Hemoglobin is composed of heme (iron + protoporphyrin) and globin polypeptide chains. It is present in the red blood cells of all vertebrates except Channichthyidae (the family of fish: white-blooded fish also called crocodile fish found in southern South America and the Southern Ocean around Antarctica). It carries oxygen from the lungs to the tissues and carbon dioxide from tissues to the lungs.
In humans, hemoglobin is not homogeneous and normally different variants and derivatives exist. Normal hemoglobin variants are fetal hemoglobin (Hb F), adult hemoglobin (Hb A), Hb A2 and embryonic hemoglobins (Gower I, Gower II and Portland). They differ from each other on the basis of the structure and the type of polypeptide chains.
INDICATION FOR HEMOGLOBIN ESTIMATION
- Screening for polycythemia: Polycythemia is a disease state in which the hemoglobin level and hematocrit (HCT) or packed cell volume (PCV) value is elevated. It may be primary, secondary or relative.
- To determine presence and severity of anemia: Anemia is a disease state in which the hemoglobin concentration or oxygen-carrying capacity of blood is low. Clinical signs and symptoms (conjunctival vessels, polar of skin, mucosal membranes) are unreliable for the diagnosis of anemia. Anemia is best determined by estimation of hemoglobin and hematocrit (HCT) or packed cell volume (PCV).
- To assess response to specific therapy in anemia.
- Estimation of red cell indices (along with hematocrit (HCT) or packed cell volume (PCV) and red cell count) i.e. mean cell volume (MCV), mean cell hemoglobin (MCH) and mean cell hemoglobin concentration (MCHC).
- Selection of blood donors in the blood bank.
METHOD FOR ESTIMATION OF HEMOGLOBIN
There are different methods for estimation of hemoglobin. These are:
(1) Colorimetric methods: In these methods, the color comparison is made between the standard and the test sample, either visually or by colorimetric methods.
- Visual methods: Sahli’s acid hematin method, WHO hemoglobin color scale and Tallqvist chart.
- Photoelectric or colorimetric methods: Cyanmethemoglobin (hemoglobin-cyanide) method, oxyhemoglobin method, and alkaline hematin method.
(2) Gasometric method: In this method, oxygen-carrying capacity of red blood cells (RBCs) is measured in a Van Slyke apparatus. The amount of hemoglobin is then derived from the formula that 1 gram of hemoglobin carries 1.34 ml of oxygen. However, this method measures only physiologically active hemoglobin, which can carry oxygen. It does not measure methemoglobin, sulfhemoglobin, and carboxyhemoglobin. Also, this method is expensive and time-consuming, and the result is about 2% less than other methods.
(3) Chemical method: In this method, iron-content of hemoglobin is first evaluated. The value of hemoglobin is then derived indirectly from the formula that 100 grams of hemoglobin contain 374 mg of iron. This method is tiresome and time-consuming.
(4) Specific gravity method: In this method, an approximate value of hemoglobin is estimated from the specific gravity of blood as determined from copper sulfate technique. This method is simple and rapid. This method is useful and most common in mass screening like the selection of blood donors. See procedure.
Tallqvist Hemoglobin Chart
Tallqvist hemoglobin chart consists of a series of lithographed colors said to correspond to hemoglobin values ranging from 10% to 100%. In this method, a drop of blood obtained by finger puncture is placed on a piece of absorbent paper. The color produced is matched against the color on the chart and the corresponding reading is taken. The room of error is 20-50%. Although this method is very cheap and simple.