This study validates the caregiver-rated Pediatric Acute-onset Neuropsychiatric Syndrome (PANS) Global Impairment Score (GIS), a single-item, 0-100 scale, for use in PANS. We collected longitudinal data from community patients meeting PANS criteria. We included 128 patients with 1926 GISs, each from a unique clinic visit. To assess discriminant validity, we compared GISs from patients with PANS with scores from a population of healthy controls. To evaluate external validity, we compared global impairment with a clinician-reported global measure-the Child Global Assessment Scale (CGAS)-using the Bland-Altman plots and correlation coefficients. Then, we evaluated associations between the PANS GIS and symptom-specific disease severity variables by fitting mixed models with repeated measures. The GIS shows excellent discriminant validity, distinguishing patients with PANS from healthy controls. The scores on the GIS show an acceptable level of agreement with the clinician-reported CGAS. The regression line in the Bland-Altman plot had a positive slope, indicating that parents tend to report higher disease severity than clinicians at higher levels of disease severity. Correlation was higher during disease remissions than during disease flares (r = -0.69 vs. r = -0.48). All disease severity scales predicted GIS in the expected direction. The GIS has excellent discriminant validity and acceptable construct validity.