It’s easy to let the numbers go to your head. Even a small drop in T cells or a tiny viral load blip can make it seem like ‘someone up there’ doesn’t like you. After all, for the last three months you’ve taken your pills on time, looked after your health, eaten well and avoided stress — so why aren’t your blood tests going along with the plan?
If this sounds like you, your doctor’s probably already explained that individual results can fluctuate for lots of reasons, sometimes for no reason at all, and usually that’s no cause for alarm. By understanding the factors that can influence your blood test results, and by focusing on the big picture rather than individual results, you and your doctor can make the best use of these important tools.
There is no single blood test that shows how well you’re doing, but most doctors and people with HIV use the CD4 and viral load tests as their primary guide. Together, these two tests come under the general heading of immunology. (The other routine blood tests that most of us have are called biochemistry or haematology — we’ll look at them in the second part of this series).
Lymphocyte counts — CD4 and CD8
The precious red stuff that your doctor or nurse syphons out of your veins every few months (*blood*) is made up of different types of cells, classified into three groups: white cells (*leukocytes*), red cells (*erythrocytes*) and platelets.
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CD4 cells are part of a group of white blood cells called lymphocytes, which includes many of the important immune system cells, including a class=“glossary-term” href=”/glossary/term/409”>B cells, T cells and NK (‘natural killer’) cells.
Your results will usually include a figure for the total lymphocyte count (the total number of all types of lymphocytes in each cubic millimetre of blood) and breakdowns for the two most important T cells — CD4 and CD8 cells.
CD4 Count
Your CD4+ T Cell Count gives an indication of your overall immune system health. The normal range for someone without HIV is 500-1350 CD4 cells per cubic millimetre of blood (cells/mm3), slightly higher for women and much higher for kids under six.
(An average-sized adult male has about five litres — one million cubic millimetres — of blood, so you can multiply your CD4 count by a million to guesstimate the total number of CD4 cells in your bloodstream. Because only about 2 percent of your immune cells are in your bloodstream at any time, an average-sized male with a CD4 count of 500 has about 25 billion CD4 cells.)
As HIV infection progresses, the CD4 count usually falls, and it usually rises when HIV replication is suppressed by antiviral treatment. But CD4 counts can vary from day to day, and even from hour to hour, for lots of reasons.
If you’ve recently had an infectious illness such as the ‘flu or herpes, or if you’ve recently been given some types of vaccinations, your total lymphocyte count, CD4 count and CD8 count may all rise. So if you’re ill, or you’ve been ill just before your blood tests are due, it may be sensible to wait a week or two to let your CD4 count settle down.
Women should be aware that their menstrual cycle can affect their CD4 count — for this reason, try to always schedule the test around the middle of your cycle.
Fluctuations can occur for other reasons — the season, time of day, stress levels, smoking, fatigue and the different equipment used to do the test at different laboratories can all affect the result.
The CD4 tests are useful because they tell you how your immune system is doing; they’re a good indicator of the likelihood that you’ll develop HIV-related illness in the near future. If your CD4 count is consistently below 350, your doctor will probably want to discuss starting treatment, and if it falls to 200 or lower, you’ll be encouraged to take drugs to prevent some common HIV-related infections.
CD4 T Cell Percentage
The CD4 percentage (CD4%) is calculated from your CD4 count and your total lymphocyte count — it’s the percentage of all lymphocytes that are CD4s. Because it doesn’t fluctuate as much as the CD4 count, it may give a more stable picture of how your immune system is doing than your CD4 count.
The normal range for CD4% is between 28 and 58 percent. As the CD4 count falls, the percentage usually falls too.
Another way of using your CD4 percentage is to make sense of a CD4 count that has gone down (or up) without explanation. Fluctuations in CD4 count that aren’t accompanied by a change in the CD4 percentage are common and usually no cause for concern.
CD8 Count
Your test report will usually also show the CD8+ T Cell Count and percentage. CD8 cells are another important part of your immune system.
