KRAS mutations are common in colorectal cancer (CRC). In this setting, mutation status determination in circulating-free DNA from blood samples (liquid biopsy) has been shown to be a viable alternative to tissue testing. The objective of this study was to compare the sensitivity of two liquid biopsy methods for detecting KRAS mutations in plasma samples from metastatic CRC patients. Samples with a positive (KRAS-MUT+) result and a mutant allelic fraction (MAF) < 5% according to the OncoBEAM RAS CRC assay were pairly analyzed by the Idylla ctKRAS Mutation Test (n = 116). In a cohort of 71 patients with at least 1 year of follow-up, the progression-free survival (PFS) was determined according to MAF values. Idylla detected KRAS mutations in 81/116 OncoBEAM KRAS-MUT+ samples with MAF < 5% and in 48/79 samples with MAF < 1%. Concordance between OncoBEAM and Idylla significantly improved at higher MAF values. PFS rates at 6 and 12 months tended to be lower in patients with MAF levels between 1% and 5% than in those with levels <1%. OncoBEAM demonstrated greater sensitivity for plasma detection of KRAS mutations than Idylla. Importantly, our data identified a "gray zone" below 1% MAF where Idylla showed reduced KRAS mutation detection, highlighting the importance of an accurate method to provide the mutational status of CRC patients.