Nurses Revision

Nurses Revision

HAEMATOLOGICAL INDICES - PCV, MCV, MCH

HAEMATOLOGICAL INDICES: PCV ESTIMATION

Student Objectives (PCV Experiment)

At the end of this experiment, you should be able to:

  • Identify all equipment and reagents used in the determination of PCV.
  • Define hematocrit, and explain its clinical significance.
  • Briefly describe physiological/pathological factors that cause decrease PCV.
  • List the possible sources of error in the determination of PCV.

Instruments & Reagents

For Venous Blood

  • Wintrobe tube
  • Pasteur pipette
  • Centrifuge
  • Anticoagulant: Potassium Oxalate crystals (EDTA can also be used)

For Capillary Blood

  • Heparinized capillary tubes
  • Micro-centrifuge

Procedure for Venous Blood PCV

Using Wintrobe Method

  1. Blood Collection: Perform venipuncture to collect blood into a tube with a pinch of oxalate crystals mixture.
  2. Mixing: Mix the blood with anticoagulant by rolling the tube between the palms of both hands.
  3. Transfer: Draw blood into a Pasteur pipette and introduce it into the Wintrobe tube.
  4. Wintrobe Tube Details:
    • Special centrifuge tube with uniform diameter throughout.
    • Holds about 1 ml of blood.
    • Graduations are scaled in reversed directions on each side so either plasma or cell volume can be read.
  5. Filling: Fill the Wintrobe tube with blood from a fine teat pipette up to the 100 mark (equivalent to 100%).
  6. Centrifugation: Centrifuge the tube.
  7. Reading: Read the PCV as a percentage of the total volume.

Procedure for Capillary Blood PCV

Using Microhematocrit Method

  1. Labeling: Using labeling paper, mark two micro capillary tubes as X and Y.
  2. Blood Sample: Place blood into watch glass X.
  3. Tube Filling:
    • Dip one end of tube X into the blood at an angle.
    • Allow tube to fill to 3/4 full by capillary attraction.
  4. Sealing:
    • Close the open end with index finger.
    • Lift tube off the blood and seal the end with plasticine wax.
    • Open the tip to remove excess wax.
  5. Centrifuge Setup:
    • Open micro centrifuge lid and unscrew top to expose segment carrier.
    • Fix micro capillary tubes (sealed end first) in segments X and Y.
  6. Centrifugation:
    • Close lid and start centrifuge.
    • Centrifuge for 5 minutes.
    • Gradually increase speed to 10,000 rpm.
  7. Reading:
    • Remove segments and place into micro hematocrit reader.
    • Position tube so total blood column reads from 0% to 100%.
    • Place movable arm so line cuts the interface between cells and plasma.
    • Record results in % volumes.

RESULTS

Measurements
PCV of Male:
PCV of Female:
Thickness of Buffy Coat:
Components Separated
Plasma (Top)
Buffy Coat (Middle)
Red Cells (Bottom)

DISCUSSION TOPICS

  • Comparison of both methods: Discuss the differences, advantages, and disadvantages between Venous (Wintrobe) vs Capillary (Microhematocrit) methods.
  • Clinical Application: Describe the use of PCV (Packed Cell Volume) in clinical practice.

CLINICAL SIGNIFICANCE OF ABSOLUTE CORPUSCULAR VALUES

Knowledge of hemoglobin level, RBC count, and PCV (Hematocrit) alone does not provide information about:

  • Average red blood cell volume.
  • Hb content per cell.
  • Percentage saturation with hemoglobin.

These parameters are crucial for diagnosing anemia types. While not obtainable directly through experimental methods, they can be calculated from three basic values: Hemoglobin (Hb), RBC count, and PCV.

Student Objectives (Corpuscular Values)

  • Explain the clinical significance of calculating absolute corpuscular values.
  • Describe the macro-corpuscular values and different formulas used in calculations.
  • Describe the classification of anemia based on hematological indices.

Calculations & Formulas

Required Basic Measurements: 1. Hb (g/100ml)
2. RBC count (×10⁶ cells/mm³)
3. PCV (% per 100ml blood)

1. Mean Corpuscular Volume (MCV)

Definition: Average volume of a single red blood cell, expressed in femtoliters (fl).

Formula:

MCV = (PCV × 10) / RBC count

OR: MCV = PCV per liter / RBC (10¹²/L)

Normal Range: 74 - 95 femtolitres

2. Mean Corpuscular Hemoglobin (MCH)

Definition: Average hemoglobin content (weight) in a single red blood cell, expressed in picograms (pg).

Clinical Use: Basis for classifying anemia into hypochromic, normochromic, and hyperchromic types.

Formula:

MCH = (Hb in g/100ml) / RBC count

(RBC count in million/mm³)

Normal Range: 27 - 32 pg

3. Mean Corpuscular Hemoglobin Concentration (MCHC)

Definition: Relationship between hemoglobin and volume in red blood cells, expressed as percentage saturation of cells with Hb (not whole blood).

Key Principle: RBCs cannot exceed ~36% Hb concentration due to limitations in Hb synthesizing machinery.
Formula:

MCHC = (Hb × 100) / PCV

Normal Range: 30 - 36%

Other Hematological Indices (for further reading):
  • Mean Corpuscular Diameter (MCD)
  • Color Index (CI)

QUESTIONS

1. Reliability

Giving a reason, state which of the corpuscular values (MCV, MCH or MCHC) is most reliable and useful clinically?

2. Physiological Limits

Why can't RBCs be filled beyond 36% with Hb?

3. Classification

How can you classify anemias on the basis of MCV and MCH?

Hematology Steeplechase: Hb, PCV & Indices

Hematology Steeplechase

Hb Estimation, PCV & Clinical Indices

Exam Focus:

  • Calculations: Know your formulas for MCV, MCH, and MCHC.
  • Equipment: Identify Sahli's vs. Wintrobe's tubes.
  • Layers: Locate the Buffy Coat.
  • Principles: Acid Hematin vs. Cyanmethemoglobin.
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