Diagnostic Approaches and Differential Considerations in Septic Arthritis

Show Notes

Episode Produced by Audrey Bree Tse, MD

Background

• Bacteria enters the joint by hematogenous spread due to absence of basement membrane in synovial space from invasive procedures, contiguous infection (e.g. osteomyelitis, cellulitis), or direct inoculation (e.g. plant thorns, nails)
  • WBCs migrate into joint → acute inflammatory process → synovial hyperplasia, prevents new cartilage from forming, pressure necrosis on surrounding joint, purulent effusion
• Why do we care? 
  • irreversible loss of function in up to 10% & mortality rate as high as 11%
  • Cartilage destruction can occur in a matter of hours
  • Complications include bacteremia, sepsis, and endocarditis

Etiology

• Risk factors: extremes of age, RA, DJD, IVDA, endocarditis, GC, immunosuppression, trauma, or prosthesis
• Organisms: 
  • Staph: staph aureus (most common), MRSA, Staph epidermis
  • N gonorrhea: young healthy sexually active adults
  • Strep: group A & B
  • GNRs: IVDA, diabetics, elderly
  • Salmonella: sickle cell disease
  • Cutibacterium acnes: prosthetic shoulder infection
  • Consider mycobacterial & fungal in more indolent courses

Presentation

• Typically a single, warm, erythematous, tender joint (#1: knee (50% of cases) → hip, shoulder, ankle)
  • *Any joint can be involved!
  • IVDA can involve sacroiliac, costochondral, & sternoclavicular joints 
  • Classic teaching: very painful with ROM, but this is not always present!
  • Joint usually held in position of maximum joint volume
  • Prosthetic joints may have less pain than expected for a septic joint given changed anatomy and disrupted nerve endings
• In 10-20% of cases, can see polyarticular involvement
  • GC typically monoarticular but commonly polyarticular
• Often have fever & separate infection as well (only see fever in ~60% of cases)

Diagnostics

Arthrocentesis: 

• Gold standard 
• Tap joint even if acceptable ROM: septic joints can have normal motion so it does not exclude the diagnosis!
• Use ultrasound if possible
• Relative contraindications: overlying cellulitis (risk of seeding joint) or severe coagulopathies (weigh risk of creation or worsening of iatrogenic hemarthrosis)
• Keep in mind that a “dry tap” may occur due to incorrect needle placement, absent/ minimal joint effusion, ort mechanical obstruction
• Note: talk to ortho colleagues if prosthesis present prior to performing arthrocentesis 
  • Ortho team may want to perform the arthrocentesis themselves because scar tissue formation and altered anatomic relationships make the procedure more challenging
  • Usually want to perform washout in OR plus/ minus antibiotic spacer
• Send fluid for protein, glucose, cell count with differential, gram stain, culture, and crystals
  • Often see decreased glucose and elevated protein
  • The presence of crystals does not rule out septic arthritis
  • No clear number of synovial WBCs to define septic arthritis, but in general: >30 to 50K/ mm3 synovial WBCs with PMN predominance (>75%) seen in septic arthritis
  • A 2011 meta-analysis suggests +LRs of 4.7 (95% CI = 2.5 to 8.5) and +LR of 13.2 (95% CI = 3.6 to 51.1) for a sWBC count of >50L × 109 or >100K, respectively
  • Use the synovial WBC count plus the whole clinical picture to rule in or out the diagnosis of septic arthritis (do not use the synovial WBC in isolation)
  • Different threshold for prosthetic joints: WBC > 1100 or >64% PMNs = septic arthritis
• Gram stains only identify causative organisms 1/3 of the time
• Culture negative arthrocentesis can be seen in cases where abx have been given prior to arthrocentesis, or in TB/ brucella/ nocardia/ other indolent organisms like fungi

Labs:

• No studies have demonstrated an acceptable sensitivity or overall diagnostic accuracy of peripheral WBC count for SA, but usually see leukocytosis with left shift
• ESR and CRP are reasonably sensitive but there is no cutoff that significantly increases or decreases the pretest probability
• UA, urine cultures, blood cultures: send even if no fever
  • Blood cultures are positive in 50-70% of nonGC SA
• If GC suspected, do GC NAAT from throat/ rectal/ urethral/ cervical discharge

Imaging:

• XRs: effusion, baseline status of joint, contiguous osteomyelitis, fractures, foreign body
• US: effusion
• CT, MRI: not really used in ED

Differential

• Viral arthritis
• RA
• gout/ pseudogout
• HIV associated arthritis
• Reactive arthritis
• Lyme
• Osteo
• Septic bursitis
• Trauma

