Background: Albeit showing great benefit in individual cancer patients, only the minority of patients benefit from checkpoint inhibitor immunotherapies (IMTs). Mutation load and PD-L1 staining are used for response prediction, but are imperfect predictors. A universal method to quantify tumor cell-free DNA (TcfDNA) enables the early and effective evaluation of individual IMT efficacy[1], by comparing TcfDNA to pre-therapeutic values. Here we present a health economic evaluation of test usage. Methods: This cost-minimization study determines the economic efficiency of TcfDNA-test from the perspective of the statutory health insurance in Germany. The assumption is that the effectivity of the intervention (TcfDNA monitoring) and of the comparator (no test), is comparable with regards to the patient-relevant-endpoints (morbidity, mortality, quality of life). The model simulates the course of treatment for each patient with and without TcfDNA testing, calculating the respective cost. Treatment details, outcome (RECIST) and TcfDNA results are derived from an earlier clinical trial.[1] Costs are obtained from publicly accessible data bases. Two testing strategies are explored. Strategy 1: Testing all patients only before the second cycle. Strategy 2: Same as strategy 1 plus a confirmation test before the third cycle, in patients with initial result in a defined grey zone. Results: Both testing strategies correctly classified 68% of progressive disease patients. Testing strategy-1 misclassifies about 8% of disease control patients, which would lead to discontinuation of successful therapies in those. In contrast strategy-2 correctly classifies all patients with disease control, whilst requiring only 24% more tests, since ~50% of patients are undoubtedly classified after one cycle. Assuming six cycles at average costs of 4,781 EUR in the no-test-setting, average costs savings per patient with strategy-1 are 5,735 EUR (20% of treatment costs), with strategy-2 are 2,010 EUR (7%) compared to no testing. Conclusions: TcfDNA monitoring is a cost-saving strategy. However, a confirmatory strategy is desirable to avoid early discontinuation of successful IMT. [1] Weiss GJ et al.: Clin Cancer Res: 2017;5074-81.