A diagnostic test strip for determining the presence of a specified analyte in a fluid sample is described. The test strip has a test membrane sandwiched between two layers of a plastic sheath. The upper layer of the sheath has a sample well into which a liquid sample is placed. The test membrane has a sample receiving zone typically containing a buffer and a fatty acid sarcosinate. Adjacent to the sample receiving zone is a reagent zone containing reagent chemicals including gold colloid particles coated with antibodies to the specified analyte. In fluid connection with the reagent zone is a test zone containing immobilized molecules of the specified analyte. Preferably, in fluid connection with the test zone is a control zone containing an indicator that changes color when wetted by the sample and a liquid sink zone to absorb excess liquid in the sample.