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Case
53 yo female comes in with 2 day history of increasing R knee pain, now giving her a limp. Does not recall injuring it
That knee is always ‘a little sore’ from running injuries years ago
Case
PMHx: HTN GERD Smoker Gout (toes, L ankle) – hasn’t had a flare in years
Meds Allopurinol Ramipril Ranitidine
Case
Phx No fever, normal vitals Knee looks swollen, no
cellultis Joint warm, ROM is
painful but patient can do it
Labs
Serum WBC 14 ESR 32 CRP 17 Uric Acid 400 Synovial Fluid
WBC 36 x109/L, PMN’s 65% Low glucose Negative for crystals
Overview
Value of serum labs Value of synovial fluid analysis Gram’s Stain & Cultures Prosethetic Joints Course of Action for Dry Taps
Review – The Swollen Joint
Non-inflammatory Trauma OA
Inflammatory RA Crystal arthropathies Seronegative arthropathies Septic joint
Why is this Important?
Joint destruction can occur within 2-3 days if untreated infection
Patients can become systemically septic from a joint infection rather easily
We need to make decisions before cultures come back
Serum Labs
Serum WBC >10 x 109/L sensitivity of 50% for infection LR 1.4 Many sterile but inflammatory joints give
elevated serum WBC
Bottom Line: Not sensitive
Serum Labs
Serum ESR ‘Elevation’ in most studies >30 mm/h Sensitive but not specific LR 1.3
Bottom Line: Only useful to track resolution of the infection over time
Serum Labs
Serum CRP ‘Elevated’ in most studies >100 mg/L Sensitivity 75%, poor specificity LR 1.6
Bottom Line: Although CRP shows promise, there is insufficient evidence for its sensitivity to be high enough to rule out septic arthritis.-Best Bets 2008
Synovial Fluid
What’s Normal? Normal knee has avg 4cc synovial fluid Normal synovial WBC <0.2x109/L Glucose same as plasma Uric Acid same as plasma Protein <25% of plasma
Hemarthrosis
Trauma #1 cause Anticoagulation therapy Hemophilia Synovioma
Rarely, infection and hemarthrosis co-exist. If concerned, send for culture.
Synovial Fluid
Findings Normal Non-Inflamm Inflammatory Septic
Colour Clear Yellow Yellow Yellow
Clarity Transparent Transparent Opaque Opaque
WBC (x109/L) <0.2 0.2 - 2 2 - 150 20 - 200
PMN’s <25% <25% >50% >75%
Synovial Fluid
Glucose and Protein Synovial / Serum Glucose <0.5-0.75, low
sensitivity Synovial Glucose <1.5 mmol/L sensitivity 38-64% Synovial Protein >25% of plasma, low sensitivity
Bottom Line: Glucose and Protein levels have no role in the work up of a septic joint
Synovial Fluid
LDH >250 U/L was 100% sensitive in retrospective study
on 8 confirmed cases, prospectively was not as strong
Lactic Acid 90-97% NPV, but low powered studies
Bottom Line: Insufficient data to date
Synovial Fluid
Tumour Necrosis Factor – α Jeng et al, Am J Emerg Med 1997 Prospective, n=75 Synovial TNF-α >36.2 pg/mL sens 95%, spec
50% for bacterial infection
Bottom Line: Needs more study before routine order
Synovial Fluid
30% of immunocompetent people with culture confirmed septic joint have synovial WBC <50 - McGillicuddy et al, Am J Emerg Med. 2007
50% of immunocompromised people with culture confirmed joint infection had WBC <28-McCutchan et al, Clin Orthop Relat Res 1990
Synovial Fluid
WBC Bottom Line Cut-off of 50 x109 /L too insensitive rule-out
infection Use in clinical context The diagnostic cut-off that maximized the
sensitivity / specificity was a synovial WBC count of 17.5 x109/L (Sens 83%, Spec 67%)
- Li et al, Emerg Med J 2007
Combined Value?
