Asbestos Fibers and Other Elongate Mineral Particles: State of the ...

pleural plaques as evidence of exposure when
diagnosing asbestosis); and (3) exclusion of alternative
diagnoses [ATS 2004]. The specificity
of an asbestosis diagnosis increases as the
number of consistent clinical abnormalities increases
[ATS 2004]. In practice, only a small
proportion of cases are diagnosed on the basis
of tissue histopathology, as lung biopsy is an
invasive procedure with inherent risks for the
patient. Thus, following reasonable efforts to
exclude other possible diagnoses, the diagnosis
of asbestosis usually rests on chest imaging abnormalities
that are consistent with asbestosis
in an individual judged to have sufficient exposure
and latency since first exposure.
Chest radiography remains the most commonly
used imaging method for screening exposed
individuals for asbestosis and for evaluating
symptomatic patients. Nevertheless, as
with any screening tool, the predictive value of
a positive chest radiograph alone depends upon
the underlying prevalence of asbestosis in the
screened population [Ross 2003]. A widely accepted
system for classifying radiographic abnormalities
of the pneumoconioses was initially
intended primarily for epidemiological use
but has long been widely used for other purposes
(e.g., to determine eligibility for compensation
and for medicolegal purposes) [ILO 2002].
A NIOSH-administered “B Reader” Program
trains and tests physicians for proficiency in
the application of this system [NIOSH 2007c].
Some problems with the use of chest radiography
for pneumoconioses have long been recognized
[Wagner et al. 1993] and recent abuses
have garnered substantial attention [Miller
2007]. In response, NIOSH recently published
guidance for B Readers [NIOSH 2007d] and for
the use of B Readers and ILO classifications in
various settings [NIOSH 2007e].
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In developed countries, conventional film radiography
is rapidly giving way to digital radiography,
and work is currently under way to
develop digital standards and validate their use
in classifying digital chest radiographs under
the ILO system [Franzblau et al. 2009; NIOSH
2008a]. Progress on developing technical standards
for digital radiography done for pneumoconiosis
and ILO classification is under
way [NIOSH 2008a]. In a validation study involving
107 subjects with a range of chest parenchymal
and pleural abnormalities typical of
dust-induced diseases, Franzblau et al. [2009]
compared ILO classifications based on digital
radiographic images and corresponding conventional
chest X-ray films. The investigators
found no difference in classification of small
parenchymal opacities. Minor differences were
observed in the classification of large parenchymal
opacities, though more substantial differences
were observed in the classification of
pleural abnormalities typical of asbestos exposure
[Franzblau et al. 2009].
Computerized tomography, especially highresolution
computed tomography (HRCT),
has proven more sensitive and more specific
than chest radiography for the diagnosis of
asbestosis and is frequently used to help rule
out other conditions [DeVuyst and Gevenois
2002]. Standardized systems for classifying
pneumoconiotic abnormalities have been proposed
for computed tomography but have not
yet been widely adopted [Kraus et al. 1996;
Huuskonen et al. 2001].
In addition to documenting structural tissue
changes consistent with asbestos-caused
disease, usually assessed radiographically
as discussed above, the diagnosis of asbestosis
relies on documentation of exposure
[ATS 2004]. In clinical practice, exposure
is most often ascertained by the diagnosing
NIOSH CIB 62 • Asbestos