Methods, compositions, and apparatus for detecting the presence of caffeine in a liquid sample are provided. In certain embodiments, an internally referenced competitive assay allows a very precise determination of a threshold value of caffeine for use in semiquantitative types of ligand-receptor assays. By using a detection means that participates in two assays, sensitivity is doubled in the maximum sensitivity range and the range can be adjusted to match the predicted concentration range of an analyte. This format and the materials described herein allow the assay to complete within three minutes. In addition, this format accommodates common attributes of liquid samples for detecting caffeine, such as the inclusion of milk or sugar in a coffee-type beverage.