Medical Laboratory Test FAQ

What are the most frequently ordered tests? The most frequently ordered tests are: 

  • a panel of chemistry tests performed on blood samples to assess the kidneys and liver, glucose control, and overall metabolic state
  • hematology tests that examine red cells, white cells, and platelets for signs of anemia, infection, bleeding tendency, or leukemia 
  • urinalysis to detect infections and other diseases of the kidneys and urinary tract
  • coagulation tests to monitor doses of the medicine coumadin, which is prescribed to prevent blood clots
  • therapeutic drug measurements, such as the heart medicine digoxin, to adjust its blood level to an effective one and prevent overdose
  • bacterial cultures of suspected infections (sore throats, wounds, abscesses, sputum, stool, blood)
  • tests for infection with viruses such as hepatitis and HIV.

How will my medications affect my blood tests? Many lab tests are designed to measure the effect of drugs and prevent toxicities from too high a level. Those tests should be done using blood specimens collected at precise times, such as just before the next dose, to make the right interpretation. 

Will smoking and/or drinking affect lab tests? Smokers have slightly higher white cell counts than non-smokers because of irritation to the lungs and probably to blood vessels too. Smokers also have higher levels of carbon monoxide. With chronic lung disease from smoking, oxygen levels fall in the blood, and carbon dioxide rises. 

Even mild drinking causes elevation of enzymes in the blood from the liver, which can be confused with early stage hepatitis. For that reason, blood donors are asked to refrain from drinking any alcohol days before donating blood. Alcoholics are at risk to develop serious liver disease such as cirrhosis, which produces marked abnormalities in many laboratory tests (e.g., bilirubin, AST, ALT, LDH, albumin, and others). 

Does the time of day affect my lab tests? A few endocrine tests (especially cortisol from the adrenal cortex) are strongly influenced by time of day, which should be specified by the ordering physician. Some tests are best done after overnight fasting (cholesterol, triglycerides, and other lipids). 

Why do they test my blood so much in the hospital? Patients in the hospital frequently receive fluids through intravenous lines. They should be monitored by blood tests to make sure their electrolytes, glucose, and calcium are kept in balance. This is especially true for critically ill patients in ICUs. Surgical patients or those injured in accidents may have lost blood. Their need for transfusion is established from blood cell counts, and another sample must be used to crossmatch the patient's correct blood type. 

What do the numbers mean? All lab test results are reported with a normal range that is expected in good health. If the numerical result is outside that range, it may indicate illness or organ damage that requires medical treatment. The more out-of-range the results, the more likely it is to be a serious change. 

When do I have to be re-tested? How often should I be re-tested? Mild abnormalities may lead a physician to repeat a test on the next visit or to order a confirmatory test that makes a diagnosis more certain. If the result is normal, it may not be necessary to re-test for a few years if the patient feels well. 

How often are the tests wrong? Laboratory measurements have become highly reliable due to advances in instrumentation. However, the quality of the specimen (How well was it collected? How was it stored and handled before testing?) may lead to abnormal results that should be re-tested before embarking on a new course of treatment. Laboratory technologists at VCUHS maintain the practice of checking severely abnormal results or results that change a great deal in a patient before reporting them to the doctor. 

Does what I eat affect the test? The body's metabolism goes through cycles every time food is eaten. Glucose rises in the blood, phosphates fall, lipids rise, etc. Accurate timing of blood collection around meals is very important for some diagnostic tests. These include glucose and lipids. Some other tests are not affected much by eating and can be collected in the morning or in the afternoon after lunch. These include blood cell counts, coagulation tests, many tests for kidney and liver function, and most common blood chemistry tests. 

How does family history affect my need for lab tests? A family history of high cholesterol and heart disease is strong stimulus for a person to have lipids measured. Other conditions such as excess bleeding (hemophilia) or clotting are inherited and should be investigated with special coagulation tests. Many new genetic tests on DNA are emerging to assess a person's risk of hereditary disease. Examples are cystic fibrosis, fragile X, thrombophilia (clotting), hemochromatosis (excess iron), neurologic conditions (amyloidosis and neuropathy; Huntington's disease), colon cancer, breast and ovarian cancer.

How many units of blood are needed each year? Throughout the US, an average of 38,000 units of red blood cells are needed on any given day. Blood transfusions are needed for trauma victims, surgical cases, organ transplants, and patients with leukemia, cancer and sickle cell disease. In 1999, 26.5 million units of blood components were transfused.

Who can donate blood? All healthy individuals who are at least 17 years of age or older may donate a unit of blood. Fewer than 5% of those healthy individuals eligible to donate blood actually volunteer to donate.