storage of blood), the HCT will be falsely increased, but usually do not exceed the upper reference limit (although this may mean a mildly anemic animal may no longer appear anemic). Studies in cows show a small difference in PCV with centrifugation times of 2 versus 4 minutes (decrease of 0-2% in 4 samples), with little change thereafter (Fisher 1962). Artifact : Insufficient centrifugation speed may falsely increase the PCV if the RBC do not pack properly.Test interpretation PCV measurement Increased values (erythrocythemia, erythrocytosis) In an animal with true intravascular hemolysis versus in vitro hemolysis, the PCV or HCT is a better indicator of the oxygen carrying capacity of blood than the hemoglobin (which includes free hemoglobin and that within intact red blood cells, with the latter being the oxygen carriers). This is considered an artifactual decrease with in vitro hemolysis. Hemolysis: Will decrease the HCT and PCV.Studies in lactating and non-lactating dairy cows show an average difference in PCV of 4% and 2%, respectively, in blood taken from the jugular vein and mammary gland, with lower PCV in the mammary gland ( Fisher 1962). Also red blood cells start to hemolyze with storage, resulting in false decreases in both the HCT and PCV. After this time, RBC tend to swell which increases the MCV, falsely increasing the HCT and PCV potentially (RBC do not pack as well). HCT and PCV are optimally stable for 24 hours at 4☌. Heparinized whole blood can also be used. Although citrate can be used, the volume of citrate in the tube (10% of the collection volume) will dilute the PCV or HCT accordingly. Whole blood, body cavity fluids (PCV only) AnticoagulantĮDTA is the preferred anticoagulant. % ÷ 100 = L/L Sample considerations Sample type The conversion formula to SI units is as follows The “buffy coat smear” has the advantage of providing a concentrated preparation of nucleated cells, which can be useful if looking for low-incidence infectious agents (e.g., Anaplasmaorganisms in leukocytes).īoth are expressed as % of the blood (SI units are L/L). Additionally, one can score and break the tube as desired to remove the plasma for refractometric protein estimation, or to extrude the buffy coat for smear-making. Examination of the “crit tube” can also provide subjective information about the color and clarity of the plasma (icterus, hemolysis, lipemia), and the size of the “buffy coat” (which contains WBC and platelets). At Cornell University, we centrifuge microhematocrit tubes for 10 minutes in ruminants versus 3 minutes in other species, because it is thought that red blood cells of ruminant do not “pack” as well as other species, however this remains to be tested. Unlike the HCT, this measurement is affected by plasma trapping and how the red blood cells pack within the column. It is the quickest and most readily available measure of the red blood cell component of blood. The PCV is measured as the height of the red cell column in a microhematocrit tube after centrifugation (see image at right). Packed cell volume (PCV): This is a directly measured value obtained from centrifuging blood in a microhematocrit tube in a microhematocrit centrifuge. hemolysis will decrease) will affect the HCT, but not necessarily, the PCV. storage of RBC may result in RBC swelling with an increased MCV, thus falsely increasing the HCT) or RBC count (e.g. Thus, anything that falsely increases or decreases the MCV (e.g. The formula used to calculate the HCT is as follows: Therefore, if there are any inaccuracies in measurement of the MCV or RBC count, the HCT will reflect those inaccuracies. It is the product of the mean cell volume ( MCV) and the red blood cell ( RBC) count, both of which are directly measured by the analyzer. Hematocrit: This is actually a calculated value obtained from modern automated hematology analyzers.Method of measurement PCV tubeĪlthough used synonymously, HCT and PCV actually represent different ways of measuring the proportion of blood composed of red blood cells. This can normalize a HCT/PCV in an anemic animal or may increase the HCT/PCV above the upper reference limit in a non-anemic animal, resulting in a relative erythrocytosis. Because HCT and PCV are affected by changes in water, the HCT and PCV may be high due to water losses in a dehydrated animal (depending on the extent of dehydration). An increase in red blood cell mass is equivalent to erythrocytosis and a decrease indicates an anemia. Hematocrit (HCT) and packed cell volume (PCV) are used to measure red blood cell mass.
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