The YGTSS has been widely utilized as an outcome measure in treatment studies for individuals with tic disorders and has utility for practitioners in gauging treatment response to pharmacological and psychosocial interventions. To date, however, there are no empirical guidelines for determining optimal tic symptom reduction and cutoff scores that correspond with clinical improvement. Consequently, the present study sought to formally establish the optimal criteria for judging treatment response using the YGTSS.
The optimal YGTSS total tic severity score percent reduction cutoff for determining response was 35%, when balancing the costs of false positives and false negatives equally (which is the goal in clinical trials). In a clinical setting, however, sometimes false negatives are more problematic than false positives. That is, a clinician does not want to abandon a treatment course for a patient that is actually responding. Given these scenarios, a 25% reduction could be used to determine response in clinical settings, given the slightly poorer performance on K(.5) (which weights false positives and negatives equally), but displays better sensitivity (and stronger (K(1) performance), meaning that fewer true responders are missed by this cutoff than at the 35% cutoff. Inflexibly requiring a higher percent YGTSS total tic severity score reduction in clinical contexts may result in changing interventions prematurely. In assessing the performance of raw difference YGTSS cutoffs to denote treatment response, YGTSS total tic severity scores of 6 or 7 were considered optimal. Raw score differences are less influenced by initial symptom severity and are readily communicable (i.e., it is easy to quickly assess how many points a patient has reduced when making treatment planning decisions).
Despite offering suggested cutoff scores and percent symptom reduction targets, ROC and QROC analyses found that YGTSS total tic severity score did not perform optimally in detecting treatment response. This may be a product of the inherent heterogeneity of tic presentation, with patients differing in motor and phonic tic severity; in creating a total score, patients with predominantly one symptom type may not be measured similarly to patients with equal motor and phonic tic severity. However, simply breaking down analyses by YGTSS Motor and Phonic score cutoffs can suffer from the same problem, where a patient who has all motor tics may be measured accurately but one who has additional phonic tics will not be measured as accurately when using only one of the two component scales. For example, if two patients have a YGTSS total tic severity score of 25, one patient who has a YGTSS Phonic score of 25 (the maximum attainable score) may present differently than a patient with a YGTSS Motor score of 13 and a YGTSS Phonic score of 12. In considering these two patients in the context of treatment response, if the first patient has a YGTSS Phonic reduction of approximately 50% and goes from a score of 25 to a score of 12, this change may be qualitatively different than the same YGTSS total tic percent reduction in the second patient that could render a score of 6 on both the YGTSS Motor and Phonic subscales. Thus, although the YGTSS is adequately able to detect clinical response, these data indicate that reconceptualization of the YGTSS scoring structure may enhance its measurement sensitivity.
Several limitations of this research are noteworthy. First, participants were drawn from several different studies offering different interventions. However, the aim of the study was to identify optimal YGTSS scores and percent reductions based on clinical response. The focus of these analyses are to detect clinical change; the mechanism of change (i.e., the intervention type) is not empirically relevant unless it is hypothesized that the YGTSS detects clinical change differently depending on its cause (e.g., a patient responding to pharmacotherapy will not be as accurately detected by the YGTSS than a patient who responds to psychotherapy). Given the conventional use of the YGTSS to assess outcome across pharmacotherapy and psychotherapy studies, there is no current evidence to indicate this is the case. In fact, sample heterogeneity tends to improve generalizability and the variation in treatment approaches may better reflect how the YGTSS is used in a variety of settings. Second, we were unable to examine optimal remission cutoffs associated with the YGTSS. However, unlike some psychiatric disorders, clinically significant improvement rather than remission may be the more common and realistic target in youth with tic disorders. Third, we were unable to group our ROC analyses by age, gender, or illness characteristics (e.g., YGTSS subscales) given the current sample size; however, there is no a priori reasoning to expect why the YGTSS should perform differently among these populations. Fourth, the sample only included children and as such, these findings may not be generalized to adults with tics. Fifth, unfortunately data were not available to examine the degree to which CGI-I ratings and YGTSS total tic severity scores covary at different time points during treatment. Finally, the present analysis focuses on the YGTSS total tic severity score as a predictor of treatment response. It is important to consider that patients may present with both phonic and motor tics or with either predominantly phonic or motor tics. These differing classes of patients may not be reflected in the YGTSS total tic severity score (e.g., a child with severe motor tics [no phonic tics] may have a similar YGTSS total tic severity score as a child with moderate phonic and motor tics). Nevertheless, although this inherent limitation of the YGTSS total tic scoring may impact the measurement of tic severity, it does not impact the signal detection analysis which examines change in relation to the CGI-I.