Berikut merupakan kutipan ilmiah kedokteran tentang kesehatan sel darah yang sangat bermanfaat sehingga disusun dan digunakan sebagai referensi pribadi.
Perpustakaan Keluarga Helmut Todo Tua Simamora dan dr. Olga Y.V Hutapea.
Hematocrit (he-MAT-uh-krit) is the proportion of your total blood volume that is composed of red blood cells. A hematocrit (Hct) test indicates whether you have too few or too many red blood cells — conditions that can occur as the result of certain diseases. Red blood cells, or erythrocytes (uh-RITH-roe-sites), transport oxygen throughout your body.
A hematocrit test is done using a sample of your blood. A lab technician puts the sample in a device called a centrifuge that spins the blood very quickly in a test tube. This motion separates your blood into three parts: the fluid component (plasma), red blood cells and other blood cells. When the blood is separated, the technician can determine what proportion of the cells are red blood cells. Hematocrit is also called packed-cell volume (PCV).
A hematocrit test is part of a complete blood count (CBC). The proportion of red blood cells compared with all blood cells may help your doctor make a diagnosis or monitor your response to a treatment.
A lower than normal hematocrit may indicate:
- An insufficient supply of healthy red blood cells (anemia)
- A large number of white blood cells — usually a very small portion of your blood — due to long-term illness, infection, leukemia, lymphoma or other disorders of white blood cells
- Vitamin or mineral deficiencies
- Recent or long-term blood loss
A higher than normal hematocrit may indicate:
- Dehydration
- A disorder, such as polycythemia vera, that causes your body to produce too many red blood cells
- Lung or heart disease — if the body senses low oxygen levels, it will make more red blood cells in an effort to increase the amount of oxygen in the blood
The hematocrit is a simple blood test. You won't need to fast before the test or make other preparations.
A nurse or medical assistant may collect a blood sample by pricking your finger and collecting the blood into a narrow glass tube or by drawing blood with a needle from a vein in your arm. You may feel some tenderness at the site, but you'll be able to resume normal activities after the sample is drawn.
Results from your hematocrit are reported as the percentage of blood cells that are red blood cells. The normal range is 38.8 to 50 percent for men and 34.9 to 44.5 percent for women. The normal range for children 15 years of age and younger varies by age and sex. The lower and upper thresholds for a normal hematocrit value may vary somewhat from one medical practice to another.
The result of your hematocrit test is just one piece of information that helps your doctor check your health. Talk to your doctor about what your hematocrit test result means in light of the symptoms you're experiencing and the results of other diagnostic tests.
Accuracy of test results
A number of conditions can affect the outcome of a hematocrit test and yield inaccurate or misleading results. These complicating factors include:
- Living at a high altitude
- Pregnancy
- Significant recent blood loss
- Recent blood transfusion
- Severe dehydration
Your doctor will take into account possible complicating factors when interpreting the results of your hematocrit test. Your doctor may want to repeat the hematocrit test and do other blood tests if results provide conflicting or unexpected information.
Hematocrit
If the hematocrit must be determined quickly, as is often the case when a patient hemorrhages, it may be necessary to measure the hematocrit directly without the use of an automated counter. The materials needed are:
- Lancets
- Alcohol prep pads
- Gauze pads
- Microhematocrit tubes (heparinized)
- Sealant ("Seal-Ease," "Crit-Seal," etc)
- Microhematocrit centrifuge
- Microhematocrit reader
- If venipuncture is required: tourniquet, syringe, tube containing anticoagulant (EDTA, citrate)
For hematocrits obtained by fingerstick, wipe the fingertip pad of the fourth finger of the nondominant hand with the alcohol prep pad. Make certain the area is allowed to dry. Prick the fingertip with the lancet. Place the hematocrit tube near the incision site and allow the blood to flow via capillary action into the hematocrit tube until it is two-thirds to three-fourths full or to a predesignated mark on the tube. Avoid "milking" the finger if possible; this causes the expression of tissue fluids and may result in a falsely low hematocrit. Always fill at least three tubes. For hematocrits obtained by venipuncture, draw a sample of blood into the tube containing anticoagulant and mix well. Dip the hematocrit tube into the blood and allow the blood to rise to the desired two-thirds to three-quarters level. Because blood cells naturally sediment, a prior thorough mixing of the blood in the tube is necessary to ensure accurate reading.
