This study was based on data from the Danish Colorectal Cancer Group’s national prospective database, which has a data completeness rate of over 96%,15 and electronically registered medical records with detailed information on postoperative treatment. Using these registries, we aimed to compare the risk of anastomotic leakage among patients receiving regular postoperative NSAIDs (cases) with those not receiving regular NSAID treatment (controls). In Denmark, electronic medical records were introduced to hospitals from 2003 onwards. With these new recording systems, all treatments administered at a hospital were documented. The medical staff is not allowed to administer treatment without electronic registration and, therefore, data completeness is 100%.
We included patients from the six major centres responsible for colorectal cancer surgery in eastern Denmark (population 2.6 million, about half of the country’s entire population). This area was chosen for logistical reasons, since registrations from medical records had to be performed with our physical presence in the different areas. Inclusion criteria were patients with available electronic medical records who had undergone an elective operation for colorectal cancer between 1 January 2006 and 31 December 2009 with either colonic or rectal resection, and receiving a primary anastomosis.
From the database, we retrieved information on resection type (coded as either colonic or rectal resection), demographic variables, comorbidities (pre-existing diabetes mellitus, ischaemic heart disease, chronic obstructive lung disease, or hypertension), alcohol and tobacco use, tumour T stage, intraoperative blood loss (mL) and transfusion (whether it occurred or not), open or laparoscopic procedure, and anastomotic leakage. Alcohol consumption was registered as units of alcohol per week (1 unit=12 g ethanol) in the following categories: 0 units, 1-14 units, 15-21 units, and more than 21 units. Tobacco use was registered as active smokers, previous smokers, and non-smokers. We defined anastomotic leakage, according to the definition previously proposed and used,16 (
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17 (
link) as clinical leakages requiring acute surgical intervention such as re-laparoscopy or re-laparotomy. Radiological or endoscopic drainage was not considered surgical intervention.
Data for postoperative NSAID consumption from electronic medical records were registered for each patient by three observers blinded for the presence of anastomotic leakage. Furthermore, the type of NSAID (that is, the active component) was recorded. In the electronic records, this information was registered if and when a dose of a given drug was prescribed and if this dose was taken by the patient. Only doses registered as taken were included. We defined regular postoperative consumption of NSAIDs as at least two days’ treatment with a relevant daily dose of an NSAID in the first seven days after surgery. This period was chosen because clinical anastomotic leakage occurs after a median of seven days postoperatively.18 (
link) We defined the relevant daily dose as at least 50 mg for diclofenac and at least 800 mg for ibuprofen. We retrieved data for 30 day postoperative mortality from the Danish Central Person Registry by looking up each patient, and related these data to the date of surgery. No information on cause of death was available; therefore, mortality was defined as all cause mortality.
Statistical analyses were performed with SPSS (version 17). We tested for distribution of variables between groups with χ
2 and two sided Fischer’s exact tests for dichotomous variables and with Mann-Whitney’s test for continuous variables. To identify possible risk factors for anastomotic leakage, we planned to perform univariate logistic regression analyses on all variables with less than 10% missing data. These variables included NSAID use and drug type, intraoperative transfusion, colonic or rectal resection, sex, surgical centre where surgery was performed, age at time of operation, intraoperative blood loss, American Society of Anesthesiologists’ score, open or laparoscopic surgery, and tumour T stage. We included all variables with P<0.1 in a multivariate logistic regression analysis (method: backwards, likelihood ratio). Furthermore, we planned to test for interactions between the variables included in the multivariate analysis—we included any significant interactions in the multivariate analysis. In the multivariate analysis, we excluded patients if they had missing data for a variable included in the model. We provided the number and percentage of patients excluded from the multivariate analysis, and presented results as odds ratios or proportions with 95% confidence intervals and P values, unless stated otherwise. Differences between independent proportions were calculated as absolute risk increase with confidence intervals and calculated according to method 10 in reference 19.19 (
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Klein M., Gögenur I, & Rosenberg J. (2012). Postoperative use of non-steroidal anti-inflammatory drugs in patients with anastomotic leakage requiring reoperation after colorectal resection: cohort study based on prospective data. The BMJ, 345, e6166.