We did a prospective study at a district general hospital in Buckinghamshire in the United Kingdom. Roads in the county of Buckinghamshire are almost universally surfaced in tarmac and are smooth, with any speed bumps raised from the road surface in a variety of designs and elevations. All patients aged 16 or over who had been referred to the on-call surgical team as part of their usual care, by either a general practitioner or an emergency department doctor, with suspected appendicitis were eligible. They were identified consecutively over a six month period between February and August 2012.
We asked participants to complete a questionnaire survey about their symptoms, including four specific questions related to their journey into hospital: mode of transport, whether they had travelled over speed bumps, whether they had had pain on the journey, and whether the pain changed when they went over a speed bump. We defined patients as “speed bump positive” if they had a worsening of pain from baseline over speed bumps and as “speed bump negative” if their pain stayed the same, if they were unsure, or if their pain improved on going over speed bumps. To minimise recall bias, patients had to complete the questionnaire within 24 hours of arrival in hospital and before they had been to theatre. We also recorded examination findings on admission from their notes. Two of the authors entered data on to a spreadsheet, and a third author double checked transcription.
We then followed participants through their admission to determine the outcome and whether they were taken to theatre for presumed appendicitis. For those who had been to theatre, we obtained the subsequent histology report. We used histological diagnosis of appendicitis as the reference standard, which is the usual practice in studies of appendicitis.6
One of the authors, who was blinded to all clinical details of the participants, corroborated interpretation of the histology findings. We also asked participants to provide contact details so that, if an alternative diagnosis or no diagnosis was made, we could contact them after their admission to ensure that their symptoms had resolved, to avoid missing cases of subacute or “grumbling” appendicitis. A positive or negative histological diagnosis of appendicitis was made in participants who went to theatre and had their appendix removed. We assumed participants whose symptoms resolved without surgery to have a negative diagnosis. We confirmed resolution of symptoms by telephone follow-up between two weeks and three months after admission.
In pilot data (11 cases and 21 controls) collected in 2009, the sensitivity was 82% (95% confidence interval 48% to 98%) and the specificity was 67% (43% to 85%). We used the R software package to simulate studies of varying sizes on the basis of these estimates. We calculated that 100-150 participants in the main study would be sufficient to show a likelihood ratio greater than 1.8-2.0.
We calculated the sensitivity, specificity, positive and negative predictive values, and positive and negative likelihood ratios, with 95% confidence intervals, for the outcome diagnosis of appendicitis. When a sign was recorded as “unsure,” we considered it absent for the purposes of calculation. We restricted the primary analyses to those patients reported to have travelled over speed bumps on the route to the hospital. We also planned to compare the diagnostic accuracy of worsened pain over speed bumps with more conventional diagnostic features of appendicitis, such as migratory pain and rebound tenderness. We used the “diagt” command in Stata (Release 11) for calculations.