After cleaning the outside of the hematocrit tubes of excess blood, invert the tube slowly so that the blood migrates just short of the bottom end of the tube. Seal the bottom of the tube with sealant. Make certain that little or no air is interspersed in the column of blood. If the seal is incomplete, leakage will occur during centrifugation and false readings will be obtained.
Place the tubes in a microhematocrit centrifuge and spin for 3 to 5 minutes at high speed. A shorter spin will not allow for complete sedimentation.
Using either a hematocrit reader or any ruled apparatus, measure the length of the column of the packed red cells and divide it by the length of the whole column of blood (cells and plasma), as in Figure 151.1. To obtain the hematocrit, multiply this number by 100%. Average all readings obtained from the different microhematocrit tubes.
Figure 151.1
Microhematocrit tube after sedimentation. The hematocrit is a ratio of the packed cells to total volume.
Example: If the column of packed red cells measures 20 mm and the whole blood column measures 50 mm, the hematocrit is 20/50 = 0.4 or (0.4 × 100%) = 40%.
Hemoglobin
Hemoglobin determinations will usually be performed by an automated cell counter from a tube of well-mixed EDTA-anticoagulated blood filled to a predetermined level. In this assay, all forms of hemoglobins are converted to the colored protein cyanomethemoglobin and measured by a colorimeter. An inadequate sample, whether due to insufficient volume or inadequate anticoagulation, may give false readings. If it is necessary to determine the level of anemia quickly, the hematocrit is an easier, more convenient test.
Hemoglobin Electrophoresis
Hemoglobin electrophoresis measures the mobility of hemoglobin in an electric field; it can therefore detect only those abnormalities in hemoglobin that alter the charge. Electrophoretic mobilities are affected by pH and by the medium in which the test is conducted. Screening tests typically use a hemolysate of anticoagulated blood electrophoresed on cellulose acetate at pH 8.6 to 8.8. If necessary, a further electrophoresis in starch gel at pH 6.2 to 6.8 is performed. At that stage, the work will usually be performed by a specialized laboratory.
Hemoglobin electrophoresis will not readily assess situations where there are neutral amino acid substitutions or where the hemoglobin is normal but the constituent chains are not produced in equal numbers (thalassemias). The diagnosis of alpha thalassemia of a mild to moderate degree cannot be made by hemoglobin electrophoresis; the diagnosis of beta thalassemia may be made by inference from an increase in the Hb A2.
A standard electrophoresis would look like Figure 151.2.
Basic Science
The molecular weight of hemoglobin is approximately 64,500 daltons. Hb is composed of two pairs of dissimilar chains, α and β, each defined by a specific amino acid sequence and incorporating an iron-containing heme group. Two α–β dimers combine to form a hemoglobin tetramer. This allows for the "heme–heme" interaction necessary for effective oxygen uptake (deoxyhemoglobin → oxyhemoglobin) and delivery (oxyhemoglobin → deoxyhemoglobin). The oxygen affinity of hemoglobin is a function of this heme–heme interaction and of pH (Bohr effect), and is a measure of how many hemoglobin molecules have oxygen bound to them for a given level of oxygen tension. In a normal individual the major hemoglobin is Hb A, constituting approximately 97% of the total hemoglobin. Variations and/or amino acid substitutions in these chains exist. Some are deleterious to the normal function of hemoglobin, whereas others may have relatively normal oxygen affinity and stability. Hemoglobins containing different types of chains make up the remainder of the hemoglobin content in red cells (α2δ2 = Hb A2 approximately 2%; α2γ2 = Hb F approximately 1%).
Substitutions in the normal hemoglobin amino acid sequence may result in hemoglobins that have different sub-unit interactions and varying affinities for oxygen. For example, a substitution of the sixth amino acid on the beta chain causes Hb S, or sickle hemoglobin. Hb S has a lower oxygen affinity and surrenders its oxygen more readily. Hb F, a normal minor hemoglobin constituent, has a higher oxygen affinity.
