Immunotherapy has revolutionized cancer treatment; however, the availability of cost-effective blood-based biomarkers for prognostic and predictive factors of immune treatment in patients with solid tumors remains limited. Due to low cost and easy accessibility, blood-based biomarkers should constitute an essential component of studies to optimize and monitor immunotherapy. Currently available markers that can be measured in peripheral blood include total monocyte count, myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs), relative eosinophil count, cytokine levels (such as IL-6, IL-8, and IL-10), lactate dehydrogenase (LDH), C-reactive protein (CRP), soluble forms of CTLA-4 and PD-1 or PD-L1, as well as circulating tumor DNA (ctDNA). In our mini-review, we discuss the latest evidence indicating that routinely accessible peripheral blood parameters-such as the neutrophil-to-lymphocyte ratio (NLR), lymphocyte-to-monocyte ratio (LMR), platelet-to-lymphocyte ratio (PLR), and rheological parameters, which so far have been rarely considered for such an application, may be used as non-invasive biomarkers in cancer immunotherapy. Rheological parameters such as whole blood viscosity are influenced by several factors, such as hematocrit, aggregability and deformability of erythrocytes, and plasma viscosity, which is largely dependent on plasma proteins. Especially in cases where the set of symptoms indicates a high probability of hyperviscosity syndrome, blood rheological tests can lead to early diagnosis and treatment. Both biochemical and rheological parameters are prone to become novel and future standards for assessing immunotherapy among patients with solid tumors.