CD28 and activated PI3K-delta syndrome: After 90-minute anti-CD3/CD28 stimulation, phosphatidylinositol 3,4,5-trisphosphate (PIP3) and phosphatidylinositol 4,5-bisphosphate (PIP2) were measured using high-performance liquid chromatography-mass spectrometry.<h4>Results</h4>Higher levels of PIP3 (measured as the ratio of PIP3/PIP2) in stimulated T cells distinguished APDS patient from other subjects providing a useful biomarker of increased PI3Kδ activity.