What is Useful in the ED to Help Diagnose or Rule Out Septic Arthritis?


There are many risk factors for septic arthritis including age >80, Diabetes, Rheumatoid Arthritis, recent joint surgery, prosthesis, cellulitis.  The absence of risk factors does not make septic arthritis less likely in an acute monoarticular arthritis


Monoarticular arthritis is often characterized as a warm, painful, swollen joint with limited range of motion.  No studies to date have quantified specificity data on the physical exam.  Therefore, clinicians must use their own clinical gestalt when interpreting physical exam findings.

Serum Tests

Useless – unless ESR and CRP are negative.  In this case, low negative likelihood ratios may help to exclude septic arthritis

Synovial Fluid Analysis

  • sWBC > 50,000 - essentially rules IN septic arthritis
  • sWBC < 50,000 - unhelpful for clinical diagnosis
  • Gram Stain: useful when positive, unhelpful when negative
  • Culture: not available when we need it in the ED
  • Lactate >10 - essentially rules IN septic arthritis

In the end, clinicians must determine their own pre-test probabilities and interpret history, physical exam, and labs as a whole picture.  When in doubt, it is safest to treat the patient with antibiotics and consult Orthopaedic Surgery for further management.

For more detailed information check out this article by Margaretten, et al (2007) - "Does This Adult Patient Have Septic Arthritis"

Arthrocentesis Approaches

Glenohumeral Joint (Shoulder) – Posterior Approach

  1. Internally rotate
  2. Palpate posterolateral edge of acromion
  3. Enter 1 cm distal and 1 cm medial
  4. Direct needle toward coracoid

Posterior Approach for Shoulder Joint Arthrocentesis

Glenohumeral Joint (Shoulder)– Anterior Approach

  1. Slightly externally rotate
  2. Palpate coracoid and humeral head
  3. Enter 0.5-1 cm lateral to coracoid
  4. Direct needle posteriorly aiming slightly laterally and superiorly

Shoulder Joint - Anterior Approach

Radiohumeral Joint (Elbow)

  1. Flex arm to 45 degrees, hand in neutral position
  2. Palpate olecranon, lateral epicondyle, and radial head 
  3. Enter soft spot in the middle
  4. Direct needle toward medial epicondyle

Elbow Joint Arthrocentesis Landmarks

Radoiocarpal Joint (Wrist)

  1. Slightly flex wrist
  2. Palpate distal radius and extensor pollicis longus (EPL) tendon
  3. Enter soft spot just ulnar to EPL and distal to the radius
  4. Direct needle perpendicularly to skin

Wrist Joint Arthrocentesis Landmarks

Knee – Superolateral Approach

  1. Fully extend knee
  2. Palpate superolateral edge of patella
  3. Enter 1cm superior and 1 cm lateral
  4. Direct needle 45 degrees distally and under the patella

Knee Joint - Superolateral Approach for Arthrocentesis

Knee - Inferolateral Approach

  1. Flex to 90 degrees
  2. Palpate inferior pole of patella, lateral aspect of patellar tendon, and proximal lateral tibial plateau
  3. Enter soft spot 1 cm proximal to lateral tibial plateau, just lateral to patellar tendon
  4. Direct needle 30 degrees medially toward intercondylar notch

Knee Joint - Inferolateral Approach for Arthrocentesis

Tibiotalar Joint (Ankle)

  1. Slight plantar flexion, may use towel bump under the ankle
  2. Palpate the tibial plafond, anteromedial border of medial malleolus, and tibialis anterior tendon
  3. Enter soft spot between tibialis anterior tendon and medial malleolus
  4. Direct needle posterolaterally

Ankle Joint Arthrocentesis Landmarks


  1. Mary E. Margaretten, MD; Jeffrey Kohlwes, MD, MPH; Dan Moore, PhD; Stephen Bent, MD JAMA. 2007;297(13):1478-1488. doi:10.1001/jama.297.13.1478.
  2. Parrillo, S., Morrison, D., Panacek, E. Arthrocentesis. Roberts & Hedges Clinical Procedures in Emergency Medicine, 5th edition. 2009. ISBN: 978-1-4160-3623-4. p 971-985.
  3. Egol, Kenneth and Strauss, "Eric. Emergency Room Orthopaedic Procedures: An Illustrative Guide for the House Officer."  Jaypee Brothers Medical Publishers.  New Delhi, India. 2012.
  4. Carpenter, Christopher et al. “Evidence-based Diagnostics: Adult Septic Arthritis.” Acad Emerg Med.” 2001; 18(8): 781-796