BACKGROUND: Symptom assessment is a critical component of concussion diagnosis and management, with item selection primarily driven by clinical judgment or expert consensus. We recently demonstrated that concussion symptoms assessed by the Sport Concussion Assessment Tool (SCAT) are essentially unidimensional, implying that overall symptom severity may be accurately estimated with relatively few questions. Briefer, evidence-based forms for symptom assessment would provide clinicians flexibility. PURPOSE: To develop and validate an abbreviated assessment of general concussion symptom severity using item response theory analyses. STUDY DESIGN: Cohort study (diagnosis); Level of evidence, 2. METHODS: Broad clinical assessments (SCAT3, Immediate Post-concussion and Cognitive Testing, Balance Error Scoring System, and Brief Symptom Inventory-18 Global Severity Index) were completed by 265 injured athletes and 235 matched teammate controls at 24 to 48 hours and 8, 15, and 45 days after concussion. Symptom checklist short forms (3-14 items from the original 22) were selected using item response theory item information curves. Internal consistency reliability (Cronbach alpha), correlation with criterion measures assessed concurrently (ie, acute neurocognitive performance, balance, and emotional symptoms), predictive validity (correlations with symptom duration), and differences between concussed and control groups (Cohen d) were examined across forms. Sensitivity and false-positive rates of the forms were estimated and compared using reliable change indices derived from controls. RESULTS: Across the 3- to 22-item forms, internal consistency was excellent (Cronbach alphas, 0.90-0.94). Clinical correlations were significant (P= .017) and to similar degrees for all short forms. Group difference confidence intervals overlapped across forms at 24- to 48-hour (Cohen d, 1.27-1.51) and 8-day follow-up (Cohen d, 0.31-0.44). Sensitivity remained similar across short forms, with a low false-positive rate in controls. CONCLUSION: Our findings suggest that even an ultrashort (3-item) inventory provides sufficiently reliable and valid estimates of overall concussion symptom severity 24 to 48 hours after injury. Future revisions of the SCAT could eliminate inefficient items, although replication in larger samples and extension to other postinjury time points are warranted.