Psoas abscess (also called iliopsoas abscess) is a rare condition characterized by a localized collection of pus in the iliopsoas muscle compartment. It is categorized into primary psoas abscess (caused by hematogenous or lymphatic spread of a pathogen) and secondary psoas abscess (resulting from contiguous spread from an adjacent infectious focus). The most common causes of secondary psoas abscess are infections of gastrointestinal or musculoskeletal origin. Infections are often monomicrobial, with S. aureus most frequently isolated, followed by E. coli. Polymicrobial abscesses are most commonly of gastrointestinal origin. Computed tomography is the diagnostic modality of choice. Empiric antibiotic therapy should be administered as soon as blood cultures and, if possible, abscess cultures are obtained. A small abscess (< 3.5 cm) in a nonseptic patient can be managed with antibiotic therapy alone. Larger or multiloculated abscesses should be drained under image guidance or with surgery. In a psoas abscess of any kind, the underlying cause should be evaluated and treated.
- Primary psoas abscess 
Secondary psoas abscess
- Caused by contiguous spread of a pathogen from an adjacent infectious focus
- Primary focus may be: 
- Different causative pathogens are associated with different primary infectious foci.
- Risk factors 
- Causative pathogens 
|Causative pathogens of psoas abscess|
|E. coli|| |
Bacteroides spp., S. viridans
- Classic triad: present in ∼ 30% of cases 
- Nonspecific symptoms: lower abdominal pain, malaise, weight loss
- Examination findings
- Additional features: symptoms related to underlying conditions may be present (e.g., colitis, UTI). 
Imaging is required to confirm the clinical diagnosis of psoas abscess. Laboratory studies provide supportive evidence of an acute infectious process and may be used to monitor response to therapy. Tests to evaluate for a suspected primary focus should be guided by the pretest probability of the underlying etiology, as determined by a thorough history and physical examination. If no obvious primary focus of infection can be identified, the psoas abscess is presumed to be a primary psoas abscess. 
Laboratory studies 
Routine tests for suspected psoas abscess
- CBC: ↑ WBC; anemia may be present
- Inflammatory markers: ↑ ESR, ↑ CRP
- ↑ Creatine kinase
- Procalcitonin: not routinely indicated; usually elevated in patients with sepsis
- Gram stain and culture of blood and abscess aspirate
- Tests to evaluate for an underlying etiology
CT abdomen and pelvis with IV contrast (axial section) 
- Indication: imaging modality of choice 
- Characteristic findings
- Characteristic findings
Abdominal ultrasound 
- Asymmetrical swelling of the psoas muscle
- The abscess appears as a hypoechoic collection within the psoas muscle.
- Signs of intra-abdominal or pelvic primary focus
Imaging to evaluate for a suspected primary focus
- Consider the following:
If no primary focus of infection can be identified, primary psoas abscess is most likely. 
See also differential diagnosis of low back pain.
The differential diagnoses listed here are not exhaustive.
General principles 
- Antibiotic therapy and drainage of the abscess are the mainstays of therapy.
- Consider a trial of antibiotic therapy alone in small abscesses (< 3.5 cm). 
- Abscess drainage (either image-guided or surgical) is indicated in septic patients or if the abscess is large or multiloculated.
- Identify and treat the underlying condition.
Empiric antibiotic therapy for psoas abscess 
- Coverage: empiric therapy should provide coverage of S. aureus and E. coli.
- One of the following beta-lactams with beta-lactamase inhibitor:
- OR combination therapy with metronidazole PLUS one of the following:
- PLUS consider one of the following for MRSA coverage :
- Additional considerations
- Duration of antibiotic therapy: usually 4–6 weeks 
Image-guided percutaneous drainage (PCD)
- Procedure: open surgery or laparoscopic drainage
- Identify and treat sepsis, if present.
- Obtain blood cultures and, if possible, abscess fluid cultures before administering antibiotics.
- Administer empiric antibiotic therapy for psoas abscess. 
- Consult surgery and, if available, interventional radiology for abscess drainage.
- Provide supportive care (e.g., analgesia, antiemetics, IV fluids)
- Consider ICU admission in patients with sepsis.