Complete Blood Count, Hematocrit


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Complete Blood Count, Hematocrit

Synonym/acronym: Packed cell volume (PCV), Hct.

Common use

To evaluate anemia, polycythemia, and hydration status and to monitor therapy.

Specimen

Whole blood from one full lavender-top (EDTA) tube, Microtainer, or capillary. Whole blood from a green-top (lithium or sodium heparin) tube may also be submitted.

Normal findings

(Method: Automated, computerized, multichannel analyzers)
AgeConventional Units (%)SI Units (Conventional Units × 0.01)
Cord blood42–620.42–0.62
0–1 wk46–680.46–0.68
2–3 wk40–560.41–0.56
1–2 mo32–540.32–0.54
3 mo–5 yr31–430.31–0.43
6–8 yr33–410.33–0.41
9–14 yr33–450.33–0.45
15 yr–adult
 Male38–510.38–0.51
 Female33–450.33–0.45
Older adult
 Male36–520.36–0.52
 Female34–460.34–0.46

Description

Blood consists of a liquid plasma portion and a solid cellular portion. The solid portion is comprised of red blood cells (RBCs), white blood cells (WBCs), and platelets. It is important to be able to assess whether there is a sufficient number of circulating RBCs to transport the required amount of oxygen throughout the body. The hematocrit (Hct) is a mathematical expression of the number of RBCs, or packed cell volume, expressed as a percentage of whole blood. For example, a packed cell volume, or Hct of 45% means that a 100-mL sample of blood contains 45 mL of packed RBCs, which would reflect an acceptable level of RBCs for a patient of any given age. The Hct depends primarily on the number of RBCs, however the average size of the RBCs influences Hct. Conditions that cause RBC size to be increased (e.g. swelling of the RBC due to change in osmotic pressure related to elevated sodium levels) may increase the Hct while conditions that result in smaller than normal RBCs (e.g. microcytosis related to iron deficiency anemia) may decrease the Hct. Hematocrit can be estimated directly by centrifuging a sample of whole blood for a specific time period. As the blood spins it is separated into fractions.The RBC fraction is read against a scale. Most often the Hct is measured indirectly, by multiplying the RBC count and mean cell volume (MCV), using an automated cell counter. Hct can also be estimated by multiplying the hemoglobin by three.

The Hct level is part of the complete blood count (CBC). It is also frequently requested together with hemoglobin (Hgb) as an H&H. Hgb and Hct levels parallel each other and are the best determinant of the degree of anemia or polycythemia. Polycythemia is a term used in conjunction with conditions resulting from an abnormal increase in Hgb, Hct, and RBC counts. Anemia is a term associated with conditions resulting from an abnormal decrease in Hgb, Hct, and RBC counts. Results of the Hgb, Hct, and RBC counts should be evaluated simultaneously because the same underlying conditions affect this triad of tests similarly. The RBC count multiplied by 3 should approximate the Hgb concentration. The Hct should be within 3 times the Hgb if the RBC population is normal in size and shape. The Hct plus 6 should approximate the first two figures of the RBC count within 3 (e.g., Hct is 40%; therefore 40 + 6 = 46, and the RBC count should be 4.6 or in the range of 4.3 to 4.9). There are some cultural variations in Hgb and Hct (H&H) values. After the first decade of life, the mean Hgb in African Americans is 0.5 to 1 g lower than in whites. Mexican Americans and Asian Americans have higher H&H values than whites.

This procedure is contraindicated for

    N/A

Indications

  • Detect hematological disorder, neoplasm, or immunological abnormality
  • Determine the presence of hereditary hematological abnormality
  • Evaluate known or suspected anemia and related treatment, in combination with Hgb
  • Monitor blood loss and response to blood replacement, in combination with Hgb
  • Monitor the effects of physical or emotional stress on the patient
  • Monitor fluid imbalances or their treatment
  • Monitor hematological status during pregnancy, in combination with Hgb
  • Monitor the progression of nonhematological disorders such as chronic obstructive pulmonary disease, malabsorption syndromes, cancer, and renal disease
  • Monitor response to drugs or chemotherapy, and evaluate undesired reactions to drugs that may cause blood dyscrasias
  • Provide screening as part of a CBC in a general physical examination, especially upon admission to a health-care facility or before surgery

Potential diagnosis

Increased in

  • Burns (related to dehydration; total blood volume is decreased, but RBC count remains the same)
  • Congestive heart failure (when the underlying cause is anemia, the body responds by increasing production of RBCs with a corresponding increase in Hct)
  • Chronic obstructive pulmonary disease (related to chronic hypoxia that stimulates production of RBC and a corresponding increase in Hct)
  • Dehydration (total blood volume is decreased, but RBC count remains the same)
  • Erythrocytosis (total blood volume remains the same, but RBC count is increased)
  • Hemoconcentration (same effect as seen in dehydration)
  • High altitudes (related to hypoxia that stimulates production of RBC and therefore increases Hct)
  • Polycythemia (abnormal bone marrow response resulting in overproduction of RBC)
  • Shock

