Hemoglobin Concentration (Hb)

The reference ranges for hemoglobin (Hb) concentrations in adults are as follows[1] :

• Male: 14-18 g/dL or 8.7-11.2 mmol/L (SI units)
• Female: 12-16 g/dL or 7.4-9.9 mmol/L (SI units)
• Pregnant female: >11 g/dL
• Elderly: Slight decrease in values

The reference ranges for Hb concentrations in children are as follows[1] :

• Newborn: 14-24 g/dL
• 0-2 weeks: 12-20 g/dL
• 2-6 months: 10-17 g/dL
• 6 months-1 year: 9.5-14 g/dL
• 1-6 years: 9.5-14 g/dL
• 6-18 years: 10-15.5 g/dL

Possible critical values[1] :

• < 5.0 g/dL or >20 g/dL

Low hemoglobin

Hemoglobin concentration (Hb) is used clinically to determine the presence of anemia, which is functionally defined as insufficient red blood cell (RBC) mass to adequately deliver oxygen to peripheral tissues.[2] Anemia is considered to be present if the Hb or the hematocrit (Hct) is below the lower limit of 2 standard deviations (-2SD) or the 95% confidence interval for the normal population.[3, 4, 5] This definition of anemia results in 2.5% of normal individuals being classified as anemic.[4, 5] Anemia is absolute if the RBC mass is decreased and relative if associated with an increased plasma volume.[4]

Absolute anemia can be divided into 2 main categories: decreased RBC production and increased RBC destruction or loss in excess of the bone marrow’s ability to replace those losses. Causes of the first category include nutritional deficiencies (iron, folate, vitamin B12, vitamin B6), anemia of chronic disease, renal, liver, or endocrine disease, bone marrow infiltration (myelophthisic anemia), aplastic anemia, pure red cell aplasia, and sideroblastic anemia. Causes of the second category include blood loss (hemorrhage), hemolysis of various etiologies (both intrinsic and extrinsic), and hemoglobin disorders (hemoglobinopathies and thalassemias).[4, 6]

Relative anemia, on the other hand, may be seen in overhydration (volume overload), pregnancy, macroglobulinemia, and in postflight astronauts.[4, 6]

High hemoglobin

Higher-than-normal Hb may be indicative of polycythemia. The World Health Organization (WHO) classification of hematologic malignancies defines polycythemia as Hb greater than 18.5 g/dL in men, greater than 16.5 g/dL in women, Hct greater than 99th percentile of the method-specific reference range, or Hb greater than 17 g/dL in men, greater than 15 g/dL in women with a documented and sustained increase of at least 2 g/dL from baseline not attributed to correction of iron deficiency.[4, 7]

Absolute polycythemia is an increase in the total RBC mass in the body. Causes include hypoxia, inappropriate erythropoietin production, genetic polycythemia, and polycythemia vera. Relative polycythemia is an increase in Hct or RBC count resulting from a decrease in plasma volume; however, total RBC mass is not increased. Causes include dehydration, shock, diuretic therapy, and spurious polycythemia (Gaisböck syndrome).[4, 6]

See the list below:

• Specimen: Whole blood, usually collected by venipuncture

• Collection: EDTA tube (purple/lavender top; see image below) containing EDTA potassium salt additive as an anticoagulant

  Whole blood tube with lavender closure and EDTA potassium salt additive.

• Panels: Complete blood count (CBC), hemoglobin and hematocrit (Hb/Hct)

Description

Hemoglobin is the main component of red blood cells and serves as the transporter for oxygen and carbon dioxide in the blood. Hemoglobin concentration (Hb) is usually reported as grams of hemoglobin per deciliter of blood (g/dL).[8, 9]

Hb is commonly measured by the cyanhemoglobin (hemoglobin cyanide) method. A sample of blood is diluted in a solution of potassium ferricyanide and potassium cyanide, which oxidizes hemoglobin to hemiglobin (Hi; methemoglobin) and then converts it to hemoglobin cyanide (HiCN). The concentration of HiCN is then measured by spectrophotometry. The absorbance of the solution at 540 nm reflects the amount of hemoglobin present. A standard HiCN solution is also used for comparison. All forms of hemoglobin are detected except sulfhemoglobin (SHb).[8, 9]

Automated blood cell counters usually measure hemoglobin concentration by a modified cyanmethemoglobin or an alternate lauryl sulphate method.[10] With automated methods, the precision coefficient of variation for determining Hb is less than 1%.[11]

Indications/Applications

As part of a standard CBC, the Hb can be used to determine the presence of anemia or polycythemia. Measurement of Hb as part of Hb/Hct can also be used to determine an appropriate increase after red blood cell transfusion. In an adult patient, the Hb is usually expected to rise by 1 g/dL after transfusion of one unit of packed red blood cells (pRBCs).[12]

The measurement of Hb is generally considered accurate and more useful than Hct or RBC count for the diagnosis of anemia. Hb is measured directly, whereas Hct in some automated instruments is measured indirectly, calculated from the product of the RBC count times the mean corpuscular volume (MCV). Thus, factors that spuriously affect the RBC count (such as RBC clumping) and/or the MCV (such as prolonged storage) will ultimately affect the Hct measurement. Hb is also preferred because among these laboratory parameters, it is the best indicator of the oxygen carrying capacity in the blood.[10]

Although measuring the RBC mass in the body can also be an accurate method for determining anemia or polycythemia, these methods require radiolabeling of red cells or human serum albumin and are time consuming, difficult to perform, and expensive. Furthermore, Hb and Hct, which are much more readily available in medical laboratories, have been shown to correlate with RBC mass measurements.[13]

Considerations

Normal ranges for Hb vary among men and women (with different values for pregnancy), different age groups, different altitudes, and cigarette smokers.[3] Also, the aging process in adults is responsible for some mild Hb changes, such as a 1 g/dL decrease in healthy men between the ages of 70 to 88 years, partly due to decreased androgens as well as age-related decreased stem cell proliferation, which is seen in both men and women.[14, 2] Therefore, values should always be interpreted in context of established reference ranges for each individual.

Factors that cause changes in the plasma volume, without changes in the overall RBC mass, can also affect the Hb and may lead to the presence of relative anemia or relative polycythemia.

Hb is typically measured spectrophotometrically, and erroneously high results may be seen with increased sample turbidity due to hyperlipidemia, abnormal plasma proteins, large numbers of leukocytes, hemolysis, abnormal hemoglobins, or nucleated red blood cells.[8, 15] However, newer automated instruments are able to identify and minimize this type of interference.[10]