At a Glance
Why Get Tested?
To evaluate the hemoglobin content of your blood as part of a general health checkup; to screen for and help diagnose conditions that affect red blood cells (RBCs); if you have anemia or polycythemia, to assess the severity of these conditions and to monitor response to treatment
When to Get Tested?
With a hematocrit or as part of a complete blood count (CBC), which may be ordered as part of a general health screen; when your doctor suspects that you have a condition such as anemia (decreased hemoglobin) or polycythemia (increased hemoglobin); at regular intervals to monitor disease or response to treatment
A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (newborns)
Test Preparation Needed?
The Test Sample
What is being tested?
Hemoglobin is the iron-containing protein found in all red blood cells (RBCs) that gives the cells their characteristic red color. Hemoglobin enables RBCs to bind to oxygen in the lungs and carry it to tissues and organs throughout the body. It also helps transport a small portion of carbon dioxide, a product of cell metabolism, from tissues and organs to the lungs where it is exhaled.
The hemoglobin test measures the amount of hemoglobin in a person's sample of blood. A hemoglobin level can be performed alone or with a hematocrit, a test that measures the proportion of blood that is made up of RBCs, to quickly evaluate an individual's red blood cells. Red blood cells, which make up about 40% (ranging 37-49%) of the blood's volume, are produced in the bone marrow and are released into the bloodstream after they mature. The typical lifespan of an RBC is 120 days, and the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding.
Several diseases and conditions can affect RBCs and consequently the level of hemoglobin in the blood. In general, the hemoglobin level and hematocrit rise when the number of red blood cells increases. The hemoglobin level and hematocrit fall to less than normal when there is a drop in production of RBCs by the bone marrow, an increase in the destruction of RBCs, or if blood is lost due to bleeding. A drop in the RBC count, hemoglobin and hematocrit can result in anemia, a condition in which tissues and organs in the body do not get enough oxygen, causing fatigue and weakness. If too many RBCs are produced, the blood can become thickened, causing sluggish blood flow and related problems.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm or by a fingerstick (for children and adults) or heelstick (for newborns).
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Chap 31, Pp 458, 489-491.
Kasper DL, Braunwald E, Fauci AS, Hauser SL, Longo DL, Jameson JL eds (2005). Harrison's Principles of Internal Medicine, 16th Edition, McGraw Hill, Pp 329-336.
Pagana K, Pagana T. Mosby's Manual of Diagnostic and Laboratory Tests. 3rd Edition, St. Louis: Mosby Elsevier; 2006, Pp 300-303.
Harmening D. Clinical Hematology and Fundamentals of Hemostasis, Fifth Edition, F.A. Davis Company, Piladelphia, 2009, Pp 82-85, 771.
(Feb 9 2010) Dugdale D. Hemoglobin. Medline Plus Medical Encyclopedia. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003645.htm. Accessed January 2012.
(December 2005) Mayo Reference Services. How to interpret and pursue an abnormal complete blood cell count in adults. Vol. 30 No. 12. PDF available for download at http://www.mayomedicallaboratories.com/media/articles/communique/mc2831-1205.pdf through http://www.mayomedicallaboratories.com. Accessed January 2012.
(March 1, 2011) National Heart, Lung and Blood Institute. What is Polycythemia vera? Available online at http://www.nhlbi.nih.gov/health/public/blood/index.htm through http://www.nhlbi.nih.gov. Accessed Jan 2012.
(Aug 1, 2010) National Heart, Lung and Blood Institute. Anemia. Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/anemia/ through http://www.nhlbi.nih.gov. Accessed Jan 2012.
(November 4, 2011) Maarkaron J. Anemia. Medscape Reference article. Available online at http://emedicine.medscape.com/article/198475-overview through http://emedicine.medscape.com. Accessed Jan 2012.
(May 26, 2011) Kahsai D. Emergent Management of Acute Anemia. Medscape Reference article. Available online at http://emedicine.medscape.com/article/780334-overview#a1 through http://emedicine.medscape.com. Accessed Jan 2012.
(August 26, 2011) Harper J. Pediatric Megaloblastic Anemia. eMedicine article. Available online at http://emedicine.medscape.com/article/959918-overview through http://emedicine.medscape.com. Accessed Jan 2012.
(June 8, 2011) Artz A. Anemia in Elderly Persons. eMedicine article. Available online at http://emedicine.medscape.com/article/1339998-overview through http://emedicine.medscape.com. Accessed Jan 2012.
Riley R, et.al. Automated Hematologic Evaluation. Medical College of Virginia, Virginia Commonwealth University. Available onlinr at http://www.pathology.vcu.edu/education/PathLab/pages/hematopath/pbs.html#Anchor-Automated-47857 http://www.pathology.vcu.edu. Accessed Jan 2012.
Sources Used in Previous Reviews
Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.
Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 524-527.