Treatment

• Septic arthritis is an orthopedic emergency!
  • Needs IV abx + often washout of the joint
• Hold abx as much as possible prior to tap unless pt is unstable or tap cannot be performed easily
• Initiate empiric IV antibiotic therapy prior to definitive cultures based
• Transition to organism-specific antibiotic therapy once culture sensitivities result
• Start empiric abx based on gram stain if available (in non-=GC SA, grain stain is positive in 50% of cases), age group, & risk factors
  • Empiric abx: Vancomycin 15mg/kg q12h (to cover MRSA) + cefepime 2gr IV q8h (to cover gram-negatives)
  • If gram stain with GPC = Vancomycin 15mg/kg q12h
  • If gram stain with GN diplococci = ceftriaxone 1gr IV q24h + Azithromycin 1gr q24h
  • If gram stain with GN rods = cefepime 2gr IV q8h
  • If penicillin allergy: ciprofloxacin 500mg q12h or aztreonam 2gr q8h
  • No need to cover anaerobes unless human/dog/cat bite (then use Unasyn to cover eikenella, pastereulla, capnocytophaga, anaerobes, etc.)
• They usually need antibiotics for 2-6 weeks: 2 weeks for strep, up to 6 weeks if S aureus  
• Pain control: consider moderately flexed splinting
• Admit all patients with suspected septic arthritis until SA is ruled out, abx, monitoring, likely operative intervention

Take-Home Points

• Patients may present with either a single affected joint or polyarticular; they may or may not have a fever
• Have a high index of suspicion for SA, and a low threshold to tap: pts do not necessarily present w/ “classic” findings and it is difficult to distinguish SA from crystal arthropathy
• ESR, CRP, serum WBC are not definitive diagnostic tools for septic arthritis
• There is no exact cutoff for synovial WBCs for diagnosis: use whole clinical picture & keep 50K in mind for native joints, and >1100 for prostheses
• Treat with empiric abx after tap then narrow accordingly, & admit all patients with septic arthritis
• Involve your ortho colleagues early especially for prosthesis

References

Carpenter CR, Schuur JD, Everett WW, et al.  Evidence-based diagnostics: Adult septic arthritis.  Acad Emerg Med.  2011;18:781-796.

Jones D, Clements C.  Physical exam and bloodwork do not adequately differentiate infectious from inflammatory arthritis.  In: Mattu A, Chanmugam A, Swadron S, Woolridge D, Winters M.  Avoiding Common Errors in the Emergency Department. 2nd Edition.  Philadelphia, PA: Wolters Kluwer; 2017; 412-414.  

Kazzi A, Zaghrini E.  Septic Arthritis.  In: Schaider J, Barkin R, Hayden S, Wolfe R, Barkin A, Shayne P, Rosen P.  Rosen and Barkin’s 5-Minute Emergency Medicine Consult.  5th Edition.  Philadelphia, PA: Wolters Kluwer; 2015; 102-103.

Osmon D, Berbari E, Berendt A, Lew D, Zimmerli W, Steckelberg J, Rao N, Hanssen A, Wilson W.  

Diagnosis and Management of Prosthetic Joint Infection: Clinical Practice Guidelines by the Infectious Diseases Society of America, Clinical Infectious Diseases, Volume 56, Issue 1, 1 January 2013, Pages e1–e25, https://doi.org/10.1093/cid/cis803

Mlynarek C, Sullivan A.  Arthrocentesis Tips.  In: Mattu A, Chanmugam A, Swadron S, Woolridge D, Winters M.  Avoiding Common Errors in the Emergency Department. 2nd Edition.  Philadelphia, PA: Wolters Kluwer; 2017; 684-686.

Purcell D, Terry B, Sharp B.  Joint Arthrocentesis.  In: Purcell D, Chinai S, Allen B, Davenport M.  Emergency Orthopedics Handbook. 1st Edition. Cham, Switzerland: Springer; 2019; 87-104.

Sheth U, Moore D.  Septic Arthritis — Adult.  OrthoBullets.  [https://www.orthobullets.com/trauma/1058/septic-arthritis–adult].  Updated 1/9/19.  Accessed 8/2/19.  

A special thanks to our Infectious Diseases Editor:

Angelica Cifuentes Kottkamp, MD

Infectious Diseases & Immunology

NYU School of Medicine

A special thanks to our Orthopedics Editor:

Daniel Purcell, MD

Emergency Medicine

NYU Langone Brooklyn