The normal range for the CD8 count is about 250-1000. Unlike CD4s, CD8 cells don’t decrease due to HIV infection; in fact they may rise over time. The reason for this, and the role (if any) of CD8 cells in HIV disease, is still unclear.
As HIV disease progresses, the ratio of CD4s to CD8s usually falls, and in the past some doctors used the CD4:CD8 Ratio to determine the extent of HIV disease progression, but these days it’s much more likely your doctor will focus on just your CD4 count and percentage.
Viral Load
Viral load tests measure the amount of HIV in the blood sample by looking for HIV’s genetic material, called HIV RNA (ribonucleic acid) and give a result measured in copies per millilitre (copies/ml). ‘Copies’ refers to the number of individual HIV viruses measured in each millilitre of blood.
HIV activity in your body and the effectiveness of your treatments. Rises in viral load may indicate that your treatments are no longer working and may prompt a change in drugs, but as with CD4 counts, viral load can fluctuate due to infections, illness and vaccinations. Among people who aren’t taking treatments, the fluctuations are often very large.
There are a number of different viral load tests in use in Australia, and each uses one of two different methods of measuring viral load — Roche’s reverse transcriptase polymerase chain reaction (PCR) method or Chiron’s branched chain DNA (bDNA) method.
Both of these accurately measure the level of HIV in your blood, but the results may not be directly comparable. Your doctor probably always uses the same test, but if you change doctors or don’t always get your tests done at the same place, you may need to be aware of this.
Different tests also have different lower limits of detection, ranging from 400 to 25 copies/ml. If your viral load result is undetectable (or ‘not detected’) it means the viral load is lower than the lower limit of the test that was used.
How low can you go?
You’ve heard this before. ‘Undetectable’ does not mean ‘zero’. That’s because there is no viral load test with a lower limit of zero, and even if there were, there are places in your body where HIV can avoid detection by the test.
Some people feel let down if their viral load never reaches undetectable levels. You may feel a lot of pressure to get to undetectable, but it’s important to remember that many people remain well for long periods of time with low-but-not-undetectable viral load. If getting that undetectable result feels like breaking a world record, focus instead on your personal best and keep your viral load as low as you can.
The log scale
Viral load results are measured on a peculiar logarithmic scale (log scale). The table below shows what we mean:
| Viral Load | Log |
|---|---|
| 10 | 1 log |
| 100 | 2 logs |
| 1000 | 3 logs |
| 10,000 | 4 logs |
| 100,000 | 5 logs |
| 1,000,000 | 6 logs |
Because viral load fluctuates even more than CD4 counts, only large changes — greater than one log — are considered significant: if your viral load was 900 and increased to 12,500, this is an increase of 1.14 logs and may be significant. On the other hand, an increase from 12,500 to 90,000, while it seems quite a lot, is just 0.85 logs.
(How much can viral load fluctuate? I once had two viral load tests done on the same day at the same lab, the results were 294,140 and 217,450 — the difference of 76,690 copies seems like a lot, but it’s only 0.2 logs)
The big picture: combining your viral load and CD4 count
By combining your CD4 count and viral load, you can get an idea of your overall immune system health and the effectiveness of your treatments. The lower the viral load, and the higher the CD4 count, the better you’re doing.
Try to keep your eye on the big picture by looking for trends in your results over time rather than getting hung up on one result.
A simple way to think about the relationship between CD4 count and viral load is a train travelling toward a station, where that station is the point that you might become ill. The viral load is the train’s speed; the CD4 count is the distance to the station. You can delay your arrival at the station by slowing down the train, or by increasing the distance to the station.
Testing errors do occur
While all these tests are highly reliable and consistent, it’s possible that an unexplained change in a test result could be the result of a testing error. If something dramatic happens, it may be best to repeat the test and confirm the result rather than drawing conclusions from a single result.
You are not your numbers
Finally, remember that your test results only provide a guide to your and your doctor about how well you are doing. Use your test results as they are intended — as just one of many tools to manage your health. Don’t let them be your only focus.
