The filling velocity of underfilled serum and heparin tubes has a significant impact on lactate dehydrogenase (LD) levels and the hemolysis index. Studies show that underfilling blood collection tubes results in higher LD activity and increased hemolysis index when compared to fully filled tubes. According to Neuwinger et al. (2019), increasing LD levels and the hemolysis index is largely dependent on the filling velocity of underfilled tubes. In particular, the filling velocity of the first half of the blood volume is much greater than that of the second half, suggesting that different filling velocities are responsible for the elevated hemolysis index and LD levels in underfilled tubes.