The (Patrick’s) FABER Test stands for Flexion, Abduction and External Rotation. These three motions combined result in a clinical pain provocation test to find pathologies at the hip, lumbar and sacroiliac region. 
Clinically Relevant Anatomy
A (basic) knowledge of the hip, lumbar spine and sacroiliac region anatomy is recommended. The hip joint is a ball en socket joint formed by the connection of the head (ball) of the femur (thigh bone) to the hip bone in the hip socket. The hip (also called iliofemoral) joints are deep in the pelvis, located on each side of the groin area. The top part of the ischium, the top part of the pubis, and the lower part of the ilium come together is an area called the acetabulum (hip socket). The head of the femur fits into this socket to form the hip joint, which is enclosed by thick cartilage called the joint capsule and held together by ligaments and muscles. 
More information on the hip or human anatomy you can find by clicking on the anatomy link under references. 
This test is used to identify the presence of hip pathology by attempting to reproduce pain in the hip, lumbar spine and sacroiliac region. The FABER test is a passive screening tool for musculoskeletal pathologies in the middle region of the human body, like hip, lumbar, or SI joint dysfunction, or an iliopsoas spasm.
The patient is positioned in supine. The leg is placed in a figure-4 position (hip flexed and abducted with the lateral ankle (malleolus) resting on the contralateral thigh just above (/proximal to) the knee).  While stabilizing the opposite side of the pelvis at the anterior superior iliac spine, an external rotation, abduction and posterior directed force is then slightly applied to the ipsilateral knee until the end range of motion (ROM) is achieved. Further a couple of small-amplitude oscillations can be applied to check for pain provocation at the end range of motion.  A positive test is one that reproduces the patient's pain or limits their ROM. 
A video of the FABER test perform you can find by clicking on the link under references. 
- • Sensitivity for identification of hip pathology identified with arthroscopy: 0.89
• Correlation of positive test with OA on radiographs: r = 0.54
• Kappa (95% Confidence interval): 0.63 (0.43-0.83), Kappa Maximum: 0.83, Percent agreement: 84%, Prevalence: 0.37, Bias: 0.07
• Diagnostic value of FABER test compared to MR arthrogra¬phy in labral tear diagnostics: sensitivety: 41%, specificity: 100%, positive predictive value: 100%, negative predictive value: 9%
• The validity and reliability of the FABER test is very contradictory, some say it is an invalid and unreliable test , while others are staying in disagreement about the outcome.
•Sensitivity specificity Kappa (Inter-tester population examined 
0.40-0.70 0.99 0.54 consecutive pregnant
0.42-0.62 pregnant (without pelvic girdle pain)
add links and reviews of high quality evidence here (case studies should be added on new pages using the case study template)
A1: Vleeming A, Albert HB, Ostgaard HC, Sturesson B, Stuge B.European guidelines for the diagnosis and treatment of pelvic girdle pain. Eur Spine J. 2008 Jun;17(6):794-819. Epub 2008 Feb 8.
A1: Cattley P, Winyard J, Trevaskis J, Eaton S. Validity and reliability of clinical tests for the sacroiliac joint. A review of literature. Australas Chiropr Osteopathy. 2002 Nov;10(2):73-80.
B: Troelsen A, Mechlenburg I, Gelineck J, Bolvig L, Jacobsen S, Søballe K. What is the role of clinical tests and ultrasound in acetabular labral tear diagnostics? Acta Orthop. 2009 Jun;80(3):314-8.
A2: Martin RL, Sekiya JK. The interrater reliability of 4 clinical tests used to assess individuals with musculoskeletal hip pain. J Orthop Sports Phys Ther. 2008 Feb;38(2):71-7. Epub 2007 Sep 21.
Clinical Bottom Line
Knowledge of the human anatomy, pain indicators, range of motion and pathology.
Although results of a study support the use of Patrick's test as being a reliable measure of general hip motion when used by an inexperienced tester who participated in a 15-minute instructional session on how to perform Patrick's test (The results revealed that Patrick's test demonstrated high test-retest reliability (intraclass correlation coefficient = 0.93).) 
In my opinion this test is a reliable pain provocation test, but it doesn’t specify the origin, pathology or exact source/reason of the pain. So it is a good primary test to indicate pain and encourage further investigation/testing.
Recent Related Research (from Pubmed)
- ↑ 1.0 1.1 1.2 Martin RL, Sekiya JK. The interrater reliability of 4 clinical tests used to assess individuals with musculoskeletal hip pain. J Orthop Sports Phys Ther. 2008 Feb;38(2):71-7. Epub 2007 Sep 21.
- ↑ http://www.suite101.com/content/structure-and-function-of-the-hip-joints-a190399
- ↑ hip anatomy link see https://catalog.ama-assn.org/MEDIA/ProductCatalog/m890153/%20Function%20%20Anatomy%20Ch%207.pdf fckLRorfckLRhuman anatomy http://www.innerbody.com/htm/body.html
- ↑ Dutton M. Orthopaedic: Examination, evaluation, and intervention. 2nd ed. New York: The McGraw-Hill Companies, Inc; 2008.
- ↑ 5.0 5.1 Flynn T, Cleland J, Whitman J. User's guide to the musculoskeletal examination: Fundamentals for the evidence-based clinician. Buckner, Kentucky: Evidence in Motion; 2008
- ↑ 6.0 6.1 Troelsen A, Mechlenburg I, Gelineck J, Bolvig L, Jacobsen S, Søballe K. What is the role of clinical tests and ultrasound in acetabular labral tear diagnostics? Acta Orthop. 2009 Jun;80(3):314-8.
- ↑ FABER test video link http://www.myspace.com/video/vid/34178068 fckLRor http://www.youtube.com/watch?v=xcIQDMQZilMfckLRor http://www.youtube.com/watch?v=XCOD1dkcVk0&amp;amp;amp;amp;amp;feature=related
- ↑ B, McCroy P, Brukner P, et al. Hip joint pathology: Clinical presentation and correlation between magnetic resonance arthrography, ultrasound, and arthroscopic findings in 25 consecutive cases. Clin J Sports Med. 2003;(13):152-156
- ↑ Theiler R, Stucki G, Schotz R, Hofer H, Seifert B. Parametric and non-parametric measures in the assessment of knee and hip osteoarthritis: interobserver reliability and correlation with radiology. Osteoarthritis Cartilage. 1996:35-42.
- ↑ Cattley P, Winyard J, Trevaskis J, Eaton S. Validity and reliability of clinical tests for the sacroiliac joint. A review of literature. Australas Chiropr Osteopathy. 2002 Nov;10(2):73-80.
- ↑ 11.0 11.1 Vleeming A, Albert HB, Ostgaard HC, Sturesson B, Stuge B.European guidelines for the diagnosis and treatment of pelvic girdle pain. Eur Spine J. 2008 Jun;17(6):794-819. Epub 2008 Feb 8.
- ↑ Ross MD, Nordeen MH, Barido M. Test-retest reliability of Patrick's hip range of motion test in healthy college-aged men. J strength &amp;amp; cond resrch 17 (1) 156-161 Feb 2003