Hemoglobin:- Part 4 – Hematocrit (Hct), Packed Cell Volume (PCV)
- The patient blood is taken in EDTA.
- It is stable for 48 hours at 4 °C and 6 hours at 23 °C.
- Fetal blood: Collected by percutaneous blood sampling.
Precautions for Hematocrit
- Perform the test within 6 hours of the blood collection.
- EDTA is the choice of blood anticoagulant.
- Avoid hemolysis.
- Avoid clotting of the blood.
- Centrifugation must be adequate. This will give a high result.
- The buffy coat is not included in the hematocrit.
- Avoid excess of the EDTA.
- Avoid overdilution of the blood sample by the anticoagulant.
- Avoid Prolonged tourniquet, it can lead to hemoconcentration and error in the Hct.
- Drugs like penicillin and chloramphenicol decrease the Hct level.
Purpose of the test (Indications)
- It is used to diagnose anemia.
- Hct is done in patients with bleeding or blood loss.
- It is part of a complete blood count.
Definition of hematocrit (Hct)
After the centrifugation of the EDTA blood in the test tube. Now the % of the RBC column to the original whole blood volume is called hematocrit (Hct).
Principle of hematocrit (Hct)
- The word hematocrit means to separate the blood where the plasma and blood cells are separated.
- This test gives information about RBC concentration and is helpful to see hemoconcentration.
- This is basically a measurement of total blood volume and RBCs ratio as a percentage.
- In a capillary tube or Hct tube blood is centrifuged. Cells mainly RBCs will settle down and clear plasma appears on top.
- Now, this is the ratio of settled cells and upper clear plasma and calculated in %.
Quality control of the Hemoglobin:
- This hemoglobin quality control can be done by checking Hb results in g/dL by comparing the result with hematocrit results.
- Hct results in % units, using the following formula:
- One g/dL Hb unit = 3 Hct unit
- For example, if the Hb is 13 g/dL
- Then Hct is = 13 x 3 = 39 %
- Now compare the result with the Hct value.
Factors that will affect Hematocrit are :
- Abnormalities of RBCs morphology will affect Hct.
- Raised values of WBC will alter the Hct.
- People from high altitudes have increased Hct.
- After the hemorrhage values are not reliable.
- Dehydration and hemodilution will affect the Hct.
- Pregnant ladies will have low values due to hemodilution.
- Chloramphenicol and Penicillin decreased value.
Pathophysiology of Hematocrit (Hct)
- Hct is a macroscopic observation where the percentage volume of the packed RBCs is measured.
- This is also called packed cell volume or PCV.
- The Hct closely reflects the hemoglobin and RBC values.
- Hct mostly depends upon the number of RBCs
- The word hematocrit means to ‘separate blood’ where blood cells and plasma are separated by centrifugation.
- While in electronic counters, it is calculated by MCV.
- A calculated Hct is lower 2% than the microhematocrit due to trapped plasma in a centrifuged column of RBCs.
- Hct is lower in pregnant women than nonpregnant ladies.
- Females have less Hct than men.
- Hct is usually three times the Hb concentration in grams/dl.
- Decreased values indicate anemia.
- Increased values indicate erythrocytosis.
- In dehydration, Hct will be falsely high due to less plasma volume and the number of RBCs is the same.
- In the case of large-size RBCs, the Hct will be falsely high.
- Blood transfusion is not recommended if the Hb is >8 g/dl or Hct is >24 %.
- Hct is more reliable and useful than the RBCs count.
Method to estimate the Hct:
- Microhematocrit tube method. These are as correct as of the tube method.
- Wintrobe hematocrit method is the macro method.
- Automated method.
- In an automated machine, it is calculated RBCs count and MCV.
- Hct = Hb x 3
- Hb = RBC in million x 3
- Hct = RBC in million x 9
- This formula depends upon the value of MCHC (33).
- This factor varies depending upon the MCHC from 2.7 to 3.2.
|Newborn||44 to 64|
|2 to 8 weeks||39 to 59|
|2 o 6 months||35 to 50|
|6 to 12 months||29 to 43|
|1 to 6 years||30 to 40|
|6 to 18 years||32 to 44|
|Male||42 to 52|
|Female||37 to 47|
|Old people||Values may slightly decrease|
|Fetal ( 26 to 30 weeks )||41 to 54 %|
|cord blood ( 9 months+ )||32 to 40 %|
|0 to 2 weeks||44 to 64 %|
|2 to 6 months||35 to 49 %|
|6 months to 1 year||29 to 43 %|
|1 to 6 years||30 to 40 %|
|6 to 18 years||32 to 44 %|
|Men||42 to 52%|
|Women||36 to 48%|
|Pregnant females||> 33 %|
- Decreased values are an indicator of Anemia.
- Decreased values are also seen in, Leukemia, Lymphoma, Hodgkin’s disease. and Myeloproliferative disorders.
Raised Hct is seen in:
- Polycythemia Vera.
- Extreme physical exercise or excitement.
- High Altitude.
- Dehydration leads to Hemoconcentration e.g. diarrhea, burns, and vomiting.
- Congenital heart failure.
- Severe chronic pulmonary obstructive disease (COPD).
Decreased Hct is seen in:
- Hemolytic anemia (Erythroblastosis fetalis, drug-induced hemolytic anemia, and paroxysmal nocturnal hemoglobinuria.
- Bone marrow failure
- Renal diseases.
- Normal pregnancy.
- Autoimmune diseases.
- malignancies like lymphoma, leukemia, multiple myeloma, and Hodgkin’s diseases.
- Normal pregnancy.
- Bone marrow failure.
Critical value of Hct = <15 % or >60 %.