Decreased in

    Anemia (overall decrease in RBC and corresponding decrease in Hct) Blood loss (acute and chronic) (overall decrease in RBC and corresponding decrease in Hct) Bone marrow hyperplasia (bone marrow failure that results in decreased RBC production) Carcinoma (anemia is often associated with chronic disease) Cirrhosis (related to accumulation of fluid) Chronic disease (anemia is often associated with chronic disease) Fluid retention (dilutional effect of increased blood volume while RBC count remains stable) Hemoglobinopathies (reduced RBC survival with corresponding decrease in Hgb) Hemolytic disorders (e.g., hemolytic anemias, prosthetic valves) (reduced RBC survival with corresponding decrease in Hct) Hemorrhage (acute and chronic) (related to loss of RBC that exceeds rate of production) Hodgkin’s disease (bone marrow failure that results in decreased RBC production) Incompatible blood transfusion (reduced RBC survival with corresponding decrease in Hgb) Intravenous overload (dilutional effect) Fluid retention (dilutional effect of increased blood volume while RBC count remains stable) Leukemia (bone marrow failure that results in decreased RBC production) Lymphomas (bone marrow failure that results in decreased RBC production) Nutritional deficit (anemia related to dietary deficiency in iron, vitamins, folate needed to produce sufficient RBC; decreased RBC count with corresponding decrease in Hct) Pregnancy (related to anemia) Renal disease (related to decreased levels of erythropoietin, which stimulates production of RBCs) Splenomegaly (total blood volume remains the same, but spleen retains RBCs and Hct reflects decreased RBC count)

Critical findings

  • Adults & children
  • Less than 19.8% (SI: Less than 0.2 L/L)
  • Greater than 60% (SI: Greater than 0.6 L/L)
  • Newborns
  • Less than 28.5% (SI: Less than 0.28 L/L)
  • Greater than 66.9% (SI: Greater than 0.67 L/L)
  • Consideration may be given to verifying the critical findings before action is taken. Policies vary among facilities and may include requesting immediate recollection and retesting by the laboratory or retesting using a rapid Point of Care instrument at the bedside.

  • Note and immediately report to the health-care provider (HCP) any critically increased or decreased values and related symptoms.

  • It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.

  • Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

  • Low Hct leads to anemia. Anemia can be caused by blood loss, decreased blood cell production, increased blood cell destruction, and hemodilution. Causes of blood loss include menstrual excess or frequency, gastrointestinal bleeding, inflammatory bowel disease, and hematuria. Decreased blood cell production can be caused by folic acid deficiency, vitamin B12 deficiency, iron deficiency, and chronic disease. Increased blood cell destruction can be caused by a hemolytic reaction, chemical reaction, medication reaction, and sickle cell disease. Hemodilution can be caused by congestive heart failure, renal failure, polydipsia, and overhydration. Symptoms of anemia (due to these causes) include anxiety, dyspnea, edema, hypertension, hypotension, hypoxia, jugular venous distention, fatigue, pallor, rales, restlessness, and weakness. Treatment of anemia depends on the cause.

  • High Hct leads to polycythemia. Polycythemia can be caused by dehydration, decreased oxygen levels in the body, and an overproduction of RBCs by the bone marrow. Dehydration from diuretic use, vomiting, diarrhea, excessive sweating, severe burns, or decreased fluid intake decreases the plasma component of whole blood, thereby increasing the ratio of RBCs to plasma, and leads to a higher than normal Hct. Causes of decreased oxygen include smoking, exposure to carbon monoxide, high altitude, and chronic lung disease, which leads to a mild hemoconcentration of blood in the body to carry more oxygen to the body’s tissues. An overproduction of RBCs by the bone marrow leads to polycythemia vera, which is a rare chronic myeloproliferative disorder that leads to a severe hemoconcentration of blood. Severe hemoconcentration can lead to thrombosis (spontaneous blood clotting). Symptoms of hemoconcentration include decreased pulse pressure and volume, loss of skin turgor, dry mucous membranes, headaches, hepatomegaly, low central venous pressure, orthostatic hypotension, pruritus (especially after a hot bath), splenomegaly, tachycardia, thirst, tinnitus, vertigo, and weakness. Treatment of polycythemia depends on the cause. Possible interventions for hemoconcentration due to dehydration include intravenous fluids and discontinuance of diuretics if they are believed to be contributing to critically elevated Hct. Polycythemia due to decreased oxygen states can be treated by removal of the offending substance, such as smoke or carbon monoxide. Treatment includes oxygen therapy in cases of smoke inhalation, carbon monoxide poisoning, and desaturating chronic lung disease. Symptoms of polycythemic overload crisis include signs of thrombosis, pain and redness in the extremities, facial flushing, and irritability. Possible interventions for hemoconcentration due to polycythemia include therapeutic phlebotomy and intravenous fluids.

