Septic arthritis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief:Venkata Sivakrishna Kumar Pulivarthi M.B.B.S [2]


Patients with history of chronic disease with concurrent septic arthritis can be misdiagnosed as acute flareup of underlying chronic disease which often delays the treatment for septic arthritis. So, patients with acute flare of one or two new inflamed joints with underlying chronic joint diseases or with another connective tissue disease, it should be assumed that the joint is septic until proven otherwise, should always rule out concurrent septic arthritis with appropriate diagnostic studies.[1] In patients with acute effusion of unknown etiology, might have concurrent crystal-induced arthritis and septic arthritis. So, the synovial fluid should always be cultured and examined for crystals in the evaluation of an acute effusion.[2]

Laboratory Findings

Serum markers

Serum markers such as peripheral white cell count, erythrocyte sedimentation rate, C-reactive protein are useful to determine infectious or inflammatory response and also useful to monitor therapeutic response. Absence of raise in these parameters may not be good correlate for the diagnosis of septic arthritis, as these tests are neither sensitive nor specific.[3]

  • Peripheral WBC count: Peripheral WBC count increases in septic arthritis, especially in children where as in adults it varies with severity.[4] It is not sensitive or specific to diagnose septic arthritis.
  • Erythrocyte sedimentation ratio: Patients with septic arthritis may have ESR > 30 mm/hr, but normal ESR may not rule out septic arthritis.[3]
  • C- reactive protein: Elevated CRP of > 100 mg/L increased the likelihood of septic arthritis slightly.[5]

Synovial Fluid Analysis

Diagnosis of septic arthritis mainly depends on arthrocentesis and isolation of the pathogen from aspirated joint fluid.[6] Clinical suspicion of joint sepsis should prompt for immediate synovial fluid aspiration. Septic arthritis should not be excluded even though the patient have low fever and normal WBC. The definitive diagnosis of septic arthritis requires identification of bacteria in the synovial fluid by Gram stain or by culture.[1] If synovial fluid cannot be obtained with closed needle aspiration, the joint should be aspirated again with imaging guidance such as ultrasound guidance, computed tomography or fluoroscopic guidance.[1] Synovial fluid analysis include:

  • Synovial WBC count with differential
  • Crystal analysis
  • Gram stain
  • Culture and sensitivity

Normal synovial fluid appears as clear, transparent, thick in viscosity with WBC count less than 200 mm3 and < 25% of PMN, where as in septic arthritis and other arthritis synovial fluid analysis will be as follows:[7][8][9][10][11][12][13][14]

Type of


Color Transparency Viscosity WBC count

(per mm3)


cellcount (%)

Gram stain Gram Culture polymerase chain reaction

(PCR) test

Normal Clear Transparent High/thick < 200 < 25 Negative Negative Negative Negative
Gonococcal arthritis Yellow Cloudy-opaque Low 34,000 to 68,000 > 75 Variable (< 50 percent) Positive (25 to 70 percent) Positive (> 75 percent) Negative
Non-gonococcal arthritis Yellowish-green Opaque Very low > 50,000 (> 100,000 is

more specific)

> 75 Positive (60 to

80 percent)

Positive (> 90 percent) -- Negative

crystalline arthritis

(e.g.Gout, Pseudogout)

Yellow Cloudy Low/thin 2,000 to 100,000 > 50 Negative Negative Negative Positive

non-crystalline arthritis

(e.g. Rheumatoid arthritis, reactive arthritis)

Yellow Cloudy Low/thin 2,000 to 100,000 > 50 Negative Negative Negative Negative
Noninflammatory arthritis

(e.g. Osteoarthritis)

Straw Translucent High/thick 200 to 2,000 < 25 Negative Negative Negative Negative
Lyme's arthritis Yellow Cloudy Low 3,000 to 100,000

(mean: 25,000)

> 50 Negative Negative Positive (85 percent) Negative
  • Synovial fluid glucose level < 40 mg/dl and increased lactate level may represent septic arthritis, but these parameters are very less sensitive.[15][9]
  • Presence of crystals may not exclude septic arthritis, as the co-existent infection might be possible along with crystalline disease.[9][16]
  • Polymerase chain reaction (PCR) test is useful in diagnosing gonococcal arthritis especially in patients with high suspesion of septic arthritis with negative culture. PCR has high specificity (96%) and high sensitivity (76%) with less low false positivity (<4%).[14]


For a detailed approach on performing arthrocentesis watch the video below: {{#ev:youtube|fZ2dcZhoGP8}}

Culture From Other Sites

Culture from sites other than synovial fluid is useful in the diagnosis of gonococcal arthritis or disseminated gonococcal infection gram stain and culture of synovial fluid for Neisseria gonorrhoeae is less reliable.[8][10]

Site of Culture Positivity
Endocervix ~90% of women
Urethral 50-75%
Pharyngeal 20%
Rectal mucosa 15%
  • Culture of derivatives from skin lesions and blood are rarely positive.


