There is an official scratch and sniff test developed at the U of Pennsylvania. If anyone is curious or unsure, you can order a test online. It is purported to be 90% accurate in diagnosing Parkinson's. What does that mean for those of us who retain our sense of smell? The author's of this article make a strong case that our clinical diagnostic criteria be expanded or improved on by looking at things beyond the "cardinal signs". They point out that...

Is Parkinson's disease a primary olfactory disorder?

Viewed alone the UPSIT result places 81% of patients outside the 95% limit for controls. When this information is combined with OEP data (4/10 abnormal where UPSIT score was normal) the fraction of olfactory abnormality approaches 90%. Such frequency is higher than that of tremor, which is usually quoted at 70%16 and nearly equals that of rigidity and akinesia. At least for a hospital population, olfactory dysfunction is common and potentially as important as the cardinal motor signs of PD.

A final point to consider is what we mean by a disease. The frequency of the three cardinal signs of PD is not well established, and was addressed only by Hoehn and Yahr at a time when parkinsonian syndromes were less well appreciated. No single sign is pathognomic of PD. Defective smell sense is found in about 80% of IPD classified clinically. What if there were patients with tremor, rigidity and akinesia, of whom 80% were blind? Would this be classified as movement or visual disorder? If the blindness came first the disease would probably be classified as a visual disorder and it would then be proposed that the condition was a visual problem with motor accompaniments. If blindness is replaced by anosmia, the analogy is complete. The most disabling symptoms and signs (and indeed the initial symptoms and signs) do not necessarily equate with the primary causative mechanism.