Fishing for T Cells

A common occurrence that confuses and upsets patients is an unexpected change in their T-cell count. It is especially mystifying to patients when I have left messages saying that their count is stable but they later get a copy of their labs and the numbers do not appear nearly the same as on their previous lab report. Let me try to clarify this issue, starting with terminology. In normal HIV-speak, when physicians talk about T cells we are talking about the T helper cells. These are the cells that HIV attacks and destroys. Their function is to coordinate a variety of immune responses, and when T-helper counts are low the immune system starts to fail. However, to be scientifically accurate, the term T cells refers to the combination of T helper (CD4) and T suppressor (CD8) cells. I make this distinction because it is important in understanding how T-helper counts are measured and thus why an appearance of change in the counts may not be meaningful. For the rest of this article, when I say simply T cells I will mean the total count of both CD4 and CD8 cells. T helpers are actually a subset of a subset of a subset of white cells. So to start the process of getting a T-helper count, we first run a complete blood count, which your doc probably refers to as a CBC. This counts the total number of white cells. The white cells are then broken down into five types based on how they look under the microscope and how they function. One of these subsets is called lymphocytes and is composed of T cells and B cells. If a patient starts with a normal white-cell count of 4,000 to 10,000'and lymphocytes are normally 20% to 40% of the white cells and T cells are about 50% to 80% of lymphocytes and T helpers run 30% to 60% of the T cells overall'then T-helper counts calculate out to a range of about 450 to 1,500. But because the white-cell and subset percentages may vary from moment to moment, the T-helper counts may bounce around without any change in immune status. Here is how this can happen. The white cells are not evenly distributed in the blood system. They sit in pools on the margins of the blood vessels and rush into the circulation whenever needed. This may happen when you have an adrenaline surge because you were almost in an accident on the way to your doctor appointment or because you ran up three flights of stairs because the elevator was not working. I like to think of white cells as schools of fish in the ocean. When you cast a net into the ocean, you might capture a big school of fish swimming past. Likewise, when you stick a needle into a vein and draw out some blood, you might capture a big school of white cells swimming past. Or maybe it is a slow day and the fish are not swimming around much. The percentage of lymphocytes in that school of white cells may shift. If you have a viral infection, like a cold, the lymphocyte percentage often goes up. But with a bacterial infection, like strep throat, the percentage may go down. So if at a previous visit you had just come from spinning class after recently recovering from the stomach flu, you may have seen a nice big spike in your T-helper count. Yet on the current visit the white cells were quietly not circulating while you were starting to get a sinus infection. Suddenly your T-helper count seemed to really drop. This is why it is important to be seen every three to four months for lab work. As your doctor sees more and more counts, he can see what your range is. With T-helper counts over 500, swings of several hundred are not uncommon. But with T-cell counts at 200, the swing is probably about 50 either way. Over time you can see if there is a general drift in the counts and not overreact to every shift. Another general rule of thumb is to look at the percentage of T cells that are helpers. This number tends to be more stable. Bowers is board-certified in family practice and is a senior partner with Pacific Oaks Medical Group, one of the nation's largest practices devoted to HIV care, located in Beverly Hills, Calif.