Hair Analysis Panel Discussion: Section 5.3
5.3 Choosing the Best Biological Marker
Panelists briefly discussed if and when hair may be more advantageous than other biological samples, such as blood or urine. From both an exposure and clinical perspective, panelists considered which approaches were most productive. Generally, based on current science, they concluded that hair may be used to provide historical exposure perspective within a fairly small window of time (i.e., 1 year). Panelists' views are highlighted below:
Two panelists emphasized that the following question needs to
be answered in making such a determination: When might a substance
be detected in hair, but not in urine (measure of excreted amount)
or blood (measure of body compartment) (MG, LW)? Another panelist
encouraged consideration of the following question: For what substances
do we have knowledge of the toxicologic implication of the measurement
of the substance in hair compared to the measurement of the substance
in other biological specimens (e.g., urine, blood, bone) (MK)?
How do we move toward establishing the "gold standard?" Could
hair samples be a better way to non-invasively get a sample? Is
it a valid measure and how does that relate back to blood or target
organ levels (LW)?
Hair samples may be considered preferable or less invasive under
certain situations (e.g., pediatric exposures) (SS). Others commented
that collecting blood or urine samples did not appear to be that
much of an obstacle (MK, LW).
Hair may be considered for retrospective purposes when blood
and urine are no longer expected to contain a particular contaminant.
Again, the distinction between the use of hair analysis as an exposure
tool, rather than a diagnostic tool, was made (LW).
From a clinical point of view, it is important to focus on what
substances are of greatest interest, then ask what is the best way
to analyze them. Is hair analysis the best way to measure body burden
(instead of blood or urine)? For example, we may be able to analyze/identify
many elements in hair, but it still may be more useful to look at
blood levels. Blood may simply be the better body compartment to
test from a scientific point of view regardless of whether we can
test for a particular substance in hair. That is, what can potential
levels in hair tell us that blood levels do not (RB)?
An acute spike in hair might help document exposure, but generally
will not help from a diagnostic perspective (MG, LW). Acute exposures
are best measured through blood or urine (RB).
Growth rate is a key consideration. Assuming growth at approximately 1 centimeter a month, the hair on the average person's head generally represents a year or less of time. Hair analysis will therefore have limited usefulness in cases where exposures occurred more than a year prior to an exposure assessment (RB). While hair analysis may provide a snapshot of exposure conditions, it is not likely to predict long-term exposures (SS).