International Headache Society classification: interobserver reliability in the diagnosis of primary headaches.

We assessed interobserver reliability of the International Headache Society (IHS) classification for diagnosis of primary headaches. The study was performed on 103 patients consecutively seen at two Headache Centres. Each patient was given a structured interview recorded on videotape. Four experienced clinicians then reviewed the interviews separately and made a diagnosis of headache according to IHS criteria at the one- and two-digit levels. At both the one- and the two-digit level the agreement was substantial (Kappa = 0.74 and 0.65, respectively). The analysis of reliability for each of nine items necessary for diagnosis showed an agreement ranging from substantial (Kappa = 0.69) to almost perfect (Kappa = 0.89). Our results indicate that the IHS classification has a good reliability for the diagnosis of primary headaches at the one- and two-digit levels.