Abnormal total T cell number

What it means

Abnormal increase or decrease of absolute number (either count per volume or proportion of total lymphocytes) of T cells or of a subset of T cells, commonly characterized as CD3+ lymphocytes, in the blood, compared to a reference range for a given sex and age-group. These may include both TCR alpha/beta and gamma/delta T cells.

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.

Rare diseases that can present with this1

The range is how often this sign appears among people diagnosed with each disease — it is not a probability of having any of them. Many of these signs also occur in common, non-rare conditions. A sign on this page is a conversation to have with a clinician, not a conclusion.

Part of the broader category

Also called: Abnormal number of T cells · Abnormal T cell count · Abnormality of T cell number