Smell threshold test is a standardized threshold test that employs a dilution series of a stimulus in an odorless diluent. In clinical application, the stimuli is presented via small sniff or squeeze bottles or felt tipped pen like devices, using a series of ascending or descending concentration trials. The odor detection threshold is the lowest concentration of a certain odor compound that is perceivable by the human sense of smell.

The smell threshold test is based upon nearly two decades of research that provides reliable detection threshold measure. It’s firmly established upon hundreds of subjects; 75%, 95%, and 99% confidence intervals provided for each decade of age. This test consists of the presentation to the patient of a dilution series of pure CN I stimulant phenyl ethyl alcohol (a proprietary absorbent, eliminating liquid stimuli) in 17 half-log concentration steps ranging from -10 log vol/vol to -2 log vol/vol. The patient must choose whether the odor is contained in one of two sniff bottles on each trial. Concentration of phenyl ethyl alcohol is moved upward in full-log steps until correct detection occurred on five sets of consecutive trials at a given concentration. If an incorrect response is given on any trial, the staircase is moved upward a full-log step. When a correct response is made on all five trials, the staircase is reversed and subsequently moved up or down in half-log increments or decrements, depending upon the patients performance on two pairs of trials (each pair consisting of a choice between a blank and an odorant) at each concentration step. The geometric mean of the first four-staircase reversal points following the third staircase reversal is used as the threshold measure. The level of threshold is converted into a numerical score, based on published norms that provide a quantitative index of olfactory function.