In capillary blood flowing through the tissues oxygen is released from hemoglobin and passes into tissue cells. Carbon dioxide diffuses out of tissue cells into erythrocytes, where the red-cell enzyme carbonic anhydrase enables its reaction with water to form carbonic acid.
The carbonic acid dissociates to bicarbonate which passes into the blood plasma and hydrogen ions, which combine with the now deoxygenated hemoglobin.
The blood flows to the lungs, and in the capillaries of the lung alveoli the above pathways are reversed. Bicarbonate enters erythrocytes and here combine with hydrogen ions, released from hemoglobin, to form carbonic acid.
This dissociates to carbon dioxide and water. The carbon dioxide diffuses from the blood into the alveoli of the lungs and is eliminated in expired air. Meanwhile, oxygen diffuses from the alveoli to capillary blood and combines with hemoglobin. Although normally present in only trace amounts, there are three species of hemoglobin: methemoglobin MetHb or Hi , sulfhemoglobin SHb and carboxyhemoglobin COHb which cannot bind oxygen. They are thus functionally deficient, and increased amounts of any of these hemoglobin species, usually the result of exposure to specific drugs or environmental toxins, can seriously compromise oxygen delivery.
The principle reason for measuring c tHb is detection of anemia and assessment of its severity. The lower the c tHb, the more severe is the anemia. Anemia is not a disease entity, rather a consequence or sign of disease. The reason why ctHb is such a frequently requested blood test is that anemia is a feature of a range of pathologies, many of which are relatively common Table II. Common symptoms, most of which are non-specific, include: pallor, tiredness and lethargy, shortness of breath — particularly on exertion, dizziness and fainting, headaches, constipation and increased pulse rate, palpitations, tachycardia.
The absence of these symptoms does not preclude anemia; many mildly anemic individuals remain asymptomatic, particularly if anemia has developed slowly. Whilst anemia is characterized by reduced ctHb, a raised ctHb indicates polycythemia. Polycythemia arises as a response to any physiological or pathological condition in which blood contains less oxygen than normal hypoxemia. This so-called secondary polycythemia is part of the physiological adaptation to high altitude and may be a feature of chronic lung disease.
Primary polycythemia is a much less common malignancy of the bone marrow called polycythemia vera, which is characterized by uncontrolled production of all blood cells, including erythrocytes. Polycythemia, whether secondary or primary, is generally much less common than anemia. The first clinical test of Hb measurement devised more than a century ago [3] involved adding drops of distilled water to a measured volume of blood until its color matched that of an artificial colored standard.
A later modification [4] involved first saturating blood with coal gas carbon monoxide to convert hemoglobin to the more stable carboxyhemoglobin.
Modern hemoglobinometry dates from the s, following development of spectrophotometry and the hemiglobincyanide cynamethemoglobin method. Adaptation of this method and others for use in automated hematology analyzers followed. Over the past two decades advances have focused on development of methods which allow point-of-care testing POCT of hemoglobin.
This section deals first with consideration of some of the methods currently used in the laboratory and then with those POCT methods used outside the laboratory.
Nearly 40 years after it was first adopted as the reference method for measuring hemoglobin by the International Committee for Standardization in Hematology ICSH [5], the hemiglobincyanide HiCN test remains the recommended method of the ICSH [6] against which all new c tHb methods are judged and standardized.
The detailed consideration that follows reflects its continued significance both as a reference and routine laboratory method. Blood is diluted in a solution containing potassium ferricyanide and potassium cyanide. Potassium ferricyanide oxidizes the iron in heme to the ferric state to form methemoglobin, which is converted to hemiglobincyanide HiCN by potassium cyanide. Absorbance of the diluted sample at nm is compared with absorbance at the same wavelength of a standard HiCN solution whose equivalent hemoglobin concentration is known.
Most hemoglobin derivatives oxyhemoglobin, methemoglobin and carboxyhemoglobin, but not sulfhemoglobin are converted to HiCN and therefore measured by this method. Absorbance is read at nm against a reagent blank. The absorbance of HiCN standard is measured in the same way. The major advantage of this method is that there is a standard HiCN solution manufactured and assigned a concentration value according to very precise criteria laid down and reviewed periodically by the International Council for Standardization in Hematology ICSH [6].
