BLOOD TESTS FOR CHILDREN

Blood circulates throughout the body, so the contents of the blood can provide information both about general health and about specific diseases. The blood to be tested is drawn out of a vein with a syringe, or taken by pricking a finger. The method used to take a sample or specimen depends on how much blood is needed for the tests that are to be done, and also how “clean” the specimen needs to be. A finger-stick sample is more likely to be contaminated by contact with the surface of the finger than is a sample taken by putting a sterile needle into a vein.

Many different tests can be done on the blood, and a few of the most common ones are described here. (More complex tests can be done to find out what chemicals are present in the blood.)

Haematocrit. This test is done to find out how much of the blood is made up of red blood cells, which carry oxygen to the body tissues. The blood sample is spun in a machine called a centrifuge, which makes it separate into red blood cells, white blood cells, and plasma. The red cells are the heaviest, so they sink to the bottom. The percentage of red cells is then determined and compared to the normal range.

Hemoglobin. Hemoglobin is a protein that gives red blood cells their red color; it also combines with oxygen so that the oxygen can be carried in the blood. A hemoglobin test is often done at the same time as a haematocrit. The amount of hemoglobin in the blood is tested by adding certain chemicals to the blood and then measuring the intensity of the red color that signifies hemoglobin.

These two tests are done to check for anaemia, red blood cell deficiency.

White blood cell count. White blood cells are part of the body’s defense against infection. Too many or too few of these cells in the blood may indicate an infection or a blood disorder. To count the white blood cells, a blood specimen is diluted and put in a counting chamber, which is a slide with a grid on it. The slide is examined under a microscope and the cells are counted. This test can be done in most doctors’ surgeries.

Differential blood count. There are five different types of white blood cells; each type has a distinctive shape and appearance, and each one has a different function. To get more precise information about a disease, the doctor may need to know how many of each type of white blood cells are present in the blood. To do this, a stain is added to a blood specimen; because the stain affects each type of white blood cell differently, it is then easier to tell them apart. The stained specimen is examined under a microscope, and at least 100 white blood cells are identified and classified. The cells are also examined to see if they are a normal shape. In the course of this test the red blood cells can also be counted and checked for abnormalities.

Sedimentation rate. Sedimentation rate means the speed at which red blood cells move through the blood and settle in the bottom of a container. A rapid sedimentation rate is a sign of disease. To test the rate of settling, a chemical is added to a blood sample to keep it from clotting. Then the red cells are timed as they slowly move to the bottom of a specially marked test tube. This test is used to screen for diseases, but it does not identify exactly what is causing the cells to fall more quickly than normal. Keeping track of the sedimentation rate in successive tests can help a physician follow the progress of diseases that cause inflammation, including rheumatic fever and rheumatoid arthritis.

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Tags: General health | Posted in General health