Patients' self-reported nausea: Validation of the Numerical Rating Scale and of a daily summary of repeated Numerical Rating Scale scores

Aim and objectives: To validate the Numeric Rating Scale (NRS) for postoperative nausea assessments, and determine whether a central tendency, median, based on patients' self-rated nausea is a clinically applicable daily measure to describe patients' nausea after major surgery.

Background: Postoperative nausea causes major discomfort, risks for complications and prolonged hospital stays. The NRS is recommended for the assessment of pain but is little explored for assessing nausea.

Design: A repeated measure design was carried out on patients who had undergone major surgery in three Swedish hospitals.

Methods: Nonparametric statistical methods were used to analyse (a) associations between the NRS and a verbal scale (no, mild, moderate and severe) and (b) to analyse associations between Measure 1 (nausea scores postoperative Day 1) and Measure 2 (retrospective nausea scores at rest and during activity, postoperative Day 2). Reporting of this research adheres to the Strobe Guidelines.

Results: The mean age of the 479 patients (44% women) in the sample was 65 years (range, 22-93 years). Self-assessed nausea scores from the NRS and the verbal scale correlated well (r_{S pearman} = 0.79). Correlation between nausea at rest and nausea during activity was r_{S pearman} = 0.81. The calculated median scores (Measure 1) showed only moderate correlations with retrospective nausea scores (Measure 2); 4-9 ratings, r_{S pearman} = 0.41; 6-9 ratings, r_{S pearman} = 0.54.

Conclusions: Numeric Rating Scale scores showed strong associations with a verbal scale; therefore, the NRS seems to be a valid tool to measure nausea intensity. The quality of daily summarised median nausea scores needs to be further explored before clinical use.

Relevance to clinical practice: The use of the NRS in assessments of nausea in postoperative care will facilitate communication between patients and health care professionals regarding nausea intensity. When documenting nausea, it seems unnecessary to distinguish nausea at rest from nausea during activity.