The whole-mouth taste threshold test establishes an operational measure of the lowest concentration of a stimulus discernible to a subject, that is, the absolute threshold measure. The essential feature of this method is to generate a concentration that falls halfway between random and perfect performance. The efficiency of the forced choice staircase procedure has made it the test of choice for clinical assessment of gustatory function. First, the tester must select a series of concentrations of all four (sweet, salty, sour, and bitter) tastes that span the expected threshold value. These are spaced on a log scale since concentrations spaced in this way are approximately equally spaced in perceived intensity. The patient is given two small cups containing two solutions: water and a particular concentration of NaCl (or sucrose, citric acid, or quinine). The subject rinses his or her tongue with water before tasting each solution (to avoid adaptation effects) and then chooses the one with a taste. If uncertain, the subject is instructed to guess. The use of two solutions on each trial means that we must modify the up-down routine because patients will be correct 50% of the time if they simple guess which solution has the taste. If the subject is correct, then concentration is presented again on the next trial. If the subject is wrong the concentration is raised for the next trial. If the subject is correct a second time, then concentration is lowered for the next trial. This will produce a series of runs of correct (i.e., two trials of correct choices) and incorrect (i.e., one trial with an incorrect choice) responses. The concentration at which the responses change form correct to incorrect or vice versa is called a reversal. The first reversal is usually discarded and the next six reversal are averaged (geometric mean) to produce the threshold. This threshold represents the concentrations chosen correctly.