Clinical value of soluble IgG Fc receptor type III in plasma from patients with chronic idiopathic neutropenia.

Previous studies have shown that the plasma level of soluble IgG Fc receptor type III (sFcgammaRIII) is a measure of the total body neutrophil mass. The aim of this study was to determine whether the plasma level sFcgammaRIII is associated with the risk of contracting bacterial infections in patients with neutropenia. We collected blood from 66 patients suffering from acquired idiopathic neutropenia, whose blood was sent to our laboratory for diagnostic evaluation of neutropenia (neutrophil count <1,500 cells/microL). Soluble FcgammaRIII levels were measured in plasma. Genotype distibutions of FcgammaR polymorphisms were determined. Clinical data were obtained from the patient files. Patients were assessed as to whether or not they had suffered from a bacterial infection 3 months before to 3 months after a single sFcgammaRIII measurement. In addition, longitudinal data were obtained from 21 patients. Of the 66 neutropenic patients who were included, 15 had suffered from a bacterial infection in the period 3 months before to 3 months after sFcgammaRIII measurement. The age and sex distribution was equal among the groups with and without infections, as were the genotype frequencies of neutrophil FcgammaR polymorphisms. Both neutrophil count and plasma level sFcgammaRIII were significantly lower in the patient group with infections, compared with the noninfected group (P = .03 and P < .0001, respectively). No infections were reported for patients who had plasma sFcgammaRIII levels above 100 arbitrary units (AU; normal value, 30 to 200). After matching each infected patient with two noninfected patients having the same neutrophil count, sFcgammaRIII plasma levels remained significantly lower in the group with infections (P = . 0001). For the patients who were followed in time, no infections were reported when sFcgammaRIII levels were above 100 AU. In conclusion, our population of patients with chronic idiopathic neutropenia with plasma sFcgammaRIII levels above 100 AU did not show an increased risk of contracting bacterial infections.