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Like
virtually all other psychiatric conditions, the diagnosis of ADHD is a
judgment made by clinicians based on various information collected during the
diagnostic evaluation. In the case of ADHD, parent and teacher reports of a
child’s behavior figure heavily in this judgment, and some critics have
contended that the entire process is heavily influenced by subjective factors
that render the diagnosis unreliable. Although it is true that parent and
teacher reports of a child’s behavior can be influenced by factors other than
the child’s actual behavior, and that the diagnosis of ADHD, like other
psychiatric diagnoses, depends on a clinician’s judgment, most experts agree
that ADHD can be diagnosed in a reliable and valid manner when careful and
systematic procedures are used. Nonetheless, many argue that the development of an "objective" test
for ADHD would be an important addition to current diagnostic procedures, and
various efforts have been made in that area.
The most well known and widely used of the objective measures for diagnosing
ADHD are called Continuous Performance Tests (CPT). In a typical CPT, an
individual sits in front of a computer terminal and is required to press (or
not press) certain keys depending on the stimulus that flashes on the screen.
The test typically lasts between 14 and 20 minutes and is purposely designed
to be repetitive and boring. Good performance requires the child to sustain
attention to a rather uninteresting task and to refrain from responding
impulsively. Both errors of omission (failing to press the designated key in
response to the target stimulus flashing) and errors of commission (pressing
the key to a non-target stimulus), along with several other variables (such
as reaction time and reaction time variability) are computed, and a child’s
score can be compared to how children of the same age and gender typically
perform. Several studies have shown that children with ADHD perform poorly on
these tests relative to non-ADHD children, and many clinicians routinely
incorporate the CPT into their ADHD evaluation procedures.
An important problem with many studies using the CPT is that children with
ADHD have been compared directly to children without any psychiatric disorder
(non-clinical controls) rather than to children with an alternative
psychiatric diagnosis (clinical controls). This is a serious limitation. For
a test like the CPT to be useful, it must not only differentiate children with
ADHD from "normal" children, but must also discriminate between
children who have ADHD and children with other psychiatric disorders (such as
anxiety, oppositional behavior, or depression) and children with learning
difficulties. In most instances where a child is being evaluated, clinicians
do not simply have to decide whether a child has ADHD or not, but are
involved in the more challenging task of determining the best explanation for
the difficulties that a child is displaying. Thus, clinicians are generally
required to determine whether a child’s symptoms reflect ADHD or some other
type of problem (differential diagnosis).
How useful is the CPT for this purpose? This question was addressed in a
recent study published in the Journal of Abnormal Child Psychology (McKee,
R.A. et al., Vol. 28, 2000). Participants in this study included 100 children
between the ages of 6 and 11 who had been consecutively referred for
assessment of potential ADHD to an outpatient child mental health clinic over
a 2-year period. Consistent with the pattern of referrals to psychiatric
clinics in general, most of the children (79%) were males. All children
received a thorough evaluation from a multi-disciplinary team that included
interviews with parents and the child, the collection of standardized
behavior rating scales from parents and teachers, and behavioral observations
of the child. In addition, as part of the diagnostic work up, children were
given the Conners’ CPT and several other tests designed to evaluate children for
reading disability.
The Conners’ CPT is one of several commercially available CPT programs and is
perhaps the most widely used by clinicians. It differs from other CPTs in
that it requires the child to respond by pressing a designated key when all
stimuli except the predetermined target are flashed on the screen. Other CPTs
require the child to respond only when the target is flashed. The importance
of this distinction is that the Conners’ CPT places a greater emphasis on the
child’s ability to inhibit themselves from responding when they are not
supposed to do so. Because a deficit in being able to inhibit behavior has
been proposed as the core deficit in ADHD (Phil: Link to article on Barkley’s
theory of ADHD) the Conners’ CPT is believed by many to be more useful in
evaluating children for ADHD than other available CPT programs.
Based on the diagnostic work-up, children were divided into 4 groups: those
with ADHD alone (n=42); those with only a reading disability (RD) (n=14);
those with ADHD and RD (n=14); and those with another psychiatric diagnosis
(n=32). The latter group consisted primarily of children diagnosed with
oppositional defiant disorder (ODD) or conduct disorder (CD), although a
variety of other conditions were also represented. In arriving at the
diagnosis of ADHD, children’s results on the Conners’ CPT were not used. This was because the researchers wanted to
compare the CPT results of children in the four groups who were diagnosed by
standard clinical procedures. If CPT results were used in assigning the
original diagnoses, these comparisons would be confounded.
Results
Although a number of different scores are computed for the Conners’ CPT, the
authors focused on the overall index in their analyses. The overall index
provides a global summary of how the child did on the test. According to the
manual which accompanies the Conners’ CPT, an overall index score above 11 is
considered a conservative cutoff for attention problems, and children who
score above this are considered to have "failed" the CPT. Thus, if
the Conners’ CPT is useful in helping clinicians make differential diagnostic
assessments, one would expect children with ADHD to have significantly higher
scores than children in the other groups. However, this was not the case. Children
with ADHD + RD performed worse on the Conners’ CPT than children in the other
groups, but those with ADHD alone, RD alone, and the psychiatric controls did
not differ. In fact, the data indicate that children with RD are more likely
to fail the Conners’ CPT than children with ADHD.
The authors also examined what percentage of children diagnosed with ADHD
were considered to have failed the CPT. This was the case for a little over
50% of the children. This means that among those with a carefully established
ADHD diagnosis, the likelihood of them performing above the clinical cut-off
on the Conners’ CPT was no better than chance.
Note: Because a number of other important variables besides the overall index
are computed on the Conners’ CPT, the authors repeated their analyses using
several other variables as well. The pattern of results obtained for these
other measures did not differ substantially from those reported above.
Summary And Implications
The primary conclusion from this study is that the Conners’ CPT has
questionable value as a diagnostic instrument. Although the authors recognize
that supporters of the Conners’ CPT may criticize their study for
oversimplifying their interpretation of the instrument, they correctly point
out that the inability of the overall index, as well as several other
measures derived from the test, to distinguish ADHD subjects from clinical
controls speaks to the test’s need for considerable refinement. Although
these data do not refute the potential utility of using instruments like the
CPT as one component of a diagnostic evaluation, they certainly underscore
the need to avoid using CPT results as a primary basis for deciding whether
or not a child has ADHD. In many cases, doing so is likely to result in
diagnostic errors that result in the selection of ill-advised treatments.
In general, parents and clinicians are advised to be cautious in attributing
too much significance to how a child performs on a CPT or on other so-called
"objective" indicators of ADHD symptoms. The
American Academy of Pediatrics and the American Academy of Child and Adolescent
Psychiatry recently published guidelines for the evaluation of ADHD and
neither advocates that any such tests be routinely incorporated into ADHD
evaluations, although recent research suggests that QEEG procedures for
diagnosing ADHD show promise. Thus, for the time being, the use of careful clinical interviews that
incorporate information from multiple sources will remain the cornerstone of
a comprehensive ADHD evaluation.
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