Infectious complications are defined according to Centers for Disease Control and prevention (CDC) criteria and International Sepsis Forum consensus definitions. A comprehensive list of diagnostic criteria, as well as an assessment of the interobserver agreement associated with these, has previously been published in Klein Klouwenberg, Crit Care Med 2013; 41:2373-8. Since these definitions are in agreement with the CDC criteria used in the EPCO definitions, they did not have to be modified for the PLUTO project. For all events, the post hoc probability of true infection will be categorized using a four-point scale (none, possible, probable, and definite infection). This enables us to distinguish cases with true infection from cases with, for example, postoperative systemic inflammation.

Postoperative pulmonary complications are registered according to the European Perioperative Clinical Outcome (EPCO) definitions. A postoperative pulmonary complication is registered if (1) the patient has a saturation below 90%, (2) the patients oxygen consumption is exceeding 5L/min or (3) the patient's saturation is below 94% while on oxygen therapy. In addition to clinical signs and symptoms, radiology findings, diagnoses and instituted therapies part of the EPCO definitions, duration of the episode and diagnostic modalities used to obtain the diagnosis (e.g. chest x-ray, CT, arterial blood gas, etc.) are registered.

Major Adverse Cardiac Events (MACE) are registered according to the StEP definitions. In addition, the items of the EPCO definition for MACE are included in the registration and all cardiovascular complications included in both these consensus definitions can therefore be reconstructed from the PLUTO database and easily be compared to other perioperative outcome studies. For every patient of 60 years and older, daily troponins are obtained every morning on the first three postoperative days. Daily, an anaesthesiologists assess the troponin values and institutes further diagnostic testing as indicated. For the remaining postoperative days and for patients younger than 60 years of age, troponin is measured on clinical indication.

Severity of all surgical complications is assessed according to the Clavien-Dindo classification.

Daily pain scores, both in rest and during activity, are measured via the Numeric Rating Scale (NRS). This score ranges from 0 to 10, with 0 being absolutely no pain at all and 10 being the worst imaginable pain.