In many parts of the world, medical providers care for their patients who have HIV/AIDS without the benefit of laboratory tests -- sometimes even the most basic ones. Fortunately, in the United States (and various other countries), medical providers almost always have available to them a wide range of diagnostic tools. Below are some of the most important ones.
Complete Blood Count (CBC)
This tells you whether you are anemic (too few red blood cells), neutropenic (too few of the white blood cells called neutrophils that fight bacterial infections), or thrombocytopenic (too few platelets, or thrombocytes, for your blood to clot normally) -- all of which occur commonly in people with HIV. The CBC also gives you something called the "differential," which is an individual count of all the different kinds of white blood cells: neutrophils, monocytes, lymphocytes, basophils, and eosinophils. It is also crucial in determining your total CD4+ T-cell (or CD4) count, a critical measure of immune function.
CD4 Cell Count
The FACS (not "fax," although pronounced the same way) or Fluorescence Activated Cell Sorter machine identifies CD4 cells and directly measures what their proportion (percent) is of all your lymphocytes (white blood cells) when your blood was drawn. So, the CD4 percent is what the machine actually measures, and the total (or "absolute") CD4 cell count is derived from multiplying the percentage of CD4s by the total lymphocyte count. That is why the differential from the CBC is necessary -- in order to obtain your CD4 cell count.
Over time, the CD4 percent is a more stable measure of your immune system function because the total CD4 count is influenced by your white blood cell count at the moment your blood was drawn, a number that varies constantly. Nonetheless, by habit and tradition, health care providers have used the total CD4 cell count for key decisions, such as when to start HIV meds or preventive therapy for opportunistic infections, like PCP (pneumocystis pneumonia).
Viral Load, or HIV RNA PCR (HIV Ribonucleic Acid Polymerase Chain Reaction)
This key test tells us how much virus is in a milliliter (abbreviation: mL) of your blood, one-fifth of a teaspoon. Note that it does not tell us how much HIV is in your entire body, where most of the virus is -- in tissues, such as your lymph glands or nodes. Even though this test uses a tiny quantity of blood, it has proven to be a very good indicator of response to antiretroviral therapy.
The most commonly used test is made by Roche, and has established cut-off values for the upper limit of how much virus can be detected and the lower limit, which is referred to as "undetectable" virus, a shorthand way of saying "below the limit of detection." Note that "undetectable" means the amount of HIV is less than the technical ability of the test to detect it and does not mean that HIV is not there. The Ultrasensitive test has a low-end cut-off of less than 20 copies of HIV RNA/mL.
There are other types of viral load tests (see Positively Aware July/August 2011, "Undetectable -- Says Who?"), such as the branched DNA (bDNA) assay, that are sometimes used. There are various technologies, but the measurement principles, with cut-offs for the upper and lower limits, are similar to the RNA PCR tests above.
Serum Chemistries -- Liver and Kidney Function Tests
Liver Function Tests (LFTs): These tests measure some key functions of the liver, a complex organ that performs many important tasks in the body. The liver manufactures proteins that are essential to blood-clotting and to keeping fluid in your bloodstream instead of leaking out into your tissues and causing swelling (edema). The liver breaks down most environmental poisons (toxins) and drugs to rid the body of them. It also forms bile, which is important for digestion.
Amino aspartate transaminase (AST) and alanine aspartate transaminase (ALT) are key enzymes that indicate how well liver cells (hepatocytes) are functioning. The levels of an enzyme called alkaline phosphatase and a protein by-product called bilirubin indicate how well the liver is producing and excreting bile, which is then stored in the gall bladder. Albumin, a protein made in the liver, is critical for keeping fluid in the bloodstream and is an overall measure of nutritional status.
Liver function can be damaged by alcohol abuse, environmental toxins (including street drugs), viral infection of the liver (viral hepatitis), and a long list of diseases and prescription medications. Since people with HIV sometimes also have chronic hepatitis B or C, drink too much, or experience liver damage (hepatotoxicity) from medications, LFTs are important to monitor.
Kidney (renal) function tests: These tests measure how well your kidneys are doing their primary job, which is to rid the body of protein waste (blood urea nitrogen, or BUN) and regulate blood volume by filtering out the waste and extra water to form urine. The two main kidney function tests look at the level of waste as a way to measure how efficiently your kidneys are operating. These tests can provide clues that someone might have HIV-associated nephropathy (HIVAN) or kidney malfunction due to other causes, such as dehydration, diabetes, or drug toxicity.
Since some drugs are excreted from the body by the kidneys, dose adjustments need to be made when a person develops altered kidney function. Awareness of kidney dysfunction is also important when selecting an ARV regimen because some medications, like Viread (tenofovir), are not preferred for someone with underlying kidney disease.
Kidney function should be checked when someone enters HIV care. Thereafter, people at high risk of developing kidney disease (primarily African Americans and diabetics or people with a family history of diabetes) should have their kidney function checked at regular intervals. Routine checks at least annually are also recommended for people on ARV therapy.