Table 1.

Alternative methods used to estimate thymic output and their limitations.

Method	Limitation
Naïve T cells (CD45RA, CD62L, CCR7)	Have longer life span, may proliferate or may convert into memory T cells also; memory cells can acquire naïve phenotype
Imaging	Provides a semi quantitative estimate of thymic output
T cell receptor excision circles (TREC)	Influenced by other factors such as the longevity of naïve T cells, peripheral expansion or apoptosis of T cells and intracellular degradation
T cell receptor diversity	Provides indirect information about thymic activity. More expensive and laborious
Novel markers such as CD31	Not all CD31+ naïve T cells are necessarily recent thymic emigrants (RTE). Furthermore, CD31 expression can be maintained during cytokine-driven proliferation of CD4 T cells