Not all T-lymphocytes express CD4 on their surface, so the total lymphocyte (white blood cell) count is an important measurement your doctor will monitor was well in your routine blood tests. The CD4 percentage takes into consideration the total lymphocyte count when assessing immune system health. The CD4 percentage may be an additional (or secondary) clinical marker (indicator) to help guide treatment decisions. Generally a CD4 percentage below 20% suggests declining immune system health, so it is best to have a percentage results above this. Below 15% can be considered an increased risk of HIV disease progression and opportunistic infections.