Interfering factors

  • Drugs and substances that may cause a decrease in Hct include those that induce hemolysis due to drug sensitivity or enzyme deficiency and those that result in anemia (see monograph titled “Complete Blood Count, RBC Count”).
  • Some drugs may also affect Hct values by increasing the RBC count (see monograph titled “Complete Blood Count, RBC Count”).
  • The results of RBC counts may vary depending on the patient’s position: Hct can decrease when the patient is recumbent as a result of hemodilution and can increase when the patient rises as a result of hemoconcentration.
  • Leaving the tourniquet in place for longer than 60 sec can falsely increase Hct levels by 2% to 5%.
  • Traumatic venipuncture and hemolysis may result in falsely decreased Hct values.
  • Failure to fill the tube sufficiently (i.e., tube less than three-quarters full) may yield inadequate sample volume for automated analyzers and may be a reason for specimen rejection.
  • Clotted or hemolyzed specimens must be rejected for analysis.
  • The results of a CBC should be carefully evaluated during transfusion or acute blood loss because the body is not in a state of homeostasis and values may be misleading. Considerations for draw times after transfusion include the type of product, the amount of product transfused, and the patient’s clinical situation. Generally, specimens collected an hour after transfusion will provide an acceptable reflection of the effects of the transfused product. Measurements taken during a massive transfusion are an exception, providing essential guidance for therapeutic decisions during critical care.
  • Abnormalities in the RBC size (macrocytes, microcytes) or shape (spherocytes, sickle cells) may alter Hct values, as in diseases and conditions including sickle cell anemia, hereditary spherocytosis, and iron deficiency.
  • Elevated blood glucose or serum sodium levels may produce elevated Hct levels because of swelling of the erythrocytes.

Nursing Implications and Procedure

Pretest

  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this test can assist in evaluating the body’s blood cell volume status.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s cardiovascular, gastrointestinal, hematopoietic, hepatobiliary, immune, and respiratory systems; symptoms; and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Note any recent procedures that can interfere with test results.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.

Intratest

  • Potential complications: N/A
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate tubes with the corresponding patient demographics, date, and time of collection. Perform a venipuncture; collect the specimen in a 5-mL lavender-top (EDTA) tube. An EDTA Microtainer sample may be obtained from infants, children, and adults for whom venipuncture may not be feasible. The specimen should be mixed gently by inverting the tube 10 times. The specimen should be analyzed within 24 hr when stored at room temperature or within 48 hr if stored at refrigerated temperature. If it is anticipated the specimen will not be analyzed within 24 hr, two blood smears should be made immediately after the venipuncture and submitted with the blood sample. Smears made from specimens older than 24 hr may contain an unacceptable number of misleading artifactual abnormalities of the RBCs, such as echinocytes and spherocytes, as well as necrobiotic white blood cells.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis.

Post-Test

  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Nutritional Considerations: Nutritional therapy may be indicated for patients with increased Hct if iron levels are also elevated. Educate the patient with abnormally elevated iron values, as appropriate, on the importance of reading food labels. Patients with hemochromatosis or acute pernicious anemia should be educated to avoid foods rich in iron. Iron absorption is affected by numerous factors that may enhance or decrease absorption regardless of the original content of the iron-containing dietary source (see monograph titled “Iron”). Iron levels in foods can be increased if foods are cooked in cookware containing iron. Consumption of large amounts of alcohol damages the intestine and allows increased absorption of iron. A high intake of calcium and ascorbic acid also increases iron absorption. Iron absorption after a meal is also increased by factors in meat, fish, and poultry.
  • Nutritional Considerations: Nutritional therapy may be indicated for patients with decreased Hct. Iron deficiency is the most common nutrient deficiency in the United States. Patients at risk (e.g., children, pregnant women, women of childbearing age, and low-income populations) should be instructed to include in their diet foods that are high in iron, such as meats (especially liver), eggs, grains, green leafy vegetables, and multivitamins with iron. Educate the patient with abnormally elevated iron values, as appropriate, on the importance of reading food labels. Iron absorption is affected by numerous factors, enhancing or decreasing absorption regardless of the original content of the iron containing dietary source (see monograph titled “Iron”). Iron absorption is affected by numerous factors, enhancing or decreasing absorption regardless of the original content of the iron containing dietary source (see monograph titled “Iron”). Iron absorption is decreased by the absence (gastric resection) or diminished presence (use of antacids) of gastric acid. Phytic acids from cereals; tannins from tea and coffee; oxalic acid from vegetables; and minerals such as copper, zinc, and manganese interfere with iron absorption.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family. Educate the patient regarding access to nutritional counseling services. Provide contact information, if desired, for the Institute of Medicine of the National Academies (www.iom.edu).
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include biopsy bone marrow, CBC, CBC hemoglobin, CBC RBC indices, CBC RBC morphology, erythropoietin, ferritin, iron/TIBC, reticulocyte count, and US abdomen.
  • Refer to the Cardiovascular, Gastrointestinal, Hematopoietic, Hepatobiliary, Immune, and Respiratory systems tables at the end of the book for related tests by body system.
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