Reprinted from www.antimicrobe.org

Septic Arthritis - Diagnosis

CLINICAL PRESENTATION

                Clinical signs and symptoms may be variable and imprecise. The key to diagnosis is thorough clinical history, physical examination along with laboratory studies of the synovial joint fluid from the affected joint. Pain and limited motion of the involved joint is present in more than 80% of the patients. Fever is seen in 60% to 80% of the patients unless masked by the use of anti-inflammatory agents and steroids.

        Joint Distribution: 

Knee is the most commonly involved joint.  45% to 50% of the septic arthritis in adults involves the knee.  Other large joints include hip 15%, ankle 9%, elbow 8%, wrist 6% and shoulder 5%.  Polyarticular disease is seen in 10% to 20% of cases.  It is usually asymmetric and involves an average of four joints. Major risk factors are steroid therapy, rheumatoid arthritis, lupus and diabetes mellitus.

LABORATORY DIAGNOSIS

                The laboratory abnormalities suggestive septic arthritis includes peripheral leucocytosis, elevated sedimentation rate and C-reactive protein. The synovial fluid cell count greater than 50,000/mm3 with more than 75% polymorphonuclear leucocytes is a commonly used threshold for empiric therapy. Blood cultures are likely to be positive in non gonococcal bacterial arthritis. A presumptive diagnosis of septic arthritis may be made with positive blood culture and a negative synovial fluid culture in an appropriate clinical setting.

                The definitive diagnosis of bacterial arthritis requires identification of bacteria from synovial fluid obtained through athrocentesis. This procedure should be performed in all patients with inflammatory arthritis in whom septic arthritis is suspected. Synovial WBC count >25000/μL, percentage of polymorphonuclear cells ≥ 90% and LDH> 250 U/L provide the high sensitivity in diagnosing septic arthritis (sensitivity: 88%, 92% and 100%, respectively) (Table 7).

                 Microscopic examination of the synovial fluid may eliminate crystal-induced inflammatory arthritis. Gram stain of the fluid is highly specific but not sensitive and may vary based upon the pathogen (Table 7). Culture of the synovial fluid is essential and the yield for positive culture may be maximized with inoculation into blood culture medium rather than plating on solid media. Direct inoculation of synovial fluid into blood culture medium bottles may improve the recovery of pathogens.

               Polymerase chain reaction (PCR) can be used in microbiological confirmation of septic arthritis. The problem of false-positive results needs to be addressed by more refined techniques which may replace the current methods of diagnosis.

Table 7: Differential Diagnosis of Synovial Fluid Characteristics

 

  Septic arthritis Acute inflammatory Non-inflammatory Normal
  Appearance

Turbid yellow

Turbid yellow

Clear, straw

Clear, colorless

Fibrin clot

Positive

Positive

Negative

Negative

  WBCs count      (/μL)   Septic Arthritis a) Gonococal Arthritis

TB  Arthritis

 Acute Gouty Arthritis Rheumatic Fever Rheumatoid Arthritis   <5000   <200
Range    15600- 21,300  1500-10,800  2500-10,500  750-45,000  300-98,000  300-75,000
Average  65400  14000  23500  13500  17800  15500
PMN (%) Range    ≥90  b)  2-96  29-96  48-94  8-98  5-96   <25   <25
Average  95  64  67  83  46  65

Blood- synovial glucose difference

(mg/dL) c)

Low glucose d)     

Range    40-122  0-97  0-108    0-41    0-88

<10

<10

Average  71  25  57  12  6  31
LDH (U/L)   >250 e)    >100  <250  <250
Gram stein/ Culture   Positive f)  Negative  Negative  Negative
Crystals  Negative  Monosodium urate (gout), Calcium pyrophosphate (pseudogout), Hydroxyappatite  Negative  Negative

a)    Sensitivity/Specificity (%) for septic arthritis

      >10,000                  40/99

      > 50,000                 70/92

      > 25,000                 88/71

 

   b) Highly sensitive (Sensitivity/Specificity  92/78 (%)) for septic arthritis

 

   c) Glucose concentration may give spurious results unless obtained after prolonged fasting, and differences between joint and blood samples

        may not be significant unless >50 mg/dL.  Joint tap should be performed, preferably after the patient has been fasting for >4 hrs, and a

        blood glucose determination should be performed simultaneously.

 

d) Serum/synovial fluid glucose ratio of less than 0.5 or 0.75, synovial fluid glucose level of less than 1.5mmol/mL, or both Sen/Spe  64/85(%) 

       for septic arthritis

 

 e) Highly sensitive (Sensitivity/Specificity  100/51 (%)) for septic arthritis.

       Gas liquid chromatography assay is superior to enzymatic analysis of LDH

 

 f) Gram stain: highly specific, low sensitive (Sensitivity/Specificity 50-70/100(%)).

       Gram-negative diplococci for gonococcal arthritis are demonstrated in only 10-25%

       Culture                               Sensitivity           Specificity

        Non-gonococcal                75-95                      >90

        Gonococcal                         10-50                     >90