Diagnostic Approach for High WBC and Fever

Initial Assessment

For a patient with leukocytosis and fever, the American College of Critical Care Medicine recommends immediately obtaining a chest radiograph, performing a manual differential to assess for left shift, and collecting blood cultures if bacteremia is suspected, followed by targeted diagnostic testing based on clinical symptoms pointing to respiratory, urinary, abdominal, or skin/soft tissue sources 1, 2
The Society of Critical Care Medicine suggests using central temperature monitoring when these devices are already in place, and using oral or rectal temperatures rather than less reliable methods like axillary, tympanic, temporal artery, or chemical dot thermometers 1

The American College of Critical Care Medicine recommends obtaining a complete blood count with manual differential immediately to accurately assess band forms and immature neutrophils, as automated analyzers are insufficient for this critical determination 2, 3
A left shift of ≥16% band neutrophils has a likelihood ratio of 4.7 for bacterial infection, even with normal total WBC, according to the Infectious Diseases Society of America 2, 3
An absolute band count ≥1,500 cells/mm³ has the highest likelihood ratio (14.5) for bacterial infection, as stated by the American College of Critical Care Medicine 2, 4

The American College of Critical Care Medicine recommends performing a chest radiograph on all patients who develop fever during ICU stay or present with fever and leukocytosis 1
The Society of Critical Care Medicine suggests performing formal bedside diagnostic ultrasound of the abdomen if the patient has recent abdominal surgery, abdominal symptoms, abnormal physical examination/POCUS, or elevated transaminases, alkaline phosphatase, or bilirubin 1

The Infectious Diseases Society of America recommends obtaining pulse oximetry and chest radiography if hypoxemia is documented in patients with respiratory symptoms 3
The American College of Critical Care Medicine suggests considering thoracic ultrasound if chest radiograph is abnormal in patients with respiratory symptoms 1
The American Urological Association recommends performing urinalysis for leukocyte esterase/nitrite and microscopic examination for WBCs in patients with urinary symptoms 3

The American Geriatrics Society notes that in older adults in long-term care facilities, typical symptoms and signs of infection are frequently absent, and basal body temperature decreases with age and frailty 2
The American College of Critical Care Medicine states that left shift has particular diagnostic importance in older adults due to these atypical presentations 3