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.