ObjectiveTo evaluate the performance of a newly developed assay to assess neutrophil function capacity. After optimization, the assay was performed on samples derived from patients with septic shock and compared with healthy controls and patients with a systemic viral infection.DesignProspective evaluation of the performance of a new assay.SettingsMedical intensive care unit, hospital laboratory.PatientsTen patients with septic shock, ten patients with infectious mononucleosis, and ten healthy controls.Measurements and Main ResultsWe report an assay to assess neutrophil function capacity, in which CD10 membrane expression is measured by FACS before and after in vitro stimulation with Staphylococcus aureus bacteria. This assay evaluates the early activation state of circulating neutrophils and is shown to be of value in diagnosing a sepsis syndrome. First the assay was optimized. As an anticoagulant, sodium-citrate gave the best results. Blood samples must be kept on ice to reduce activation inside the siliconized tube and can be stored in this way for at least 8 hrs without affecting the test results. Kinetic studies showed a maximal expression of CD10 on neutrophils of healthy volunteers after 15 mins of stimulation with S. aureus bacteria. Second, the test was performed on samples derived from ten septic patients and ten patients with infectious mononucleosis. Septic patients had a significantly decreased CD10 expression capacity compared with healthy controls. Patients with infectious mononucleosis have a significantly higher CD10 expression capacity compared with septic patients, but in approximately one-half of them, the expression capacity was below the range found in controls.ConclusionsThese results indicate that in circulating neutrophils, the secretory vesicles have been mobilized completely in patients with septic shock. The assay proves to be of acceptable analytical quality and can be quickly and easily performed. Regarding clinical performance, this assay may be helpful in diagnosing septic shock. (Crit Care Med 1999; 27:549-553)