The HCT is a measurement of the volume of red blood cells as a percentage of whole blood. For automated procedures, the haematocrit is the product of the RBC and the mean cell volume [see below]. For manual determinations, the haematocrit is measured after centrifugation of a microcapillary tube filled with whole blood. The percentage of blood composed of red blood cells is the haematocrit [sometimes called packed cell volume]. Manual or ‘spun’ haematocrits tend to be a few percentage points higher than calculated haematocrits, because trapped plasma is included in the apparent red blood cell volume. Haematocrit is expressed as a number without units between 0.00 and 1.00. Haematocrit values for mice are generally between 0.40 and 0.50, but may range up to 0.60 depending on sampling site and fasting status.
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Diagnostic radioisotopes in haematology
Inderjeet Dokal, S. Mitchell Lewis, in Dacie and Lewis Practical Haematology [Tenth Edition], 2006
Whole-Body and Venous Packed Cell Volume Ratio
PCV measured on venous blood is not identical with the average PCV of all the blood in the body. This is mainly because the red cell:plasma ratio is less in small blood vessels [capillaries, arterioles, and venules] than in large vessels. The ratio between the whole body PCV and venous blood PCV is normally about 0.9,9 and it is thus necessary in the calculation of TBV from measurements of RCV to multiply the observed PCV by 0.9. Thus, TBV is given by the following:
Red cell volume ×1PCV×0.9
However, the ratio varies in individuals, especially in splenomegaly, and it is better to estimate RCV and plasma volume by separate measurements rather than to attempt to calculate one of these from an estimate of the other.
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The complete blood count and white blood cell differential
Alisha D. Ware, in Contemporary Practice in Clinical Chemistry [Fourth Edition], 2020
Hematocrit
The hematocrit is the percentage of packed RBCs in the whole blood [17]. The hematocrit has the implied units of L/L, although it is usually reported as a percent. Prior to the use of modern analyzers, the hematocrit was measured by centrifuging a tube of blood, and measuring the proportion of RBCs, which would settle to the bottom of the tube, to the total volume of blood. On modern hematology analyzers, the hematocrit is a calculated value [5,18,19]. A simple equation to calculate the hematocrit percent is to multiply the RBC count [in cells/L] by the MCV [in fL].
Hematocrit=RBCCount[inRBCs/L]×MCV[infL]
Sample calculation for an RBC count of 4.68 × 1012/L and an MCV of 84.8 fL:
Hematocrit=4.68×1012/L×84.8×10–15L=0.397L/L[or39.7%].
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PRINCIPLES IN ASSESSING MUSCULOSKELETAL DISORDERS
RONALD C. EVANS D.C., F.A.C.O., F.I.C.C., in Illustrated Orthopedic Physical Assessment [Third Edition], 2009
Hematocrit
Decreased hematocrit indicates anemia, such as that caused by iron deficiency or other deficiencies. Further testing may be necessary to determine the exact cause of the anemia.
Other conditions that can result in a low hematocrit include vitamin or mineral deficiencies, recent bleeding, cirrhosis of the liver, and malignancies.
The most common cause of increased hematocrit is dehydration, and with adequate fluid intake, the hematocrit returns to normal. However, increased levels may reflect a condition called polycythemia vera—that is, when a person has more than the normal number of RBCs. Increased hematocrit can be caused by a problem with the bone marrow or, more commonly, as compensation for inadequate lung function [the bone marrow manufactures more RBCs to carry enough oxygen throughout the body].
A hemoglobin test measures the levels of hemoglobin in your blood. Hemoglobin is a protein in your red blood cells that carries oxygen from your lungs to the rest of your body. If your hemoglobin levels are abnormal, it may be a sign that you have a blood disorder.
Other names: Hb, Hgb
What is it used for?
A hemoglobin test is often used to check for anemia, a condition in which your body has fewer red blood cells than normal. If you have anemia, the cells in your body don't get all the oxygen they need. Hemoglobin tests are measured as part of a complete blood count [CBC].
Why do I need a hemoglobin test?
Your health care provider may order the test as part of a routine exam, or if you have:
- Symptoms of anemia, which include weakness, dizziness, and cold hands and feet
- A family history of thalassemia, sickle cell anemia, or other inherited blood disorder
- A diet low in iron and other minerals
- A long-term infection
- Excessive blood loss from an injury or surgical procedure
What happens during a hemoglobin test?
