The psoas sign is a classic albeit non-specific finding on the abdominal radiograph, potentially representing retroperitoneal pathology.
Normally on an abdominal radiograph, the lateral margins of both the psoas muscles are clearly visible due to adjacent fat. When the lateral edge of one, or both, psoas muscle(s) is no longer clearly demarcated, this is a positive psoas sign. Only a portion of the muscle edge needs to be obscured for the sign to be positive.
However, several unrelated findings may result in a positive psoas sign, such as scoliosis, a paucity of retroperitoneal fat, abutment of an adjacent kidney or bowel loop, or a globular-shaped psoas muscle.
A psoas sign has also been described on MRI in spondylodiscitis and this is discussed separately.
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