Abnormal T cell morphology Abnormal T cell subset number 2017-06-12T15:27:58Z 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. Abnormal increase or decrease of absolute number (either count per volume or percentage of total lymphocytes) of any T cell subpopulation, commonly characterized as CD3+ lymphocytes, in the blood, compared to a reference range for a given sex and age-group. Abnormal alpha-beta T cell morphology