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What do blood tests really show?

By the time you reach adulthood, you know that a checkup usually involves giving up a test tube or two of blood. That’s because blood offers an invaluable window on the inner workings of the body. Of the 900 tests that can be performed, here’s what some of the most common reveal:

Complete blood count (known as a CBC) is actually a series of analyses done at the same time on a single sample of blood, including:

  • A red blood cell count measures the number of red blood cells in whole blood. A high count may result from lung disease, congestive heart failure and cancer. Low counts may indicate iron-deficiency anemia, folic acid and vitamin B12 deficiencies, internal bleeding, lupus erythematosus, leukemia, kidney disease or an underactive thyroid.
  • A hemoglobin analysis measures the pigment in red blood cells. Causes of low levels of hemoglobin include congenital anemias, lead poisoning and treatments like chemotherapy.
  • Hematocrit is the ratio of red blood cells to the volume of whole blood. Severe burns, shock, severe dehydration and chronic lung disease can raise levels. Anemias, chronic blood loss and leukemia can lower levels.
  • A total white cell count measures the number of white blood cells. A high count generally indicates an infection, especially a bacterial infection. High levels also point to leukemia, pregnancy, internal bleeding, cancer, stress, heart attack and gangrene. Exposure to radiation, viral disease, an impaired immune system or bone marrow problems may account for low levels.
  • A platelet count is a measure of the blood cell fragments that promote clot formation. A high count indicates a tendency to form clots; a low count suggests a tendency toward abnormal bleeding.

Glucose tests measure blood-sugar levels. They help screen for several metabolic diseases—mainly diabetes, which produces high blood sugar levels as well as low blood sugar levels (hypoglycemia ).

A potassium test measures potassium levels. High levels may indicate kidney disease or Addison’s disease. Severe diarrhea, kidney disease and Cushing’s syndrome can cause low levels. Taking diuretics also may lower potassium levels. Both high and low levels can affect heart rhythm.

Cholesterol tests measure the levels of blood cholesterol, an important risk factor for heart disease. A low level may indicate liver disease.

Calcium and phosphorus generally are measured together because they play reciprocal roles. High levels of phosphorus suggest low levels of calcium, which, in turn, may disrupt heart muscle contraction and nerve impulse transmission.

SGOT and SGPT measure levels of two tongue-twisting enzymes, serum glutamic oxaloacetic transaminase and serum glutamic pyruvic transaminase. These enzymes are found primarily in the liver. High levels indicate kidney damage. These tests also help evaluate chest pain. That’s because soon after a heart attack, SGOT from the heart is increased while SGPT remains normal.