  1. 1.0 1.1 1.2 Goldenberg DL (1998) Septic arthritis. Lancet 351 (9097):197-202. DOI:10.1016/S0140-6736(97)09522-6 PMID: 9449882
  2. Ilahi OA, Swarna U, Hamill RJ, Young EJ, Tullos HS (1996). "Concomitant crystal and septic arthritis". Orthopedics. 19 (7): 613–7. PMID 8823821.
  3. 3.0 3.1 Gupta MN, Sturrock RD, Field M (2001) A prospective 2-year study of 75 patients with adult-onset septic arthritis. Rheumatology (Oxford) 40 (1):24-30. PMID: 11157138
  4. Jeng GW, Wang CR, Liu ST, Su CC, Tsai RT, Yeh TS et al. (1997) Measurement of synovial tumor necrosis factor-alpha in diagnosing emergency patients with bacterial arthritis. Am J Emerg Med 15 (7):626-9. PMID: 9375540
  5. Söderquist B, Jones I, Fredlund H, Vikerfors T (1998) Bacterial or crystal-associated arthritis? Discriminating ability of serum inflammatory markers. Scand J Infect Dis 30 (6):591-6. PMID: 10225388
  6. Bayer AS (1980) Gonococcal arthritis syndromes: an update on diagnosis and management. Postgrad Med 67 (3):200-4, 207-8. PMID: 7355135
  7. Goldenberg DL, Reed JI (1985) Bacterial arthritis. N Engl J Med 312 (12):764-71. DOI:10.1056/NEJM198503213121206 PMID: 3883171
  8. 8.0 8.1 O'Brien JP, Goldenberg DL, Rice PA (1983) Disseminated gonococcal infection: a prospective analysis of 49 patients and a review of pathophysiology and immune mechanisms. Medicine (Baltimore) 62 (6):395-406. PMID: 6415361
  9. 9.0 9.1 9.2 Shmerling RH, Delbanco TL, Tosteson AN, Trentham DE (1990) Synovial fluid tests. What should be ordered? JAMA 264 (8):1009-14. PMID: 2198352
  10. 10.0 10.1 Wise CM, Morris CR, Wasilauskas BL, Salzer WL (1994) Gonococcal arthritis in an era of increasing penicillin resistance. Presentations and outcomes in 41 recent cases (1985-1991). Arch Intern Med 154 (23):2690-5. PMID: 7993152
  11. Goldenberg DL (1995) Bacterial arthritis. Curr Opin Rheumatol 7 (4):310-4. PMID: 7547108
  12. Mathews CJ, Kingsley G, Field M, Jones A, Weston VC, Phillips M et al. (2008) Management of septic arthritis: a systematic review. Postgrad Med J 84 (991):265-70. DOI:10.1136/ard.2006.058909 PMID: 18508984
  13. Jalava J, Skurnik M, Toivanen A, Toivanen P, Eerola E (2001) Bacterial PCR in the diagnosis of joint infection. Ann Rheum Dis 60 (3):287-9. PMID: 11171695
  14. 14.0 14.1 Liebling MR, Arkfeld DG, Michelini GA, Nishio MJ, Eng BJ, Jin T et al. (1994) Identification of Neisseria gonorrhoeae in synovial fluid using the polymerase chain reaction. Arthritis Rheum 37 (5):702-9. PMID: 8185697
  15. Sharp JT, Lidsky MD, Duffy J, Duncan MW (1979) Infectious arthritis. Arch Intern Med 139 (10):1125-30. PMID: 485744
  16. Baer PA, Tenenbaum J, Fam AG, Little H (1986) Coexistent septic and crystal arthritis. Report of four cases and literature review. J Rheumatol 13 (3):604-7. PMID: 3735282

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