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Hematology: packed cell volume
Synonym(s): PCV, Hematocrit
Overview
- High speed centrifugation separates layers of cells:
- Erythrocytes.
- Buffy coat (thrombocytes, leukocytes and nucleated erythrocytes).
- Plasma.
- Measurement of these components indicates the patient's erythrocytic and hydration status.
- PCV is calculated from a spun sample, hematocrit is usually calculated by automated counters from cumulative cell volumes of counted RBCs (Calculated hemocrit (%) = MCV (fl) x RBC (x10*12/l)/10).
- Relationship of Hbg and PCV: Hbg x 3 = PCV.
- Relationship of RBC and PCV: PCV / 6 = RBC. Is not valid when disease is present.
Uses
Alone
- Measurement of red blood cell mass - low error of 1-2%.
- Measurement of hemoconcentration - quick and accurate, useful for monitoring cases.
Estimated red blood cell counts and hemoglobin concentrations do not hold true in iron deficiency Anemia: blood loss anemia or in regenerative anemias Anemia: overview.
- Icterus index: plasma appears yellow.
- Lipemia: plasma appears cloudy in non-fasted animals and secondary to diabetes, pancreatitis, hypothyroidism, hyperadrenocorticism and liver disease.
- Hemolysis: plasma appears red.
- Leukocyte counts may be approximated by measuring buffy coat in the Wintrobe method.
In combination
- As part of routine hematological screen.
- With RBC indices: in diagnosis and classification of anemia.
- In diagnosis of polycythemia Polycythemia: secondary.
Other points
Anemia and dehydration have opposite effects on PCV, therefore if both conditions are present, PCV may not be a true reflection of the animal's status.Non-pathological changes
- Fear/excitement. Nervous animals or breeds, eg German Shepherd Dog, Poodle, Chihuahua, may show higher counts due to splenic contraction.
- Some breeds, eg Greyhound, Whippet, Lurcher, Borzoi, have higher normal counts (upper end of normal range or just above).
- Strenuous activity.
- Late pregnancy (low PCV).
- Age (lower PCV under 12 months of age).
Sampling
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Tests
Methodologies
Wintrobe method
- Hematocrit tube of uniform bore calibrated longitudinally into 10 cm or 100 mm. Numbers on the right read from 0-10 cm and are used to determine PCV. Numbers on the left are used for erythrocyte sedimentation rate.
- Fill the tube to the 10 cm mark with a well-mixed sample of blood in anticoagulant, ensuring that there are no bubbles.
- Centrifuge sample at 3000-4000 rpm for 30 min.
- The scale is then read at the top of the packed erythrocytes adjacent to the buffy coat. The number is multiplied by 10 to express PCV as % (SI units → expressed as l/l).
Microhematocrit method
- Fill a plain capillary tube with anticoagulated blood to 1 cm from the end and wipe the outside of the tube. Seal the vacant end of the tube with clay, plastic or by heating. If blood is taken directly from the animal (ear tip, nail, etc), use heparinized tubes.
- Using a microhematocrit centrifuge, spin for 5 min at 10,000 rpm or 2 min at 16,000 rpm.
- Read the PCV using one of a variety of microhematocrit readers.
- Advantages: small volume of blood required, quick to perform.
- Automatic counters calculate the hematocrit from the cumulative volume of the cells counted.
- Standardize technique using microhematocrit method.
- Adequate centrifugal force and time of centrifugation required for maximal cell packing and minimization of trapped plasma (5 min usually adequate for canine blood).
- Measure height of packed erythrocytes relative to total height at top of plasma column.
Calculated hematocrit (larger commercial analyzers) Q-BC technique
Availability
- Widely available at commercial laboratories.
- Facilities to measure PCV should be available in all practices.
Validity
Sensitivity
- Moderate to high.
Specificity
- Low.
Predictive value
- Good.
Technique intrinsic limitations
- Does not give direct information of red cell indices or of hemoglobin concentration.
- Highly repeatable and accurate with trained staff and/or commercial equipment.
Technician extrinsic limitations
- Requires practice to accurately read microhematocrit tubes.
Result Data
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Further Reading
Publications
Refereed papers
- Recent references from VetMedResource and PubMed.
Other sources of information
- Cowell R J, Tyler R D & Meinkoth J H (1999) Diagnostic Cytology and Hematology of the Dog and Cat. 2nd edn. Mosby, USA.
- Duncan J R, Prasse K W & Mahaffey E A (1994) Veterinary Laboratory Medicine Clinical Pathology. 3rd edn. Iowa University Press, USA.
- Jain N C (1993) Essentials of Veterinary Hematology. Lea & Febiger, USA.