Background
Hemoglobin (Hb), the main component of erythrocytes (red blood cells [RBCs]), 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. Therefore, Hb determinations are important in the evaluation of anemia.
The Hb determination is part of a complete blood cell count. 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.
Normal findings
* Women: 12.0 to 16.0 g/dL or 120 to 160 g/L
* Men: 14.0 to 17.4 g/dL or 140 to 174 g/L.
Clinical alert
An Hb value less than 5.0 g/dL (50 g/L) can lead to heart failure and death. A value greater than 20 g/dL (200 g/L) can lead to obstruction of the capillaries as a result of hemoconcentration.
Clinical implications
* Decreased Hb levels are found in anemia states (a condition in which there is a reduction of Hb, hematocrit [HCT], or RBC values). The Hb must be evaluated along with the RBC count and HCT.
* iron deficiency, thalassemia, pernicious anemia, hemoglobinopathies
* liver disease, hypothyroidism
* hemorrhage (chronic or acute)
* hemolytic anemia caused by:
* transfusions of incompatible blood
* reactions to chemicals or drugs
* reactions to infectious agents
* reactions to physical agents (severe burns, artificial heat valves)
* various systemic diseases, including:
* Hodgkin disease
* leukemia
* lymphoma
* systemic lupus erythematosus
* carcinomatosis
* sarcoidosis
* renal cortical necrosis
* Increased Hb levels are found in:
* polycythemia vera
* heart failure
* chronic obstructive pulmonary disease
* Variation in Hb levels:
* occurs after transfusions, hemorrhages, burns. (Hb and HCT are both high during and immediately after hemorrhage.)
* The Hb and HCT provide valuable information in an emergency if they are interpreted not in an isolated fashion but in conjunction with other pertinent lab data.
* Hb variants can cause variation in measured Hb:
* methemoglobin
* sickle cell hemoglobin
* fetal hemoglobin
* deoxyhemoglobin.
Clinical implications of polycythemia
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 (dehydration, spurious erythrocytosis from stress or smoking)
* Absolute or true polycythemia.
* primary (polycythemia vera)
* secondary
* appropriate (an appropriate bone marrow response to physiologic conditions)
- high altitude
- cardiopulmonary disorder
- increased affinity for oxygen
* inappropriate (an overproduction of RBCs not necessary to deliver oxygen to the tissues)
- renal tumor or cyst
- hepatoma
- cerebellar hemangioblastoma
Clinical implications of anemia
Anemias are classified as follows:
* hypoproliferative anemias (inadequate production of RBCs)
* marrow aplasia
* myelophthisic anemia
* anemia with blood dyscrasias
* anemia of chronic disease
* anemia with organ failure
* maturation defect anemias
* cytoplasmic: hypochromic anemias
* nuclear: megaloblastic anemias
* combined: myelodysplastic syndromes
* hyperproliferative anemias (decreased Hb or HCT despite an increased production of RBCs)
* hemorrhagic: acute blood loss
* hemolytic: a premature, accelerated destruction of RBCs
* immune hemolysis
* hemoglobinopathies
* toxic hemolysis (physical-chemical)
* traumatic or microangiopathic hemolysis
* hypersplenism
* enzymopathies
* parasitic infections
* dilutional anemias
* pregnancy
* splenomegaly.