Micro method fir pcv

Reading File

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Reading File

Here is a complete, textbook-accurate account of the Micro Method for PCV (Packed Cell Volume / Hematocrit):

Micro Method for PCV (Microhematocrit)

Source: Henry's Clinical Diagnosis and Management by Laboratory Methods

Principle

The hematocrit (Hct) or PCV is the ratio of the volume of erythrocytes to the volume of whole blood. It is expressed as a decimal fraction (SI units, L/L) or as a percentage.

Equipment

Capillary (microhematocrit) tube: approximately 7 cm long, uniform bore of about 1 mm
For blood collected directly from a skin puncture: use heparinized capillary tubes
Microhematocrit centrifuge
Modeling clay (or capillary tube sealer)
Hematocrit reading device or millimeter rule + magnifying lens

Specimen

Venous blood anticoagulated with EDTA (K2-EDTA preferred) or dried heparin
Or direct capillary (skin puncture) blood
Before sampling, mix the blood thoroughly - at least 60 inversions for a 5 mL tube, or 2 minutes on a mechanical rotator. Inadequate mixing is the most common cause of imprecision

Procedure

Fill the capillary tube by capillary attraction from a free-flowing puncture wound or well-mixed venous sample. Fill to at least 5 cm (leave one end empty).
Seal the empty end with modeling clay.
Place the tube in the radial grooves of the microhematocrit centrifuge head with the sealed end away from the center. Rest the bottom of the tube against the rubber gasket to prevent breakage.
Centrifuge at 10,000-12,000 g for 5 minutes.
- If the Hct exceeds 50%, centrifuge for an additional 5 minutes to ensure minimal plasma trapping.
Read the result: measure the length of the total blood column and the red cell column alone using a millimeter rule + magnifying lens, or a commercial hematocrit reader.

Hct = Length of packed red cell column / Length of total blood column (cells + plasma)

Gross Examination After Spinning

After centrifugation, inspect the tube:

Note relative heights of: red cell column, buffy coat, plasma column
The buffy coat (red-gray layer between red cells and plasma) = platelets + leukocytes
Orange/green plasma = increased bilirubin
Pink/red plasma = hemoglobinemia (hemolysis)
Cloudy plasma = possible nephrosis or cryoglobulinemia

Reference Values

Group	Reference Range
Adult males	0.41 - 0.51 (41% - 51%)
Adult females	0.36 - 0.45 (36% - 45%)

Low PCV = anemia
High PCV = polycythemia

Sources of Error

Source	Effect
Inadequate centrifugation	Falsely HIGH PCV (red cells not fully packed)
Trapped plasma	Falsely HIGH PCV by 1-3% (more in sickle cell, spherocytosis, hypochromic anemia)
Excess EDTA / insufficient blood	Falsely LOW PCV (cell shrinkage) - Hb and cell counts unaffected
K3-EDTA (liquid) vs K2-EDTA	K3-EDTA shrinks red cells ~2%, lowers PCV
Failure to mix blood adequately	Imprecision
Including buffy coat in red cell column	Falsely HIGH PCV
Tourniquet stasis / posture / muscular activity	Can alter PCV

Key Points

With good technique, precision is ±1% CV
The micromethod requires correction for trapped plasma when used as a reference for calibrating automated instruments (ICSH, 1980)
PCV measures concentration of red cells, not total red cell mass - so it can be low in pregnancy (dilutional/hydremia) while total red cell mass is normal
PCV can be normal or even high in shock with hemoconcentration despite actual blood loss
PCV is unreliable immediately after blood loss or transfusion

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