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The Lowdown on Lab Tests

Important Lab Tests Everyone Should Know About

Summer 1999

A note from TheBody.com: Since this article was written, the HIV pandemic has changed, as has our understanding of HIV/AIDS and its treatment. As a result, parts of this article may be outdated. Please keep this in mind, and be sure to visit other parts of our site for more recent information!

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.


Complete Blood Count (CBC)

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).

  • White Blood Count. The total white blood count will be elevated (greater than 10,000) with bacterial and some other types of acute infections. Other less common causes of elevated white blood counts include leukemia and bone marrow diseases. On the other hand, a low white blood count can be caused by medications such as Bactrim and zidovudine (AZT) as well as many chemotherapy drugs.

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  • Red Blood Count. Only in very rare conditions are the hemoglobin and hematocrit (H & H) elevated. When both red blood cell counts are low, the condition is known as anemia. There are many causes of anemia:

    • Low red blood cell production, as seen with some iron deficiency and some vitamin deficiencies

    • Blood loss due to internal or external bleeding, or anemia associated with chronic diseases such as HIV and other chronic infections

    • Loss of red blood cells due to destruction by the body (hemolysis)

    • Decreased production of new red blood cells in the marrow due to infection, such as mycobacterium avium complex (MAC), or cancer invading the bone marrow

    • Medications

    The differential for red blood cell count should generally show the hematocrit three times the hemoglobin.

  • Platelets. The platelet count may be low due to medications, HIV infection, or destruction by antibodies made against platelets.


Electrolytes

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

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).

  • Bilirubin. Levels of this enzyme may be elevated as a result of acute and severe chronic viral hepatitis, blockage of bile flow from the liver, or some medications. There are two components to the bilirubin, direct and indirect. Concentrations of direct bilirubin may be elevated by obstruction of the bile drainage system, and by hepatitis, while the indirect component will be elevated when there is excess breakdown of red blood cells, as seen in hemolytic anemia, where the body is destroying its own red blood cells.

  • Alkaline Phosphatase. Elevated concentrations will be seen with obstruction of the bile drainage system.

  • AST (SGOT) and ALT (SGPT). These liver enzymes are normally present inside of the liver cells. When there is ongoing liver damage, the AST and ALT enzymes are released into the blood by the damaged liver cells. It is not unusual to find mild elevations (up to 2 times normal) of ALT and AST. However, levels of AST and ALT more than 2 times normal are generally considered to be significantly abnormal, and require further investigation.

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.


Kidney Function Tests

The two most commonly measured compounds reflecting kidney (or renal) function are blood urea nitrogen and creatinine.

  • Blood Urea Nitrogen (BUN). If there are stones in the kidneys or ureters (the tubes connecting the kidneys to the bladder) there is almost always blood present in the urine. It may only be a microscopic amount, or not enough to be noticeable to the eye.

  • Creatinine. If the kidney begins to function poorly, both of creatine and BUN values become elevated, but the creatinine measurement will be the more useful indicator. A relatively unusual kidney problem called proximal tubular renal dysfunction (PRTD) has been seen in the last 2 years in up to 30 percent of people who take adefovir (Preveon). The important test to monitor kidney function in all people on adefovir measures creatinine, serum phosphate, and the amount of protein in the urine (proteinuria), which is elevated in PRTD. Often times, a low phosphorus level or proteinuria may be the first indication of the onset of PRTD, which is almost always reversible if the adefovir is stopped promptly.


Muscle Tests

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.


Blood Lipids

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.

  • Triglyderides. Normal values for triglycerides are below 250 mg/dl; borderline concentrations are 250 to 500, and definitely abnormal values are above 500 mg/dl.

  • Total Cholesterol. This value is the sum of low-density lipoproteins (LDL) plus high-density lipoproteins (HDL) and should be less than 200 mg/dl. For LDL, normal levels are below 130 mg/dl. However, for HDL, it is better to be high than low. A low HDL (below 35) is associated with an increased risk of cardiovascular disease, or atherosclerosis (hardening of the arteries.)

Lipid abnormalities are being reported very often in people on protease inhibitors, and many providers now obtain baselines values when beginning a HAART regimen.


HIV RNA (Viral Load)

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.


Dr. Jeff Schouten is a former general surgeon who has been living with HIV for over 10 years. He has been co-chair of STEP's Scientific Review Committee for several years and contributes regularly to the STEP Perspective. He has also recently earned a law degree from the University of Washington, so HIV-related legal questions, as well as medical, will be accepted.


A note from TheBody.com: Since this article was written, the HIV pandemic has changed, as has our understanding of HIV/AIDS and its treatment. As a result, parts of this article may be outdated. Please keep this in mind, and be sure to visit other parts of our site for more recent information!



  
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This article was provided by Seattle Treatment Education Project. It is a part of the publication STEP Perspective.
 
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