From the Guidelines

Postoperative fever typically occurs within the first 48 hours after surgery and is often non-infectious in origin.

Fever in the early postoperative period (first 48 hours) is usually non-infectious, but after 96 hours, it is likely to represent an infection 1.
The timing of postoperative fever can vary depending on the type of surgery and the individual patient's response to the surgical procedure.
In general, postoperative fever can be categorized into three phases:
- Early postoperative fever (first 48 hours): usually non-infectious, caused by factors such as atelectasis, pain, or stress response to surgery.
- Intermediate postoperative fever (48-96 hours): may be infectious or non-infectious, requiring careful evaluation to determine the cause.
- Late postoperative fever (after 96 hours): likely to represent an infection, such as pneumonia, urinary tract infection, or surgical site infection 1.

A thorough evaluation, including physical examination, laboratory tests, and imaging studies, is necessary to determine the cause of postoperative fever.
Chest radiograph is not mandatory for evaluation of postoperative fever unless respiratory symptoms or signs suggest a high probability of utility 1.
Urinalysis or culture is not mandatory to evaluate fever during the initial 2-3 days postoperatively unless there is reason to suspect an infection at this site 1.
Wound infection is rare in the first few days after operation, except for group A streptococcal infections and clostridial infections, which can develop 1-3 days after surgery 1.
New or persistent fever days after surgery should raise a strong suspicion of persistent pathology or a new complication, requiring prompt evaluation and management 1.

From the Research

The timing of postoperative fever can vary, but most fevers that develop within the first 48 hours after surgery are benign and self-limiting 2.

Early postoperative fever, defined as a temperature greater than 100.4°F in the first 72 hours, is a common event and rarely caused by an infection 3.
Fever that develops after the first 2 days following surgery is more likely to have an infectious cause 2.
Transient elevation of temperature, occurring in the first 48 hours after surgery, is a common phenomenon 4.
Postoperative fever can be classified into different types based on the timing of occurrence, duration, and severity, and not all fevers have an infectious cause 5.

Several factors can influence the development of postoperative fever, including:

Type of surgery: Cranial surgery, prolonged duration of surgery, and the use of urinary catheters and wound drains retained beyond postoperative day 3 can predict fever 4.
Surgical trauma and blood contact with cardiopulmonary bypass circuit can cause the release of cytokines, leading to fever 6.
Noninfectious causes, such as drug fever, blood product reaction, and infections related to intravenous lines, can also contribute to postoperative fever 2.

A focused approach to evaluating postoperative fever is recommended, rather than a "shotgun" approach 2.

A brief bedside evaluation has the highest yield for determining the fever etiology, and extensive evaluations with cultures and chest imaging have little to no benefit in patients admitted for elective surgery 3.
A systematic, prudent, clinically appropriate, and cost-effective manner of evaluation and management is essential to avoid unnecessary risks and costs 6.