Li et al, Emerg Med J 2007 Retrospective chart review 156 patients Combined Sensitivity 100% if:
Serum WBC <11 Serum ESR <20 Synovial WBC <50
Bottom Line: Not powered enough, not prospectively validated, cannot use to rule out septic joint
Synovial Fluid
Crystals Gout - Monosodium Urate, 90% sensitive, LR 14 Pseudogout – PPDC, 80% sensitive, LR 2.6 Cholesterol crystals – seen in chronic
inflammatory conditions
Crystals & Infection
Crystals do not rule out infection!
Retrospective study n=265 patients with crystals, 1.5% had septic joint
-Shah et al, J of Emerg Med 2007
Literature ranges from 1-20% of infectious joints co-exist with crystals
Microbio Review
ALL AGES: #1 cause still Staph Aureus
<30, sexually active: Neisseria Gonorrhea
Elderly: Gram Negatives
Prosthetics: Careful of Pseudomonas
Gram’s Stain
Guides your antibiotic therapy while awaiting cultures
Need roughly 3-5cc for stain & culture Only 65% sensitive for non-gonococcal infections Only 25% sensitive for gonococcal infections
Bottom Line: A negative Gram stain means nothing. A positive Gram stain means you should start treatment.
Cultures
‘Gold standard’ ? Gonococcus difficult to culture
Negative 50% of the time Requires chocolate agar
Non-gonococcus will culture 90% of time If you only have enough fluid for one test, this is
what you do Blood cultures reveal pathogen 25-50% of the
time
Gonococcal Arthritis
Synovial WBC often <50
Gram stain Positive only 25% of the time
Culture Positive only 50% of the time
If you suspect it, culture at 3 mucosal sites (pharynx, genitals, anus) will increase your chance of positive culture to 80%
Generally less destructive to the joint versus other pathogens
Gram Stain Positive, Culture Negative
In reality, this is retrospective
Go with your Gram Stain treat these patients while awaiting cultures
How does this happen? Antibiotics already on board Organism difficult to culture Was infected, now clearing
Prosthetic Joints <3mos since surgery
likely Staph Epiderm
>3mos since surgery Staph, Strep, Gram Neg
Should always call Ortho before tapping these in ER
Prosthetic Joints
Trampuz et al, Amer J of Med 2004
Prospective, n=133, 34 had septic joint Synovial WBC >1.7 x109/L , sens 94% spec 88% Synovial PMN’s >65%, sens 97% spec 98%
Mason et al, J of Arthroplasty 2003
Retrospective n=86 knees Ideal sensitivity 98% for synovial WBC 2.5 x109/L and
PMN’s 60%
Dry Tap?
Makes a septic joint unlikely usually a large enough effusion for tap, but never been validated
Options U/S guided in the department Consult Ortho Fluoroscopy guided
BOTTOM LINE: You need a sample of that fluid if you are worried about infection
Hot Joint, No Organism
Fastidious organism Antibiotics begun before cultures sent Wrong Diagnosis
Help increase your yield? Use blood culture bottles for synovial fluid
(aerobic and anaerobic)- Joint, Bone, Spine 2006
Relevance to Pediatrics?
No good studies specifically on synovial fluid analysis in the pediatric population
Most use numbers from adult data
TAKE HOME MESSAGE
Cannot rely on serum values to rule out septic joint
If you believe there’s an effusion, get that fluid somehow
Unfortunately, nothing has a strong NPV
TAKE HOME MESSAGE
Synovial fluid: WBC & PMN is helpful WBC <18 is low risk but not zero WBC >50 is high likelihood PMN’S >90% is high likelihood Glucose, Protein useless
TAKE HOME MESSAGE
‘Gold Standard’ is clinical suspicion of an experienced physician, not laboratory tests (Current Opinion Rheumatology 2008)
Prosthetic Joints Lower WBC & PMN threshold
Don’t feel bad - 30% of the time reason for effusion remains ‘unknown’