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Thursday, February 8, 2007

Dengue Hemorrhagic Fever (DHF) -- The Tests

Tests may include the following:
  • Hematocrit
  • Platelet count
  • Electrolytes
  • Coagulation studies
  • Liver enzymes
  • Blood gases
  • Tourniquet test (causes petechiae below the tourniquet)
  • X-ray of the chest (may demonstrate pleural effusion)
  • Serologic studies (demonstrate antibodies to Dengue viruses)
  • Serum studies from samples taken during acute illness and convalescence (increase in titer to Dengue antigen).

Hematocrit (HTC)

The hematocrit is the percent of whole blood that is composed of red blood cells. The hematocrit is a measure of both the number of red blood cells and the size of red blood cells.

The hematocrit is almost always ordered as part of a complete blood count, which measures the number of red blood cells, the number of white blood cells, the total amount of hemoglobin in the blood, and the fraction of the blood composed of red blood cells (hematocrit). The hematocrit indicates the proportion of cells and fluids in the blood.

Performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

In the laboratory, some of the blood is centrifuged (spun in a machine). This forces the cells to the bottom of the container. The cellular portion is compared with the total amount of blood and expressed as a percent. The cellular portion is almost entirely red blood cells. The percent that is white blood cells is very small.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Prepare for the HTC test

No special preparation is necessary for this test. For infants and children, the preparation you can provide for this test depends on your child's age (infant – birth to 1 yr, toddler – 1 to 3 yr, preschooler – 3 to 6 yr, schoolage – 6 to 12yr, adolescent 12 to 18 yr), previous experiences, and level of trust.

There are some risks after got this test,

  • excessive bleeding
  • fainting or feeling light-headed
  • hematoma (blood accumulating under the skin)
  • infection (a slight risk any time the skin is broken)
  • multiple punctures to locate veins

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

The results mean

Hematocrit (varies with altitude):

  • Male: 40.7-50.3%
  • Female: 36.1-44.3%

Low hematocrit may indicate:

  • anemia (various types)
  • blood loss (hemorrhage)
  • bone marrow failure (e.g., due to radiation, toxin, fibrosis, tumor)
  • destruction of red blood cells
  • leukemia
  • malnutrition or specific nutritional deficiency
  • multiple myeloma
  • rheumatoid arthritis

High hematocrit may indicate:

  • dehydration
    • burns
    • diarrhea
  • erythrocytosis (excessive red blood cell production)
  • polycythemia vera

This test may be performed under many other conditions and in the assessment of many disease states.

Platelet count

Bleeding disorders or other bone marrow diseases, such as leukemia, require the determination of the number of platelets present and/or their ability to function correctly.

A platelet count is often ordered as a standard part of a complete blood count, which may be done as part of an annual physical examination. It is almost always ordered when a patient has unexplained bruises or takes what appears to be an unusually long time to stop bleeding from a small cut or wound.

The result mean

In an adult, a normal count is about 150,000 to 450,000 platelets per microliter (x 10–6/Liter) of blood.

If platelet levels fall below 20,000 per microliter, spontaneous bleeding may occur and is considered a life-threatening risk. Patients who have a bone marrow disease, such as leukemia or other cancer in the bone marrow, often experience excessive bleeding due to a significantly decreased number of platelets (thrombocytopenia). Low number of platelets may occur in some patients with long-term bleeding problems (e.g., chronic bleeding stomach ulcers), thus reducing the supply of platelets. Individuals with an autoimmune disorder (such as lupus or Idiopathic Thrombocytopenia Purpura [ITP], where the body’s immune system creates antibodies that attack its own organs) can cause the destruction of platelets. Certain drugs, such as heparin, quinidine, sulfa drugs, oral anti-diabetic drugs, and even alcohol, may cause decreased platelet counts. Patients undergoing chemotherapy may also have a decreased platelet count. Up to 5% of pregnant women may experience thrombocytopenia at term.

More commonly (up to 1% of the population), easy bruising or bleeding may be due to an inherited disease called von Willebrand’s disease. While the platelets may be normal in number, their ability to stick together is impaired due to a decrease in von Willebrand’s factor, a protein needed to initiate the clotting process. Many cases go undiagnosed due to the mild nature of the disease; however, the more severe form can be devastating.

Increased platelet counts (thrombocytosis) may be seen in individuals who show no significant medical problems, while others may have a more significant blood problem called myeloproliferative disorder (abnormal growth of blood cell elements). Some may have a tendency to bleed due to the lack of stickiness of the platelets, yet in others, the platelets retain their stickiness but, because they are increased in number, tend to stick to each other, forming a clump that can get stuck within a blood vessel and cause damage, including death (thromboembolism).

Living in high altitudes may cause increased platelet levels, as can strenuous exercise.

Decreased levels may be seen in women before menstruation.

Drugs that may cause increased platelet levels include estrogen and oral contraceptives.

Other inherited disorders caused by defective platelets or decreased/absent proteins that activate the platelets include Glanzmann’s Throbasthenia, Bernard-Soulier disease, Chediak-Higashi syndrome, Wiskott-Aldrich syndrome, May-Hegglin syndrome, and Down syndrome. The occurrence of these abnormalities, however, is relatively rare.

Electrolytes

Electrolytes or serum chemistries are minerals in your blood and other body fluids that carry an electric charge. It is important for the balance of electrolytes in your body to be maintained, because they affect the amount of water in your body, blood pH, muscle action, and other important processes. You lose electrolytes when you sweat, and these must be replenished by drinking lots of fluids.

Electrolytes exist in the blood as acids, bases, and salts (such as sodium, calcium, potassium, chlorine, magnesium, and bicarbonate) and can be measured by laboratory studies of the blood serum.

Tourniquet test

This test determines capillary fragility. It is also known as a Rumpel-Leede Capillary-Fragility Test or simply a capillary fragility test. It is a clinical diagnostic method to determine bleeding tendencies in a person who might have a disease such as dengue fever. It assesses fragility of capillary walls, evaluates bleeding tendencies, and identifies thrombocytopenia (a reduced platelet count).

In dengue, the test is defined by the WHO. A blood pressure cuff is inflated to a point between the systolic and diastolic blood pressures for five minutes. The test is positive if there are more than 20 petechiae per square inch (a petechia is a small red or purple spot on the body, caused by a minor hemorrhage).

Titer

A titer is a measurement of the amount or concentration of a substance in a solution. It usually refers to the amount of medicine or antibodies found in a patient's blood. Blood titer measurements can be very helpful in determining medical treatment. Antibody titers can tell the doctor if the patient has immunity to diseases such as measles, small pox, and hepatitis. Medication titers can tell if a person is receiving too much medication.

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