Impact of Investigator Gender on Perceived Pain Intensity After Acute or Scheduled Surgery
Phase: NA
Conditions: Postoperative Pain
Sponsor: Lund University

Study protocol 
 
 
Does investigator gender has an impact on patients perceived pain intensity after surgery? 
 
Aim 
The objective of this study is to investigate whether the gender of the investigator has an impact 
on patients reported levels of pain intensity after acute or scheduled surgery. 
 
Background 
Postoperative pain is a common problem after surgical procedures with many patients afflicted 
worldwide. Fundamental challenges are the complexity of measuring pain appropriately, and 
the many associated possible confounders. Over the last decades, gender of the investigator has 
been identified as a conceivable bias in the assessment and management of pain in 
experimental, as well as, clinical research. However, to our knowledge this issue has not so far 
been systematically investigated in a postoperative setting. 
 
In a preclinical setting Levine et al. showed that male subjects reported less pain when evaluated 
by a female than by a male investigator, and this was followed by other similar studies 
indicating that the gender of the investigator may have an impact on the pain report Kallai et 
al. 2004, Gijsbers et al. 2005, Aslaksen et al. 2007 and Vigil et al. 2014. In a recent review 
Chapman et al. 2018 emphasize that it may have significant impact on experiments and clinical 
trials.  
 
Methods 
In this prospective paired cross-over study, two investigators of opposite gender independently 
obtain individually reported pain intensity levels in each study patient based on three different 
methods of pain assessment the Visual Analogue Scale (VAS), the Numeric Rating Scale 
(NRS), and the Painmatcher® (PM) technique based on electrical stimulation, at Malmö 
University hospital in Sweden. 
 
Inclusion criteria is ongoing PACU care after recent surgery, age ³18 years, cognitive and 
linguistic abilities to understand instructions and participate in the study, and perceived 
postoperative pain. All study participants receive oral and written study information and sign 
an informed consent form.  
 
We have estimated to include around 50 patients per week, corresponding to around 600 
patients in total, which we value to be enough margin verifying or rejecting the primary 
hypothesis of our study. 
 
In each study session – from arrival at the PACU until approximately 15-minutes after arrival 
– the patients shortly describe, in words of their own, the main character of their postoperative 
pain. Afterwards they rate their current pain intensity level with the three different study 
techniques (VAS, PM and NRS in mentioned order). The investigators inform the patients 
equally verbally on how to carry out the pain measurements and the patients were blinded to 
the study aim of evaluating potential impact of investigator gender. Additional information on 
surgical and anaesthetic procedures, including current analgesic medication, is obtained from 
the peri-operative database of the hospital, and from individual patient records.  

 
Study value 
To further improve the postoperative care and pain relieve after acute and scheduled surgery it 
is of great importance to study biological, psychological and social factors affecting patients 
pain perception. More knowledge and awareness of investigator gender would possibly 
encourage caretaking and treatment of patients with pain and further on improved pain relief. 
 
Statistical analysis plan 
Study data is recorded in a Microsoft Excel spreadsheet. Non-parametric statistical tests will be 
used to evaluate study data reflecting pain intensity due to Visual Analogue Scale (VAS), the 
Numeric Rating Scale (NRS) and the Painmatcher®. With the Painmatcher® we obtained three 
values at each evaluation and the mean of these will be used for comparison. Pain intensity 
levels will be evaluated with respect to influence of investigator gender with the Wilcoxon 
signed-ranked test for dependent samples. Pain intensity levels will be compared between 
female and male study participants with the Mann-Whitney U-test. Reported pain descriptors 
will be analysed with qualitative methods. As is common in statistical practice we will adopt a 
significance level of P < 0.05.