Clinical Hematology Overview

Introduction

• Whole Blood Composition:
  • Comprised of erythrocytes, platelets, and leukocytes.
  • Cells are suspended in plasma, which is mainly water (approx. 90%).

Hematocrit (HCT)

• Definition:

  • The volume of packed red blood cells (RBCs) after centrifugation in a given volume of blood.
  • Expressed as a percentage (%) or liters per liter (l/l).
  • Often referred to as Packed Cell Volume (PCV).

• Measurement Method:

  • Recommended method for HCT determination is centrifugation.
  • Measures the ratio of the volume occupied by RBCs to the total volume of whole blood.

• Normal Hematocrit Values:

  • Adult Male: 42-52%
  • Adult Female: 37-47%
  • Newborn: 53-65%

Materials and Equipment

• Blood Sample:

  • Venous blood collected via venipuncture in EDTA tubes transferred into plain capillary tubes.
  • Capillary blood collected via skin puncture in heparinized capillary tubes.

• Required Equipment:

  • Microhematocrit centrifuge.
  • Sealing clay.
  • Graphic reading device.
  • Glass capillary tubes with narrow diameter.

Methodology for Testing

1. Draw blood sample into a capillary tube, filling it about 3/4 full.
2. Seal one end of the tube.
3. Centrifuge the sealed tube.
4. Measure the ratio of packed cells to plasma.

Important Considerations

• Testing Conditions and Lab Procedures:
  • Properly balance and label tubes in the centrifuge.
  • Seal end of the tube must be placed outward, open end towards center.
  • Ensure the centrifuge is covered during operation.

Reading the Results

• Visual Interpretation of Centrifugation Outcomes:
  • Identify layers: Plasma, Buffy coat (WBCs and platelets), and packed RBCs.

Sources of Error in Measurements

• Common factors affecting PCV value:
  1. Clotted or hemolysed samples.
  2. Excessive anticoagulant in the sample.
  3. Leakage from capillary tubes during centrifugation.
  4. Incorrect tube placement in centrifuge.
  5. Insufficient centrifugation time or speed.
  6. Mistakes during the reading of results.
  7. Sample obtained post-acute hemorrhage.

Indications for Hematocrit Measurement

• Request for HCT measurement may arise in conditions such as:
  1. Suspected anemia.
  2. Signs of dehydration.
  3. Evidence of bleeding.
  4. Various medical and surgical conditions.

Clinical Implications of Hematocrit Values

• Low Hematocrit:

  • Indicates low RBC count; potential causes include:
  1. Internal or external hemorrhage.
  2. Complications of chronic renal failure.
  3. Pernicious anemia (Vitamin B12 deficiency).
  4. Hemolysis associated with transfusions or autoimmune diseases.

• High Hematocrit:

  • Indicates elevated RBC count or reduced plasma volume; potential conditions include:
  1. Severe dehydration (burns, diarrhea, diuretics).
  2. Erythrocytosis (e.g., Polycythemia vera).
  3. Hemachromatosis (iron metabolism disorder).
  4. Commonly seen in newborns (especially premature infants).

Glossary

• Buffy Coat:

  • Light-colored layer of WBCs and platelets on top of red cells after centrifugation.

• Hematocrit:

  • Volume percentage of packed RBCs after centrifugation.

• Microhematocrit:

  • HCT performed in capillary tubes using small blood samples.

• Microhematocrit Centrifuge:

  • Instrument that spins capillary tubes to separate components rapidly.

• Packed Cell Column:

  • Layer of RBCs formed after centrifugation of whole blood.

References

• McKenzie, S., et al. (2019). Clinical Laboratory Hematology. 4th Ed. Pearson Education.
• Kaushansky, K., et al. (2016). Williams Hematology. 9th Ed. McGraw-Hill.
• Lewis, S., et al. (2012). Dacie and Lewis, Practical Haematology. 11th Ed. Churchill Livingstone Elsevier.
• Hoffbrand, A., & Moss, P. (2011). Essential Haematology. 6th Ed. Wiley Blackwell.
• Moss, P., et al. (2006). Essential Haematology. 5th Ed. Blackwell Science.
• Moore, G., & Knight, G. (2013). Essential Haematology. Oxford University Press.