Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
9.2.3 Available methods of analysis for house dust mites
There are three types of method for estimating the concentrations
of house dust mites or their allergens in (airborne) dust samples:
mite counts, immunochemical assays of mite allergen and guanine
determinations. The choice of a particular method depends on the
specific purpose of a study.
9.2.3.1 Mite counts
The prevalence of mites in settled house dust can be determined
by counting under a microscope after separation from the dust sample
by flotation or suspension. This technique permits the identification
of the predominant species and the recognition of live, dead, larval
or adult types. The disadvantages of this method include:
* the need for training and development of skill in determining
different mite species
* the failure to quantify faecal pellets and disintegrated mite
bodies and therefore to reflect the true extent of exposure to mite
allergen levels
* the unsuitability for large-scale (epidemiological) studies owing
to the time-consuming nature of the work (Platts-Mills & De Weck,
1989; EC, 1993).
A further limitation of this method is variation among the actual
extraction techniques. Bischoff et al. (1992) estimates that less than
10% of the mites are removed from the carpet by typical vacuuming
techniques, but this number varies with the type of surface, the type
of vacuum used and the vacuuming technique.
9.2.3.2 Immunochemical assays of dust mite allergens
Immunochemical assays are widely used to measure the
concentrations of house dust-mite allergens. The dust mite germ is
Dermatophogoides and allergens have been identified for three
species. The conventional labelling of these allergens are denoted by
the prefix "Der" followed by a letter indicating the species. These
assays are possible because the major allergens produced by house dust
mites, i.e., the group 1 allergens (Der p I, Der f I, Der m I) and the
group 2 allergens (Der p II, Der f II, Der m II) are well
characterized and purified. For immunochemical analysis, the dust
sample is extracted (e.g., in a buffered saline solution), and then
stored frozen until analysis.
Total mite allergen content can be assessed by
radioallergosorbent tests (RAST). This method provides a good estimate
of the relative potency of different allergen extracts, but cannot be
used for absolute quantification of mite allergen levels. An advantage
of the method is that it measures "relevant" antigenic determinants
that have elicited a response in allergic subjects, since human IgE is
used. Results vary with the composition of the extract used on the
solid phase and with the composition of the serum pool used for
detecting bound allergen. However, RAST inhibition results are
difficult to reproduce over an extended period of time.
http://www.inchem.org/documents/ehc/ehc/ehc214.htm
Page 154 of 284
6/1/2007