The aim of this study was to investigate validity and reliability of an instrument for patient classification in anaesthetic nursing care. The study included two parts. The first part involved two panels of experts, one internal consisting of nurse anaesthetists from the local hospital, and one external represented by nurse anaesthetists from nine different hospitals in Sweden and one in Iceland. The internal experts gave suggestions for improvements of the existing instrument. Thereafter the external experts gave their opinion about both the internal panels elucidation as well as the existing instrument. The instrument was thereafter adjusted in concordance to their suggestions. The second part involved an investigation of the inter-rater reliability, with one hundred double ratings before the changes in the instrument and one hundred ratings after the changes in the instrument. Statistical analyses for inter-rater reliability were performed with Spearmans correlation coefficient. The first assessment showed good inter-rater reliability in four of thirteen items, while the inter-rater reliability on the second version showed good agreement in eleven of thirteen items. Results indicated that the changes in the instrument were positive in terms of reliability. The instrument thus improved in terms of validity and reliability and may be of use in anaesthetic wards with similar context.