end of the capillary tube after filling

  • Procedure:

    1. Sample Collection:
      • Collect whole blood in an EDTA (purple-top) tube
      • Capillary blood (fingerstick or heelstick) can also be used, but is less accurate
    2. Filling the Capillary Tube:
      • Mix the blood sample thoroughly
      • Hold the capillary tube horizontally and touch one end to the blood sample
      • Allow the tube to fill by capillary action to at least 2/3 of its length
      • Avoid introducing air bubbles into the tube
    3. Sealing the Tube:
      • Wipe the outside of the capillary tube clean
      • Seal one end of the tube with sealing clay to prevent leakage during centrifugation
    4. Centrifugation:
      • Place the filled and sealed capillary tubes into the microhematocrit centrifuge
      • Centrifuge for the recommended time (typically 5 minutes) at the specified RCF (10,000-12,000 g)
    5. Reading the Hematocrit:
      • Remove the capillary tubes from the centrifuge
      • Carefully place the tube on the microhematocrit reader
      • Align the bottom of the packed red blood cell column with the 0% line on the reader
      • Align the top of the plasma layer with the 100% line on the reader
      • Read the hematocrit value at the interface between the packed red blood cells and the plasma
    6. Record and Report Results:
      • Record the hematocrit value as a percentage (%)
      • Report the result along with the reference range
      • Perform duplicate measurements and average the results (if duplicate results are within acceptable limits)

  • Sources of Error in Manual Hematocrit Measurement:

    • Improper Sealing of the Capillary Tube: Can lead to leakage and falsely low results
    • Inadequate Centrifugation: Insufficient packing of red blood cells can lead to falsely low results
    • Delay in Reading: Prolonged delay after centrifugation can lead to cell settling and inaccurate readings
    • Air Bubbles: Air bubbles in the capillary tube can interfere with the reading
    • Incorrect Use of the Reader: Not aligning the tube properly on the reader can lead to inaccurate results
    • Excess Anticoagulant: Overfilling a capillary tube with a blood sample in an anticoagulant collection tube is not recommended

Automated Hematocrit Measurement

  • Principle:

    • Automated hematology analyzers typically calculate the hematocrit based on the red blood cell count and the mean corpuscular volume (MCV)
    • Formula: HCT = (RBC Count x MCV) / 10

  • Procedure:

    • The automated analyzer aspirates a small amount of the whole blood sample
    • The RBC count and MCV are measured using electrical impedance, optical scatter, or other methods
    • The hematocrit is calculated using the formula above

  • Advantages:

    • Increased Speed and Efficiency
    • Improved Accuracy and Precision
    • Reduced Labor Costs

  • Limitations:

    • Can be affected by certain interfering substances or conditions (see below)
    • Does not directly measure the packed cell volume; it is a calculated value

Interfering Substances and Conditions

Several factors can interfere with both manual and automated hematocrit measurements, leading to inaccurate results. It’s crucial to be aware of these interferences and take appropriate corrective actions:

  • Lipemia:
    • Turbidity from high lipid content can falsely elevate the hemoglobin reading, which can affect the calculated hematocrit
    • Corrective Action: Perform a saline replacement procedure or use a lipemia clearing agent
  • High White Blood Cell Count (Leukocytosis):
    • Extreme leukocytosis can cause falsely elevated hemoglobin and hematocrit readings
    • Corrective Action: Dilute the sample and multiply the results by the dilution factor
  • Cold Agglutinins:
    • These antibodies can cause RBCs to clump together, leading to inaccurate cell counts and hematocrit measurement
    • Corrective Action: Warm the sample to 37°C to dissociate the agglutinins
  • Abnormal Red Blood Cell Size or Shape:
    • Spherocytes, schistocytes, and other abnormally shaped RBCs can affect the accuracy of both manual and automated hematocrit measurements
  • High Hemoglobin (Results beyond the linearity of the instrument)
    • Dilute the blood so that the Hgb result is within the linear range of the instrument

Quality Control (QC)

  • Calibrate the Instruments:

    • Perform regular calibration of the microhematocrit centrifuge and automated hematology analyzer according to the manufacturer’s instructions

  • Run Control Materials:

    • Run control materials at regular intervals (e.g., daily, with each batch of samples)
    • Use controls that are appropriate for the types of samples being analyzed (e.g., whole blood controls)

  • Review Control Results:

    • Review control results and evaluate them using statistical methods (e.g., Levey-Jennings charts, Westgard rules)
    • Take corrective action if control results are outside the acceptable range

  • Check the Rule of Three:

    • Verify that the hemoglobin, hematocrit, and RBC count are consistent with each other, using the “Rule of Three”:
      • HGB x 3 = HCT (± 3)
    • If the Rule of Three is not met, investigate potential errors

Reporting Results

  • Report the hematocrit value as a percentage (%)
  • Include the reference range for the patient’s age and sex
  • Note any abnormal results or flags
  • Document all quality control procedures and corrective actions taken

Key Terms

  • Hematocrit (HCT): The percentage of blood volume occupied by red blood cells
  • Packed Cell Volume (PCV): Another name for hematocrit
  • Microhematocrit: A manual method for measuring hematocrit using a capillary tube
  • EDTA: Ethylenediaminetetraacetic acid, an anticoagulant used in blood collection tubes
  • RCF: Relative centrifugal force, a measure of the force applied during centrifugation
  • Lipemia: The presence of excessive lipids in the blood
  • Saline Replacement: A procedure to remove lipemic plasma and replace it with saline
  • Quality Control: Procedures used to monitor the accuracy and precision of laboratory testing
  • Calibration: The process of adjusting an instrument to ensure accurate and reliable results