Aim To determine whether dental registrants can use the dental health component (DHC) and aesthetic component (AC) of the Index of Orthodontic Treatment Need (IOTN) 'accurately' to an acceptable level of agreement and diagnostic validity.Method Participants from six different registrant groups were asked to score the IOTN for 14 cases based on study models and photographs as well as completing a short questionnaire. Participants in the study were all recruited at study days and annual conferences. The main outcome measures include the different registrant groups IOTN scores compared to expert panel scores using kappa statistics. To assess for diagnostic validity, individual participants sensitivity and specificity scores were calculated.Result Overall, 229 registrants took part in the study. For the DHC the specialist orthodontist (SO), postgraduate orthodontic student (PGOS) and the qualified orthodontic therapist (QOT) groups achieved a mean kappa ≥0.60 indicating 'acceptable' agreement with the expert panel scores. The dental foundation trainee (DFT) and general dental practitioner (GDP) group achieved a mean kappa of 0.20 and 0.22 respectively indicating poor and fair agreement. The student orthodontic therapist (SOT) group achieved a mean kappa of 0.55 indicating moderate agreement. For the AC none of the registrant groups achieved an acceptable level of agreement with the mean kappa scores for the different groups ranging from kappa 0.13-0.21, indicating poor to fair agreement.Conclusion Overall agreement for the DHC was varied for the different registrant groups ranging from fair to substantial agreement. Registrants were better at applying the DHC compared to the AC with agreement ranging from poor to fair. More needs to done to help registrants use the IOTN more 'accurately'.