Lateral flow testing device with on-board chemical reactant

A lateral flow testing device is provided for testing a biological fluid for the presence of methamphetamines. The device includes a substrate element such as a nitrocellulose strip, including an antibody zone and an immobilized drug conjugate zone for detecting presence of methamphetamines, and an on-board chemical reactant for preventing undesirable cross-reactivity to ephedrine and/or pseudoephedrine that may be present in the biological fluid being tested. Preferably, the on-board chemical reactant is sodium periodate. Methods for preparing test devices are provided including the step of striping a substrate sheet with a solution containing sodium periodate during manufacture of methamphetamine lateral flow test strips.