Abnormal total CD4+ T cell number

What it means

Abnormal increase or decrease of absolute number of helper CD3+CD4+ T cells (either count per volume or percentage of total lymphocytes), compared to a reference range for a given sex and age-group. These are usually measured within the TCR alpha/beta positive population.

Absolute counts (cells per microliter) and proportions (percentage of lymphocytes or of a parent subset) capture different information, so clinical practice uses whichever is validated for a given decision. Some thresholds are count-based (e.g. Pneumocystis prophylaxis in adults); others rest on a proportion or on both, as in the SCID definition, which combines an absolute T-cell count with a naive-CD4 proportion. The choice is also age-dependent, since absolute counts vary with age more than percentages do, so percentages are often preferred in young children. Because a count equals a proportion multiplied by the total lymphocyte number, the two can dissociate: a normal proportion may accompany a low absolute count when total lymphocytes are low, and that dissociation can itself be informative.

Part of the broader category