Baseline and On-Treatment Absolute Lymphocyte Count-Defined Lymphopenia in Solid Tumors Treated With Immune Checkpoint Inhibitors: A Systematic Review With Structured Qualitative Synthesis

Immune checkpoint inhibitors are widely used in solid tumors, but patient outcomes remain variable, and practical prognostic markers are still being clarified. This systematic review examined whether lymphopenia, defined by absolute lymphocyte count, assessed before treatment and/or during therapy, is associated with survival outcomes in adults with solid tumors treated with immune checkpoint inhibitors. A total of 21 studies were included, with sample sizes ranging from 22 to 18,186 patients. Several studies reported associations between lower absolute lymphocyte count and less favorable survival outcomes, although findings were heterogeneous and context-dependent. Overall, the available evidence suggests that absolute lymphocyte count-defined lymphopenia may have prognostic relevance in some settings, although substantial heterogeneity across studies limits firm conclusions and highlights the need for more standardized research.