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
Will I need to do anything to prepare for the test?
You don't need any special preparation for a hemoglobin test. If your health care provider has ordered other tests on your blood sample, you may need to fast [not eat or drink] for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.
Are there any risks to the test?
There is very little risk to having a blood test. After the test, some people experience mild pain, dizziness, or bruising. These symptoms usually go away quickly.
What do the results mean?
There are many reasons your hemoglobin levels may not be in 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.
If any of your levels are abnormal, it doesn't always mean you have a medical condition that needs treatment. Diet, activity level, medicines, a menstrual period, and other factors can affect the results. You may also have higher than normal hemoglobin levels if you live in a high altitude area. Talk with your provider to learn what your results mean.
Learn more about laboratory tests, reference ranges, and understanding results.
Is there anything else I need to know about a hemoglobin test?
Some forms of anemia are mild, while other types of anemia can be serious and even life threatening if not treated. If you are diagnosed with anemia, be sure to talk to your health care provider to find out the best treatment plan for you.
References
- Aruch D, Mascarenhas J. Contemporary approach to essential thrombocythemia and polycythemia vera. Current Opinion in Hematology [Internet]. 2016 Mar [cited 2017 Feb 1]; 23[2]:150–60. Available from: //www.ncbi.nlm.nih.gov/pubmed/26717193
- Hsia C. Respiratory Function of Hemoglobin. New England Journal of Medicine [Internet]. 1998 Jan 22 [cited 2017 Feb 1]; 338:239–48. Available from: //www.nejm.org/doi/full/10.1056/NEJM199801223380407
- National Heart, Lung, and Blood Institute [Internet]. Bethesda [MD]: U.S. Department of Health and Human Services; Anemia; [cited 2022 Jan 4]; [about 2 screens]. Available from: //www.nhlbi.nih.gov/health/health-topics/topics/anemia/
- National Heart, Lung, and Blood Institute [Internet]. Bethesda [MD]: U.S. Department of Health and Human Services; Blood Tests; [cited 2022 Jan 4]; [about 4 screens]. Available from: //www.nhlbi.nih.gov/health-topics/blood-tests
- National Heart, Lung, and Blood Institute [Internet]. Bethesda [MD]: U.S. Department of Health and Human Services; Polycythemia Vera; [cited 2022 Jan 4]; [about 14 screens]. Available from: //www.nhlbi.nih.gov/health-topics/polycythemia-vera
- National Heart, Lung, and Blood Institute [Internet]. Bethesda [MD]: U.S. Department of Health and Human Services; Anemia; [cited 2022 Jan 4]; [about 2 screens]. Available from: //www.nhlbi.nih.gov/health-topics/anemia
- Scherber RM, Mesa R. Elevated Hemoglobin or Hematocrit Level. JAMA [Internet]. 2016 May [cited 2017 Feb 1]; 315[20]:2225-26. Available from: //jamanetwork.com/journals/jama/article-abstract/2524164
- Testing.com [Internet]. Seattle [WA]: OneCare Media; c2022. Hemoglobin; [modified 2021 Nov 9; cited 2022 Jan 4]; [about 11 screens]. Available from: //www.testing.com/tests/hemoglobin/
- University of Rochester Medical Center [Internet]. Rochester [NY]: University of Rochester Medical Center; c2022. Health Encyclopedia: Hemoglobin; [cited 2022 Jan 4] [about 4 screens]. Available from: //www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=hemoglobin
Related Health Topics
- Anemia
- Blood
- Blood Count Tests
- Blood Disorders
- Sickle Cell Disease
- Thalassemia
Related Medical Tests
- Arterial Blood Gas [ABG] Test
- Bone Marrow Tests
- Complete Blood Count [CBC]
- Ferritin Blood Test
- G6PD Test
- Hematocrit Test
- Hemoglobin Electrophoresis
- How to Cope with Medical Test Anxiety
- How to Understand Your Lab Results
- Iron Tests
- Prealbumin Blood Test
- Red Blood Cell [RBC] Indices
- Reticulocyte Count
- What You Need to Know About Blood Testing
The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.What is an assessment of RBC percentage in the total blood volume?
What does a hemoglobin test measure?
What is the percentage of haemoglobin in RBC?
Does hemoglobin measure blood volume?