Quantification of the Actin-Binding Protein Flightless-I in Human Serum by Automated Western Blot System and Investigation of Its Diagnostic Potential in Sepsis

<b>Background</b>: The actin-binding protein Flightless-I (Flii) has not been quantified in the human serum yet. We aimed to determine serum Flii levels in healthy individuals and to investigate Flii as a potential marker in patients with sepsis focusing on diagnosis, organ failures, and short-term mortality. <b>Methods</b>: A total of 30 controls and 64 septic and 22 non-septic patients were enrolled in this follow-up study. Serum Flii levels were quantified by using the capillary electrophoresis-based Simple Western™ system with chemiluminescent detection. The assay was calibrated by applying dilution series of a purified recombinant human Flii standard and a parallel internal standard. <b>Results</b>: Flii levels of healthy controls were found between 3.5 and 8.8 mg/L, while septic and non-septic patients showed significantly lower values (<i>p</i> < 0.001). First-day Flii could differentiate sepsis from the non-septic inflammatory state (AUC: 0.667; <i>p</i> < 0.05) and indicated acute respiratory distress syndrome (ARDS) among septic patients (AUC: 0.686; <i>p</i> < 0.05). Furthermore, a combination of Flii and other sepsis markers seemed to offer an improved diagnostic performance (sepsis vs. non-sepsis, AUC of Flii + gelsolin (GSN) + Gc-globulin + procalcitonin: 0.974; <i>p</i> < 0.001 and ARDS vs. non-ARDS, AUC of Flii + GSN + presepsin: 0.776; <i>p</i> < 0.001) compared with single markers even in the prediction of 14-day mortality (AUC of Flii + GSN + Gc-globulin: 0.76; <i>p</i> < 0.001). <b>Conclusions</b>: We adapted a properly precise and reproducible automated Western blot method to determine concentrations of Flii in human serum. Our results revealed the relationship between Flii and sepsis; however, Flii alone did not appear to be a prominent sepsis marker. When combined with other biomarkers, measurement of serum Flightless-I may provide additional value supporting patient care.