Loss of sense of smell as marker of COVID-19 infection

Excerpt:

In Germany it is reported that more than 2 in 3 confirmed cases have anosmia.
In South Korea, … 30% of patients testing positive have had anosmia as their major presenting symptom in otherwise mild cases
There is potential that if any adult with anosmia but no other symptoms was asked to self-isolate for seven days, in addition to the current symptom criteria used to trigger quarantine, we might be able to reduce the number of otherwise asymptomatic individuals who continue to act as vectors, not realising the need to self-isolate.

Simple clinical test for anosmia (from Davidson & Murphy, 1997):

Standard 70% isopropyl alcohol preparation pad is opened such that 0.5 cm of the pad itself is visible. The alcohol pad is placed beneath the patient’s nostrils while the patient inspires twice, to familiarize himself or herself with the alcohol odor, and the subject is asked if he or she detects an odor. Odor thresholds for alcohols are 2 or more orders of magnitude lower than trigeminal thresholds for the same stimuli.6 Thus, an anosmic will detect the presence of alcohol trigeminally only when it is extremely close to the nose. The alcohol pad is withdrawn and the threshold test begun. The subject is asked to close the mouth and eyes, breathe normally, and indicate when the odor is detected. Active sniffing and deep inspiration are discouraged. The basic procedure follows the method of limits. A standard metric tape measure is extended downward from the patient’s nares and held in place (Figure I ).

The alcohol pad is placed 30 cm below the nose and, with each expiration, is moved 1 cm closer to the nares until the subject detects the presence of odor. The distance from the anterior nares to the alcohol pad is measured in centimeters at the point at which the subject first detects the odor. The procedure is repeated 4 times and the mean distance defines the threshold.

Threshold For purposes of comparison, all of the subjects completed a standard olfactory threshold test. A series of 10 concentrations of butanol ( -butyl alcohol) was used to determine absolute olfactory threshold sensitivity. The highest butanol concentration consisted of 4% vol/vol in distilled water. Each successive dilution was one third of the preceding dilution. Two “blanks,” containing only distilled water, were also prepared. All bottles, including blanks, contained 60 mL of liquid. Olfactory threshold was assessed with a modified version 7 of a 2-alternative, forced-choice,ascending method of limits procedure.

The subject was presented with 2 bottles, one containing the odorant and the other consisting of distilled water. Each nostril was tested separately. The spout of the bottle was inserted into the nostril of interest. The subject was asked to squeeze the bottle to generate a puff of air. The subject did this with both bottles. Subjects were asked to identify which of the 2 bottles contained the stronger odor.All subjects began at the lowest concentration to avoid adaptation.9 Incorrect choices led to presentation of a higher concentration and correct choices led to continued presentation of the same concentration to a criterion of 5 successive correct responses. The presentation of the odorant and blank were randomized for each comparison trial and the nostril to be tested first was also randomly determined.

There were approximately 45 seconds between trials to allow time for recovery of the olfactory system and for the odor molecules to collect in the head space of the bottle.