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The
hair pull test
The hair pull test is, in essence, very simple. A dermatologist
takes a few strands between their thumb and forefinger and pulls
on them gently. Anagen, growing hairs should remain rooted in place
while hairs in telogen should come out easily. By knowing how many
hairs were pulled and the number that came out, you can roughly
work out the percentage of hair follicles in a telogen state. So
if you pull on 20 hairs and 2 come out, then the frequency of telogen
hair follicles is 10%. As a (very) rough guide, a 10% telogen frequency
is excellent, up to 25% is typical, over 35% is a potential problem.
However, although the hair pull test seems simple in principle,
it contains many potential pitfalls and the wrong conclusions can
be drawn if the test is not properly conducted or the limitations
fully understood. 1) The most basic problem with the hair pull test
is that the results can vary depending on what the patient did with
their hair in the preceding hours. If the patient washed their hair
this will remove many of the telogen hairs. The washing process
helps massage telogen hairs out of the follicular canal. If the
patient is subjected to a hair pull test a few hours later the result
will be a false negative one. For this reason, if you see a dermatologist
about hair loss, you should not wash your hair in the preceding
24 hours. 2) The frequency of telogen shedding varies from day to
day. So you might have a positive hair pull test result on the day
of the consultation, but a few days later the pull test might be
negative - and conversely you might have a negative test on the
consultation day but a positive one a few days later. Ideally, to
get a more accurate picture of shedding, the hair pull test has
should be done each day for a week, or even longer, and an average
of the daily results calculated to even out the day to day variability.
Of course a dermatologist will not see you each day for a week,
so the hair pull test done during a consultation only gives a limited
snap shot in time. 3) Hair pull testing may also show an increase
in shedding in spring and fall as these are natural high shed seasons.
The dermatologist, and you, should take this into account when interpreting
the results. 4) The results of a hair pull test may vary over the
scalp, so more telogen hairs may be pulled out on top than at the
sides for example. To get a global picture, the hair pull test needs
to be done a few times in different locations. 5) Finally, the hair
pull test also needs to be put into context. Hair pull tests will
vary from person to person as some people can be naturally high
shedders, but these people may also be rapid hair growers so they
have no net hair loss. Equally it is possible to have a development
of alopecia not from an increase in shedding, but rather a failure
of new anagen hairs to grow after telogen hairs have been naturally
shed. In this instance, the telogen hair follicle frequency is normal,
but the scalp hairs gets gradually, progressively thinner. Overall
then, the hair pull test provides some information, but it is of
limited value on its own. The results of the hair pull test must
be put into context with other information.
There are things that can be done to help improve the accuracy
of the basic hair pull test, but they add time and therefore money,
so more detailed investigations are not commonly done. The hair
pull test can be taken a stage further and developed into the "unit
area trichogram". In this procedure, a few hair samples are
plucked from the scalp using rubber tipped forceps - both the anagen
and telogen hairs are taken. These can be laid out on a glass slide
and examined under a microscope. The dermatologist then counts the
number of anagen hairs and the number of telogen hairs.
The dermatologist may take the anagen/telogen hair fiber count
even further and measure the diameter of the hair fibers, and look
for intermediate (unusually thin and or short) hair fibers. The
average healthy hair fiber diameter is 80 micrometers or over. In
terms of meaningful hair coverage, hair fibers need to have a diameter
of at least 40 micrometers. Below 40 micrometers the hair fibers
are too thin to have much meaningful impact on scalp coverage. To
add even more information, the area of scalp from which the hair
fibers were plucked may be measured. In this way the hair fiber
examination results can be stated in terms of hair density on the
scalp. For more detials take a look at the trichogram and unit area
trichogram page in this section of keratin.com
The
hair wash test
A few dermatologists in Europe use a variation on the hair pull
test called the wash test. It involves washing the patients hair
and collecting all the hair fibers that come out during washing.
These hairs are then counted and may be examined further under the
microscope. This test suffers from the potential problems of the
hair pull test, but it does provide the advantage that the result
is a reflection of what is going on over the entire scalp and not
just in one or two places on the scalp. However, the test is not
popular as washing a patient's hair is time consuming for the dermatologist
and it does not provide much more information than the hair pull
test. This test approach has shown that children typically shed
very few telogen hairs with around 11 hairs shed per wash on average.
However, with the onset of puberty and as we continue to age so
the number of hairs shed in the wash test gradually increases. Shedding
50 or more hairs per wash is not unusual.
The
hair pull test references
- Olsen EA,
Bettencourt MS, Cote NL. The presence of loose anagen hairs obtained
by hair pull in the normal population. J Investig Dermatol Symp
Proc. 1999 Dec;4(3):258-60.
- Guarrera
M, Semino MT, Rebora A. Quantitating hair loss in women: a critical
approach. Dermatology. 1997;194(1):12-6.
- Rampini P, Guarrera M, Rampini E, Rebora
A. Assessing hair shedding in children. Dermatology. 1999;199(3):256-7.
- Rushton DH, de Brouwer B, de Coster W, van Neste
DJ. Comparative evaluation of scalp hair by phototrichogram and
unit area trichogram analysis within the same subjects. Acta Derm
Venereol. 1993 Apr;73(2):150-3.
- Olsen EA. Alopecia: evaluation and management.
Prim Care. 1989 Sep;16(3):765-87.
- Rushton H, James KC, Mortimer CH. The unit
area trichogram in the assessment of androgen-dependent alopecia.
Br J Dermatol. 1983 Oct;109(4):429-37.
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