Hemoglobin (Hb)

Hb, the main component of erythrocytes, serves as the vehicle for the transportation of oxygen and carbon dioxide. It is composed of amino acids that form a single protein called globin, and a compound called heme, which contains iron atoms and the red pigment porphyrin. It is the iron pigment that combines readily with oxygen and gives blood its characteristic red color. Each gram of Hb can carry 1.34 mL of oxygen per 100 mL of blood. The oxygen-combining capacity of the blood is directly proportional to the Hb concentration rather than to the RBC count because some RBCs contain more Hb than others. This is why Hb determinations are important in the evaluation of anemia.

The Hb determination is part of a CBC. It is used to screen for disease associated with anemia, to determine the severity of anemia, to monitor the response to treatment for anemia, and to evaluate polycythemia.

Hb also serves as an important buffer in the extracellular fluid. In tissue, the oxygen concentration is lower, and the carbon dioxide level and hydrogen ion concentration are higher. At a lower pH, more oxygen dissociates from Hb. The unoxygenated Hb binds to hydrogen ion, thereby raising the pH. As carbon dioxide diffuses into the RBC, carbonic anhydrase converts carbon dioxide to bicarbonate and protons. As the protons are bound to Hb, the bicarbonate ions leave the cell. For every bicarbonate ion leaving the cell, a chloride ion enters. The efficiency of this buffer system depends on the ability of the lungs and kidneys to eliminate, respectively, carbon dioxide and bicarbonate. Refer to the discussion of ABGs in Chapter 14.

Women: 12.0–16.0 g/dL or 120–160 g/L

Men: 14.0–17.4 g/dL or 140–174 g/L

Children:

0–2 weeks: 14.5–24.5 g/dL or 145–245 g/L
2–8 weeks: 12.5–20.5 g/dL or 125–205 g/L
2–6 months: 10.7–17.3 g/dL or 107–173 g/L
6 months to 1 year: 9.9–14.5 g/dL or 99–145 g/L
1–6 years: 9.5–14.1 g/dL or 95–141 g/L
6–16 years: 10.3–14.9 g/dL or 103–149 g/L
16–18 years: 11.1–15.7 g/dL or 111–157 g/L

Clinical Alert

The critical Hb value is 5.0 g/dL (50 g/L), a condition that leads to heart failure and death. A value >20 g/dL (>200 g/L) leads to clogging of the capillaries as a result of hemoconcentration

Obtain 5 mL of whole blood in a lavender-topped tube (with EDTA). Fill the Vacutainer tube at least three fourths full. Automated electronic devices are generally used to determine the Hb; however, a manual colorimetric procedure is also widely used. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.
Do not allow the blood sample to clot, or the results will be invalid. Place the specimen in a biohazard bag.

Decreased Hb levels are found in anemia states (a condition in which there is a reduction of Hb, Hct, or RBC values). The Hb must be evaluated along with the RBC count and Hct.
1. Iron deficiency, thalassemia, pernicious anemia, hemoglobinopathies
2. Liver disease, hypothyroidism
3. Hemorrhage (chronic or acute)
4. Hemolytic anemia caused by:
  1. Transfusions of incompatible blood
  2. Reactions to chemicals or drugs
  3. Reactions to infectious agents
  4. Reactions to physical agents (e.g., severe burns, artificial heart valves)
  5. Various systemic diseases, including but not limited to:
    1. Hodgkin disease
    2. Leukemia
    3. Lymphoma
    4. SLE
    5. Carcinomatosis
    6. Sarcoidosis
    7. Renal cortical necrosis
Increased Hb levels are found in:
1. Polycythemia vera
2. Heart disease
3. Chronic obstructive pulmonary disease (COPD)
4. Carbon monoxide exposure
Variation in Hb levels
1. Occurs after transfusions, hemorrhages, burns. (Hb and Hct are both high during and immediately after hemorrhage.)
2. The Hb and Hct provide valuable information in an emergency situation if they are interpreted not in an isolated fashion but in conjunction with other pertinent laboratory data.
3. Hb variants can cause variation in measured Hb:
  1. Methemoglobin (Hb M)
  2. Sickle cell hemoglobin (Hb S)
  3. Fetal hemoglobin (Hb F)
  4. Deoxyhemoglobin (HHb)

Note that the same underlying conditions cause an increase or decrease in Hct, Hb, and RBC values.

Clinical Implications of Polycythemia: Increased RBC Count, Hct, or Hb

Polycythemia is the term used to describe an abnormal increase in the number of RBCs. Although there are several tests to directly determine the RBC mass, these tests are expensive and somewhat cumbersome. For screening purposes, we rely on the Hct and Hb to evaluate polycythemia indirectly. Polycythemias are classified as follows:

Relative polycythemia: an increase in Hb, Hct, or RBC count caused by a decrease in the plasma volume (e.g., dehydration, spurious erythrocytosis from stress or smoking)
Absolute or true polycythemia
1. Primary (e.g., polycythemia vera, erythemic erythrocytosis)
2. Secondary
  1. Appropriate (an appropriate bone marrow response to physiologic conditions)
    1. Altitude
    2. Cardiopulmonary disorder
    3. Increased affinity for oxygen
  2. Inappropriate (an overproduction of RBCs not necessary to deliver oxygen to the tissues)
    1. Kidney tumor or cyst
    2. Hepatoma
    3. Cerebellar hemangioblastoma

Clinical Implications of Anemia: Decreased RBC Count, Hct, or Hb

Anemia is the term used to describe a condition in which there is a reduction in the number of circulating RBCs, amount of Hb, or volume of packed cells (Hct). A pathophysiologic classification of anemias based on their underlying mechanisms follows. Anemias are further explained in Chart 2.1. Anemias are classified as follows:

Hypoproliferative anemias (inadequate production of RBCs)
1. Marrow aplasias
2. Myelophthisic anemia
3. Anemia with blood dyscrasias
4. Anemia of chronic disease
5. Anemia with organ failure
Maturation defect anemias
1. Cytoplasmic: hypochromic anemias
2. Nuclear: megaloblastic anemias
3. Combined: myelodysplastic syndromes
Hyperproliferative anemias (decreased Hb or Hct despite an increased production of RBCs)
1. Hemorrhagic: acute blood loss
2. Hemolytic: a premature, accelerated destruction of RBCs
  1. Immune hemolysis
  2. Primary membrane
  3. Hemoglobinopathies
  4. Toxic hemolysis (physical–chemical)
  5. Traumatic or microangiopathic hemolysis
  6. Hypersplenism
  7. Enzymopathies
  8. Parasitic infections
Dilutional anemias
1. Pregnancy
2. Splenomegaly

Pretest Patient Care

Explain test purpose and procedure. Assess medication history.
Refer to standard pretest care for CBC and differential count. Also, follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

Review test results; report and record findings. Modify the nursing care plan as needed.
Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Monitor for anemia or polycythemia.
Refer to standard posttest care for CBC and differential count. Also, follow guidelines in Chapter 1 for safe, effective, informed posttest care.

People living at high altitudes have increased Hb values as well as increased Hct and RBC count.
Excessive fluid intake causes a decreased Hb.
Normally, the Hb is higher in infants (before active erythropoiesis begins).
Hb is normally decreased in pregnancy as a result of increased plasma volume.
There are many drugs that may cause a decreased Hb. Drugs that may cause an increased Hb include gentamicin and methyldopa.
Extreme physical exercise causes increased Hb.

section name header

Introduction ⬇

Normal Findings ⬆ ⬇

Procedure ⬆ ⬇

Clinical Implications ⬆ ⬇

Interventions ⬆ ⬇

Interfering Factors ⬆