The LODs of the sandwich immunoassays for MN1-MN5 are at least three orders of magnitudes lower than those of competitive immunoassays, suggesting that small molecules with an epitope distance below 8.8 Å (MN5) are unlikely to be sensitively detected by sandwich immunoassay due to steric hindrance. In contrast, sandwich immunoassays exhibited very low LODs of 0.00035-0.9 nM and a wider detection range for MN6-MN9 than those of the corresponding competitive immunoassays with LODs of 0.001-38.8 nM (Table and Supplementary Table ). These results showed that the sandwich immunoassay for small molecules afforded superior sensitivity and detection range for MN6-MN9 with an epitope distance of 11.6-19.0 Å. In the case of avoiding steric hindrance as much as possible, these findings suggest the preferential use of sandwich immunoassays for the determination of small molecules due to its higher sensitivity and wider detection range.