Is an ultrasound of the abdomen necessary for a patient with suspected or confirmed Serratia marcescens infection, particularly those with signs of abdominal involvement or who are immunocompromised?

Abdominal Ultrasound for Serratia marcescens Infection

Abdominal ultrasound is NOT routinely necessary for Serratia marcescens infection unless the patient has abdominal signs/symptoms, recent abdominal surgery, abnormal liver function tests, or is critically ill with fever of unclear source. 1

Clinical Decision Algorithm

When Ultrasound is NOT Indicated:

• For critically ill patients with fever and confirmed or suspected Serratia marcescens bacteremia WITHOUT abdominal signs, symptoms, or liver function abnormalities, routine abdominal ultrasound should NOT be performed 1
• The 2023 Society of Critical Care Medicine/IDSA guidelines explicitly recommend against routine abdominal ultrasound in febrile ICU patients without abdominal findings 1

When Ultrasound IS Indicated:

Perform formal bedside diagnostic ultrasound of the abdomen if ANY of the following are present: 1

• Recent abdominal or pelvic surgery 1
• Abdominal pain, tenderness, or distention on physical examination 1
• Elevated transaminases, alkaline phosphatase, or bilirubin 1
• Abnormal point-of-care ultrasound findings 1
• Clinical suspicion of intra-abdominal abscess or source control issue 1

If Initial Ultrasound is Inconclusive:

Proceed to CT imaging with IV contrast if: 1

• Ultrasound results are negative, equivocal, or non-diagnostic AND clinical suspicion persists 1
• CT of the abdomen is the most accurate method to diagnose ongoing or recurrent intra-abdominal infection 1
• For post-surgical patients with persistent fever, CT should be performed in collaboration with the surgical service 1

Key Clinical Context for Serratia marcescens

Serratia marcescens typically causes bacteremia, urinary tract infections, and pneumonia—NOT primarily intra-abdominal infections: 2, 3

• Most commonly isolated from blood, followed by urine and cerebrospinal fluid 2
• Intra-abdominal infection accounts for only 4% of Serratia bacteremia cases 3
• Mortality rate is approximately 31-39% in invasive infections 2, 3

Important Caveats

The imaging decision is driven by clinical presentation, NOT by the specific organism: 1

• Serratia marcescens is an opportunistic pathogen affecting immunocompromised patients 2, 4
• Focus imaging decisions on anatomic source of infection rather than microbiologic identification 1
• If biliary tract infection is suspected (elevated alkaline phosphatase/bilirubin), ultrasound is the first-line imaging modality 1

Common pitfall: Ordering routine abdominal imaging for all patients with Serratia bacteremia wastes resources and delays appropriate antimicrobial therapy, which should include carbapenems or aminoglycosides with third-generation cephalosporins 2