If there are not 2 positive results, the ad- dition of the compression test, followed by the sacral thrust test also can point to the di- agnosis. The compression test is performed with vertical downward force applied to the iliac crest with the patient lying on each side, compressing the SIJ by transverse pres- sure across the pelvis. The sacral thrust test is performed with vertical force applied to the midline posterior sacrum at its apex di- rected anteriorly with the patient lying prone, producing a shearing force at the SIJs. The Gaenslen test uses a torsion force by ap- plying a superior and posterior force to the right knee and posteriorly directed force to the left knee. Omitting the Gaenslen test has not been shown to compromise diagnostic efficacy of the other tests and can be safely excluded.12