Is The Conners' CPT A Valid Tool For Diagnosing ADHD?

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.

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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