The Dowling Oration was delivered by Dr. Andrew Griffiths in Liverpool, England in March 2003 as he reflected on 35 years as a dermatologist. It is frequently used as a reference in PRP-related research.
To explain his methodology for calculating the prevalence rate of pityriasis rubra pilaris, Griffiths writes: “Most dermatologists never have a new case of PRP although they may see one at a meeting, so it is clear that PRP is a very rare disorder. We estimated, very roughly, a prevalence rate of … one case per 400,000 of the population. This would mean that there are around 150 cases in the UK at any one time. Assuming there are approximately 600 dermatologists in the UK, that means just under one in four dermatologists may be currently looking after a case. (At this point a show of hands of how many people were currently looking after a case of PRP was approximately 1 in 5).”
There are two unanswered questions:
✴ What percentage of the PRP diagnoses were supported by biopsy?
✴ When he asked for a “show of hands,” did he count the responses of dermatologists who had raised their hands in previous meetings?
Understanding the PRP Prevalence Rate
To keep the math simple, let’s start with a population of 400 million, nearly equal to the combined populations of the US, UK and Canada (420 million).
If the odds of getting PRP are one in 400,000, then there are 1,000 “active” PRP patients in this hypothetical comparison.
The odds of getting Type 1 adult onset (greater than 50%) are over one in 727,000 or only 550 of the original 400 million.
The odds of getting Type 1 adult onset PRP with remission within four years is one in 909,000 or 440.
What makes a disease a rare disease?
According to the Rare Diseases Act of 2002, a rare disease in the U.S. affects populations smaller than 200,000 individuals. Over 6,000 rare diseases currently impact over 25 million Americans.
With an estimated population of 327.5 million, the U.S., should have 818 “active” PRP patients. Unfortunately, the PRP Community Database lists only 262 as of April 24, 2018.
✴ Are we actually missing 556 PRP patients?
✴ Is the one in 400,000 and accurate indication of the PRP population?