Impaired splenic function (hyposplenism) affects more than 0.4% of the French population, half of them splenectomised, portends a serious risk of complications, including severe infections, thromboembolism and leukemia. While many splenectomised patients are fully asplenic, loss of function is partial when the spleen has been damaged by acquired or inherited diseases like sickle cell disease (SCD). In SCD, hyposplenism emerges during infancy but spleen function may persist in adults, and correlate with disease severity. The spleen clears altered red blood cells (RBC) from the circulation. Best markers of hyposplenism thus assess this ability to retain stiff RBC and “pit” those that contain rigid bodies. RBC that have not been pitted in absence of a functional spleen contain nucleic acid-positive bodies (Howell-Jolly bodies, HJB) or empty vacuoles (Pocked RBC). The proportion of these RBC in circulation is highly discriminating. Pocked RBC are 30 times more abundant than HJB and predict hyposplenism with high sensitivity and specificity. Only pocked RBC counts closely correlate with splenic scintigraphy, the reference measure of spleen function. Pocked RBC counts are the most relevant marker of hyposplenism, but current quantification methods are suboptimal. Molecular markers of their vacuoles remain to be identified, and current counting is observer-dependent and slow. Our objective is to improve the relevance and applicability of splenic function markers, as a prerequisite for clinical studies to validate their prognostic & theranostic value. Specific aims are (i) to assess the correlations between residual splenic function and outcome in hyposplenism, and (ii) to establish reliable, observer-independent methods for measuring splenic function by quantifying RBC deformability with new microsphere-based and microfluidic techniques, or set-up counting of Pocked RBC by cytometry or imaging coupled with machine learning. The main barriers to be lifted are the poor adaptation of existing markers of spleen function to the clinical lab workflow, and their elusive validation as predictors of disease severity. The final product is a routine biological test that quantifies splenic function like serum creatinine levels quantify renal function. Important knowledge and medical gaps remain to be filled. Complex physical challenges on RBC as they cross splenic slits are poorly described. The removal of rigid bodies or vacuoles from RBC without cell lysis (the pitting process) is even more complex and can be fully observed experimentally only with highly specific microfluidic devices. On the medical side, there is a pressing need for biomarkers in SCD that accurately inform key treatment decisions, such as the best timing for treatment intensification, before irreversible organ damage occurs. Because the spleen is damaged early in SCD, spleen function markers are potentially informative. Impaired spleen function may altogether reflect past severity and portend a risk of further degradation, because pathogenic subpopulations of RBC are less efficiently cleared from the circulation. In some patients splenectomised for trauma, spleen function reappears due to regrowth of splenic nodules (splenosis). Results from markers will induce a beneficial simplification of follow-up in these patients. Conversely, if hyposplenism is confirmed, patient compliance to vaccination and antibio-prophylaxis will be enhanced with positive impact on prognosis. Our consortium brings together complementary inputs from physicians, physicists, pathophysiologists, and AI specialists. This shapes a cohesive translational task force mastering medical practice and sophisticated in silico and in vitro tools, including the only microfluidic chip with spleen-mimetic slits narrower than 1 micron currently available. We will undertake 6 tasks to find either the first specific markers of pocked RBC or to count them without label. We will also assess their predictive value.