Abstract Background : to determine the operative characteristics of the Geriatric Scale 8 versus the Comprehensive Geriatric Assessment to detect older adult patients with cancer at risk of functional decline. Methods : we conducted a systematic search for studies in three major indexed databases: Medline (PubMed), Embase and Google Scholar. We selected studies from 2010-2020, considering criteria such as older adults with some type of cancer, in which the G8 scale has been applied and has been associated with functional decline. Results : we found 690 relevant titles. After reviewing the titles, abstracts, and full articles, we selected two articles that met the inclusion criteria. From these articles, we found a sensitivity of 81%, a specificity of 38%, a positive likelihood ratio of 1.21 and a negative likelihood ratio of 0.63. Conclusions : We conclude that the G8 scale has a good sensitivity for the detection of functional decline in older adults with cancer, however, the scale does not have good results in diagnostic performance tests. The use of this scale as a predictor of functional decline should be given with caution to the results presented. More studies are required to evaluate the performance of the G8 scale against predicting functional decline.