Biochemical tests for the presence of free light chains (FLCs) are used to indicate a possible diagnosis of multiple myeloma (MM). Most guidelines recommend measurement of urinary FLCs; however, recent research indicates that the presence of FLCs in blood might be a better indicator. In 113 patients with newly diagnosed MM, 100% had measurable κ/λ light chain ratios that involved FLCs in blood samples, compared with only 64% in urine samples. Furthermore, a finding of abnormal serum κ/λ light chain ratio was found to predict inferior overall survival. These data demonstrate that FLCs in blood, not urine, should be measured in patients with suspected MM.