Background It has been proposed that dysfunctional endothelial progenitor cells (EPCs) play a role in pathogenic vasculopathy in systemic sclerosis (SSc). However, there is some debate as to whether the EPC count is reduced in SSc. The European League Against Rheumatism Scleroderma Trials and Research (EUSTAR) group recently proposed recommendations for evaluating EPCs.

Objective To validate the proposed EUSTAR recommendations by a side-by-side comparison of methods for quantifying EPCs.

Methods Peripheral blood samples were obtained from 11 patients with SSc and 11 age-matched healthy controls. EPCs were simultaneously quantified by two methods: flow cytometry combined with immunomagnetic CD34+ cell enrichment or rosette-based lineage-negative (Lin−) cell enrichment. EPCs, defined as CD34+CD133+VEGFR2+ cells, were counted with and without fluorosphere calibration.

Results EPC counts measured with fluorosphere calibration correlated well with each other, regardless of the enrichment procedure used. In contrast, EPC counts from protocols that did not use fluorospheres correlated poorly with results from other protocols.

Conclusions The EUSTAR recommendations are valid when they are combined with fluorosphere calibration.