Q: I have read that in the evaluation of a patient for hair restoration surgery some doctors use a densitometer to measure miniaturization – the decrease in size of hair diameters. I read that miniaturization is a sign of genetic hair loss, but when there is miniaturization of greater than 20% in the donor area, a person may not be a good candidate for hair transplants. Is this correct and does 20% miniaturization mean that 20% of the population of terminal hairs have become fine vellus-like hairs or that there is a 20% decrease in the actual diameter of each of the terminal hairs? — B.A., New Albany, Ohio
A: Miniaturization is the decrease in hair shaft length and diameter that results from the action of DHT on healthy, full thickness terminal hairs. The hairs eventually become so small that they resemble the fine, vellus hair normally present in small numbers on the scalp and body. Miniaturized hairs have little cosmetic value. Eventually miniaturized hairs will totally disappear. Twenty percent miniaturization refers to the observation, under densitometry, that 20% of the hairs in an area show some degree of decreased diameter.
In the evaluation of candidates for hair transplantation, we use the 20% as a rough guide to include all hairs that are not full thickness terminal hairs. Of course we are most interested in the presence of intermediate diameter hairs — i.e. those whose diameters are somewhere between terminal and vellus and are clearly the result of DHT. I don’t know if one can tell the difference on densitometry between vellus hairs, fully miniaturized hairs and senile alopecia. The partially miniaturized population is most revealing.
Miniaturization in the recipient scalp (i.e. the balding areas on the front top and crown that we perform hair transplants into) is present in everyone with androgenetic hair loss. Miniaturization in the donor area, however, is less common (in men). It means that the donor area is not stable and will not be permanent. Men with more than 20% of the hair in the donor area showing miniaturization are generally not good candidates for hair transplant surgery.
Read about Miniaturization
Read about Candidacy for Hair Transplant Surgery
Posted by Robert M. Bernstein M.D. 
