The Lowdown on Lab Tests
Important Lab Tests Everyone Should Know About
It is rare to find an HIV-positive person who hasn't been poked and prodded from head to foot for countless lab tests. That makes it very easy to not look at test results or even ask what tests have been done. But with quickly changing drug therapies, new lab tests, and newly emerging side effects, there are some lab tests everyone should know about. These are the basic lab tests that most health care providers use when people have HIV and are on drug cocktails. This article will review the most commonly ordered lab tests and describe their significance. Some abnormal values will be mentioned, but in general you should ask what the normal values are for any test results, as they may vary from lab to lab.
You've probably heard a CBC called for on every episode of "ER." It's a basic test to measure total white blood cell count, "differential" (counts of the different types of white blood cells), red blood cell quantity (including hemoglobin and hematocrit), and platelets (which assist is blood clotting).
This test measures the concentration of sodium, potassium, chloride, and carbon dioxide. These tests reflect changes in the body fluids due to excess salt or potassium loss, as seen with diarrhea, and/or vomiting. Generally these are associated with acute illnesses, except for low potassium, which may be seen in people taking diuretics, and elevated potassium, which is associated with kidney failure. Significantly abnormal values may be life-threatening and require immediate medical attention. (i.e. a potassium below 3.5 or above 5.0). The chloride and carbon dioxide concentrations may be abnormal due to conditions that affect the acid/base balance in the blood. Such concentrations are generally associated with acute, severe illnesses.
Liver function tests are measurements of the levels of enzymes contained in the cells of the liver, and reflect the health of the liver. These enzymes are bilirubin, alkaline phosphatase, and the aminotransferases: serum glutamine aminotransferase, referred to as AST (SGOT), and serum alanine aminotransferase, referred to as ALT (SGPT).
The most common causes of liver damage include hepatitis A, B, or C infection, alcohol use, and medications. However, with chronic hepatitis C infection the degree of ALT and AST elevation does not correlate very well with the amount of liver damage and a liver biopsy may be necessary to determine the extent of liver damage. Also, as with HIV RNA levels, tests to determine the level of the hepatitis C virus are readily available. One note of caution: the range of hepatitis C virus levels tends to be much higher than the range seen for HIV.
The two most commonly measured compounds reflecting kidney (or renal) function are blood urea nitrogen and creatinine.
Some medications, such as zidovudine (AZT), or HIV infection may cause pain and inflammation in the muscles (myositits). The damaged muscle cells release a compound into the blood called creatine phosphokinase (CPK), elevating its concentrations. Elevated concentrations of CPK can also be caused by damage to the heart muscle, as happens in a heart attack with loss of blood flow to a part of the heart wall. However, a blood test can distinguish between elevated CPK coming from the heart CPK coming from other muscles. Intramuscular injection or vigorous exercise may also cause an elevated CPK for a day or two.
To get an accurate measurement of blood lipids (fats), the blood sample should be obtained while fasting, i.e., first thing in the morning. The two major components measured are triglycerides and cholesterol.
This test measures the number of HIV particles in the blood. The most recently approved test can detect as few as 50 copies of HIV per milliliter of blood. When there are less than 50 copies/ml, the test is not accurate enough to give a reliable number. An average person has 5 liters of blood, or about 5,000 ml. So, if you have 20 copies/ml of HIV, you would have about 100,000 copies of HIV in your blood, even though your viral load test result would be "nondetectable."
So the result when there are fewer than 50 copies/ml should properly be reported as "below the limit of detection," not "undetectable," or "no virus present." Almost all people who have a value below 50 still have some evidence, when more sensitive research tools are used, of ongoing viral replication in the blood or certainly in the lymph nodes.
This article was provided by Seattle Treatment Education Project. It is a part of the publication STEP Perspective.