Why the Psoas Sign is Positive in Right Lower Quadrant Pain
A positive psoas sign in a patient with right lower quadrant pain indicates irritation of the psoas muscle by an adjacent inflammatory process, most commonly acute appendicitis, and significantly increases the likelihood of this diagnosis. 1
Mechanism of the Psoas Sign
The psoas sign becomes positive when an inflamed or infected structure—typically a retrocecal or retroileal appendix—lies in direct contact with the psoas major muscle. 2 When the patient extends the hip or the examiner passively extends the right hip, this stretches the psoas muscle, causing pain by mechanically irritating the inflamed tissue lying against it. 1, 2
Clinical Significance in Appendicitis
The presence of a positive psoas sign, along with fever or migratory pain to the right lower quadrant, substantially increases the likelihood of appendicitis. 1, 2 This physical finding is particularly valuable because:
- It helps localize the anatomic position of the appendix (retrocecal location) 2
- It indicates direct inflammatory contact with the psoas muscle, suggesting more advanced inflammation 1
- Combined with other clinical features, it improves diagnostic accuracy beyond laboratory testing alone 1
Differential Diagnosis Beyond Appendicitis
While appendicitis is the most common cause in the acute setting, a positive psoas sign can indicate other pathology involving the psoas muscle or retroperitoneum:
- Psoas abscess: Presents with hip flexion deformity, fever, and pain on hip extension; 96% of patients with acute pyogenic psoas abscess demonstrate an associated hip flexion deformity 3
- Retroperitoneal malignancy: Can produce "malignant psoas syndrome" with proximal lumbosacral plexopathy and painful hip flexion 4, 5
- Right colonic diverticulitis or other inflammatory bowel conditions: Can irritate the psoas when located posteriorly 1
Diagnostic Approach When Psoas Sign is Present
When a patient presents with right lower quadrant pain and a positive psoas sign, proceed directly to CT abdomen and pelvis with IV contrast rather than relying on clinical assessment alone. 1 This recommendation is based on:
- CT sensitivity of 85.7-100% and specificity of 94.8-100% for appendicitis 1
- The ability to identify alternative diagnoses in 23-45% of cases with right lower quadrant pain 1
- Reduction of negative appendectomy rates from 14.7-25% (clinical diagnosis alone) to 1.7-7.7% (with preoperative CT) 6
Critical Clinical Pitfall
Do not assume that absence of fever excludes appendicitis, even with a positive psoas sign, as fever is absent in approximately 50% of appendicitis cases. 6 The psoas sign remains clinically significant regardless of temperature, and imaging should not be delayed based on normal vital signs alone. 1, 6