This international standard solution is the primary calibrant for the commercial standard solutions used in clinical laboratories around the world. Thus all those using HiCN standardization are effectively using the same standard, whose value has been scrupulously validated. Turbidity due to proteins, lipids and cellular matter is a potential problem with spectrophotometric estimation of any blood constituent, including hemoglobin.
The large dilution of sample largely eliminates the problem, but falsely raised c tHb results can occur in patients whose plasma protein concentration is particularly high [8,9,10]. Heavily lipemic samples and those containing very high numbers of white cells leucocytes can also artefactually raise c tHb by a similar mechanism [11].
Sodium Lauryl Sulphate SLS is a surfactant which both lyses erythrocytes and rapidly forms a complex with the released hemoglobin. The product SLS-MetHb is stable for a few hours and has a characteristic spectrum with maximum absorbance at nm [12]. The method has been adapted for automated hematology analyzers and is as reliable in terms of both accuracy and precision as automated HiCN methods [13,14,15].
A major advantage is that the reagent is non-toxic. It is also less prone to interference by lipemia and increased concentration of leukocytes [13]. This hemoglobin measurement method shows high specificity and sensitivity.
First, blood is pulled into a dry reagent cuvette by capillary action. Secondly, the reagent destroys the red blood cell walls. Free hemoglobin is then oxidized to a methemoglobin and finally converted to azide methemoglobin. The susceptibility of the reagent to humidity represents a challenge associated with this method. This device quantifies absorbance of oxygenated and deoxygenated hemoglobin, while turbidity is measured and compensated for at nm. Currently an EKF Diagnostics company, DiaSpect represents a leader in new technology that measures hemoglobin without a reagent based on broad spectrum photometry.
This sensor element identifies absorbance of the blood at a broad wavelength range that will provide insight into the overall absorbance spectrum resulting in a higher specificity and a lower sensitivity to interferences. Just about one second of measurement time is another benefit of this type of device. With the availability of new technologies to detect the spectral pattern and concentration of hemoglobin, non-invasive methods have become more frequently used, first introduced for monitoring during surgery and lately to provide hemoglobin spot checks in primary care and in blood donor qualification.
Some non-invasive devices use pulse oximetry while others rely on white light and the capturing of transmission data to measure hemoglobin concentrations in tissue capillaries. You can return to your usual activities immediately after the sample is taken. Normal ranges for children vary with age and sex. The range for a normal hemoglobin level may differ from one medical practice to another.
If your hemoglobin level is lower than normal, you have anemia. There are many forms of anemia, each with different causes, which can include:. If you've been previously diagnosed with anemia, a hemoglobin level that's lower than normal may indicate a need to alter your treatment plan. If you've been previously diagnosed with polycythemia vera, an elevated hemoglobin level may indicate a need to alter your treatment plan.
If your hemoglobin level is below or above normal, your doctor may want to evaluate the hemoglobin test results along with those of other tests, or additional tests may be necessary, to determine next steps. Explore Mayo Clinic studies of tests and procedures to help prevent, detect, treat or manage conditions.
Mayo Clinic does not endorse companies or products. Advertising revenue supports our not-for-profit mission. Hemoglobin tests are also frequently performed with other tests, such as: Hematocrit, which measures the percentage of red blood cells in your blood Complete blood count , which measures the number and type of cells in your blood.
Why do I need a hemoglobin test? Your health care provider may have ordered the test as part of a routine exam, or if you have: Symptoms of anemia, which include weakness, dizziness, pale skin, and cold hands and feet A family history of thalassemia , sickle cell anemia , or other inherited blood disorder A diet low in iron and minerals A long-term infection Excessive blood loss from an injury or surgical procedure.
What happens during a hemoglobin test? Will I need to do anything to prepare for the test? Are there any risks to the test? What do the results mean? There are many reasons your hemoglobin levels may be outside the normal range.
Low hemoglobin levels may be a sign of: Different types of anemia Thalassemia Iron deficiency Liver disease Cancer and other diseases High hemoglobin levels may be a sign of: Lung disease Heart disease Polycythemia vera, a disorder in which your body makes too many red blood cells. It can cause headaches, fatigue, and shortness of breath. Is there anything else I need to know about a hemoglobin test? References Aruch D, Mascarenhas J.
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