AFP and hepatocellular carcinoma: More importantly, we found serum GDF15 levels above the optimal cut-off (1.945 ng/mL) in 81.3% and 85.9% of HCC patients with AFP levels below 20 ng/mL and 100 ng/mL, respectively, demonstrating the utility of GDF15 in the diagnosis of HCC in patients with normal or mildly elevated AFP.