If the oxygen dissociation curve is abnormal, the body will adjust the hemoglobin level to ensure adequate oxygen distribution to the tissues. Thus in a rare disease like hemoglobin Hotel Dieu, the difficulty in extracting oxygen from a variant hemoglobin with increased oxygen affinity could result in a lack of oxygen for the tissues (tissue hypoxia) and a compensatory erythrocytosis. The smaller fraction of oxygen released from the hemoglobin is thereby offset by the increased number of hemoglobin molecules. Similarly, in sickle cell anemia, the decreased oxygen affinity allows these patients more tissue oxygen at any given hemoglobin level.
Clinical Significance
Many anemias are detected by routine laboratory screening performed before the patient is symptomatic. When the patient does have symptoms from an abnormality in the hemoglobin level, the symptoms are often a nonspecific weakness or fatigue. The only finding on physical examination may be pallor; additional changes in the nail beds (such as spooning), glossitis (red tongue), or hepatosplenomegaly (enlarged liver or spleen) may give a clue to the etiology of the anemia. Symptoms are usually related to the level of hemoglobin, its abruptness of onset and its duration. A patient with pernicious anemia may feel well at the same level of hemoglobin that would cause severe weakness in a patient with acute gastrointestinal hemorrhage. This is due to volume compensation by plasma and shifts in the oxygen dissociation curve which occur over time.
When first confronted with an abnormal hemoglobin or hematocrit level, the next step is to assess the red cell indices (see Chapter 152), peripheral smear (Chapter 155), and the reticulocyte count (Chapter 156) in light of the patient's history and physical examination.
What is a normal hematocrit?
Normal values for the hematocrit test vary according to age, sex,pregnancy, altitude where people live, and even vary slightly between various testing methods. The following are reported ranges of normal hematocrit levels:
- Newborns: 55%-68%
- One (1) week of age: 47%-65%
- One (1) month of age: 37%-49%
- Three (3) months of age: 30%-36%
- One (1) year of age: 29%-41%
- Ten (10) years of age: 36%-40%
- Adult males: 42%-54%
- Adult women: 38%-46%
- Adult pregnant women: about 30% - 34% lower limits and 46% upper limits
- High Altitude residents: about 45% - 61% in males; 41% - 56% in females (These levels gradually average higher as the altitude where people live increases. This is a result of the increased demand for the oxygen-carrying capacity of red blood cells at higher altitudes where there is decreased oxygen concentration in the atmosphere.)
These values may vary from authorities in the field by as much as 7%. Consequently, it is best to have a doctor explain the significance of an individual's level of hematocrit if it is not normal.
What does a low hematocrit mean?
A low hematocrit means the percentage of red blood cells is below the lower limits of normal (see above) for that person's age, sex, or specific condition (for example, pregnancy or high-altitude living). Another term for low hematocrit is anemia. Causes of low hematocrit, or anemia, include:
- Bleeding (ulcers, trauma, colon cancer, internal bleeding)
- Destruction of red blood cells (sickle cell anemia, enlarged spleen)
- Decreased production of red blood cells (bone marrow supression, cancer, drugs)
- Nutritional problems (low iron, B 12, folate and malnutrition)
- Overhydration (polydypsia, intravenous overhydration).
What does a high hematocrit mean?
A high hematocrit means the percentage of red blood cells in a person's blood is above the upper limits of normal (see above) for that person's age, sex, or specific condition (for example, pregnancy or high altitude living). Causes of a high hematocrit include:
- Dehydration (heat exhaustion, no available source of fluids)
- Low availability of oxygen (smoking, high altitude, pulmonary fibrosis)
- Genetic (congenital heart diseases)
- Erythrocytosis (over-production of red blood cells by the bone marrow orpolycythemia vera)
- Cor pulmonale (COPD, chronic sleep apnea, pulmonary embolisms)
How is a low or high hematocrit treated?
The treatment of high or low hematocrit depends on the underlying cause(s), the hematocrit level, and the overall health status of the individual. Most people are not treated with medications or procedures if the hematocrit is only slightly above or below the normal levels. Some patients with very low hematocrits may require intravenous iron or complete blood count. Some patients with very high hematocrits due to diseases, such as polycythemiarubra vera, may require blood letting (blood removal).
The patient's doctor will decide when medication or procedures are necessary for each particular individual. In general, abnormal hematocrit values are monitored by doctors with routine blood testing.
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