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differences between treatments (Breivik et al, 2000 Level II). Other limitations include personal,
cultural or linguistic differences in interpretation of the specific words chosen as descriptors
both between patients and also between patients and their clinicians.
Numerical rating scales
Numerical rating scales (NRS) have both written and verbal forms. Patients rate their pain
intensity on the scale of 0 to 10 where 0 represents ‘no pain’ and 10 represents ‘worst pain
imaginable’. The Verbal NRS (VNRS) is typically administered using a phrase such as: ‘On a
scale of 0 to 10, with 0 being no pain at all and 10 being the worst pain you could imagine,
where would you rate the pain you are experiencing right now?’. It is important that scales are
consistent, and it is recommended that the ‘no pain’ point be represented as zero (0) rather
than 1 (Scott & McDonald, 2008). Pain relief may be measured in the reverse direction with 0
representing ‘no relief’ to 10 representing ‘complete relief’. A visual form of the 11‐point NRS
with tick marks on a line or boxes with numbers may also be used (Breivik et al, 2008). This is
widely used, but some patients have difficulty representing their pain in numerical terms and
are better suited to a categorical scale. A value of 4 or more is often used as a threshold to
guide clinical intervention (Hartrick et al, 2003).
Visual analogue scales (VAS) consist of a 100 mm horizontal line with verbal anchors at both CHAPTER 2
ends and no tick marks. The patient is asked to mark the line and the ‘score’ is the distance in
millimetres from the left side of the scale to the mark. VAS are the most commonly used scales
for rating pain intensity in research, with the words ‘no pain’ at the left end and ‘worst pain
imaginable’ at the right. Pictorial versions also exist. VAS can also be used to measure other
aspects of the pain experience (eg affective components, patient satisfaction, side effects).
Assessment of pain immediately after surgery can be more difficult and lead to greater
interpatient variability in pain scores because of transient anaesthetic‐related cognitive
impairment and decreases in visual acuity. A ‘pain meter’ (PAULA) which used five coloured
emoticon faces on the front of a ruler and corresponding VAS scores on the back, and allowed
patients to move a slider to mark the pain they were experiencing, resulted in less variance
than pain scores obtained from a standard VAS (Machata et al, 2009 Level III‐2).
VAS ratings of greater than 70 mm are indicative of ‘severe pain’ (Aubrun et al, 2003 Level IV;
Jensen et al, 2003 Level IV) and 0 to 5 mm ‘no pain’, 5 to 44 mm ‘mild pain’ and 45 to 74
‘moderate pain’ (Aubrun et al, 2003 Level IV). A reduction in pain intensity by 30% to 35% has
been rated as clinically meaningful by patients with postoperative pain (Cepeda et al, 2003
Level IV; Jensen et al, 2003 Level IV), acute pain in the emergency department (Lee et al, 2003
Level IV), breakthrough cancer pain (Farrar et al, 2000 Level IV) and chronic pain (Farrar et al,
2001 Level IV).
These scales have the advantage of being simple and quick to use, allow for a wide choice
of ratings and avoid imprecise descriptive terms (Scott & McDonald, 2008). However, the
scales require more concentration and coordination, need physical devices, are unsuitable
for children under 5 years and may also be unsuitable in up to 26% of adult patients (Cook
et al, 1999).
The VAS has been shown to be a linear scale for patients with postoperative pain of mild–
moderate intensity (Myles et al, 1999 Level IV) and severe pain (Myles & Urquhart, 2005 Level IV).
Therefore, results are equally distributed across the scale, such that the difference in pain
between each successive increment is equal.
Verbal numerical rating scales (VNRS) are often preferred because they are simpler to
administer, give consistent results and correlate well with the VAS (Murphy et al, 1988 Level IV;
DeLoach et al, 1998 Level IV; Breivik et al, 2000 Level IV). Recall of pain intensity using the VNRS

Acute pain management: scientific evidence 37