|Infectious Arthritis||Bacteria, Mycobacteria, Fungi, Lyme Disease (or other spirochete disease), Virus (HIV, Hep B, others)|
|Crystal-Induced||Monosodium urate (gout), Calcium pyrophosphate dihydrate Apatite (Pseudogout, ?hyperparathyroidism/acromegaly/hemochromatosis/hypomagnesemia), , Calcium oxalate, Liquid lipid microspherules|
|Trauma||Fracture, internal derangement, hemarthrosis|
|Osteoarthritis||(Sudden worsening pain and swelling in one joint)|
|Ischemic Necrosis||(spontaneous necrosis of the knee in elderly)|
|Foreign body synovitis||(thorns, wood fragments, other foreign material)|
|Tumor||Metastastis, osteoid osteoma, pigmented villonodular synovitis|
|Systemic disease||Rheumatoid arthritis, S.L.E., Inflammatory Bowel Disease, psoriasis, Behçet's disease, Reiter's syndrome; rarely, sarcoidosis, serum sickness, hepatitis, cancer and hyperlipidemias|
Normal synovial fluid contains fewer than 180 cells/mm3, most of which are mononuclear. Fluid is considered "noninflammatory" if it contains less than 2000 cells/mm3. With greater than 2000 cells/mm3 fluids are considered to be affected by an inflammatory process. Generally, the higher the leukocyte count the higher the suspicion of an infection. The conventional number given for an infectious etiology is if the fluid contain greater than 100,000 cells/mm3.
The general appearance of the synovial fluid might help you as well in suggesting a diagnosis. The following figures illustrates this:
This bloody fluid with a thicker layer of lipid material separated after centrifugation was aspirated from a patient with a tibial fracture into the joint space.
This is the colorless, clear synovial fluid from a patient with osteoarthritis accompanied by a low synovial-fluid white cell count.
These fluid collections which serve as good samples of cloudy but translucent inflammatory synovial fluid were taken from a patient with rheumatoid arthritis (left) and gout (right) respectively.
Finally, this fluid is a good example of a cloudy, pus-like fluid aspirated from a patient with acute bacterial infectious arthritis.
The patient described in the question has a good history for infectious monoarthritis. Analysis of the fluid is supportive with a large white cell count and a left shift. The gram stain fortunately gives us the diagnosis of Gonococcal arthritis as evidenced by the gram negative diplococci. By far the most common agent are gram positive aerobes (approximately 80%) with Staphylcoccus aureus accounting for 60%, non-group A ß-hemolytic streptococci 15% and Streptococcus pneumoniae 3% (4,5). Gram negative bacteria account for 18% and anaerobes are increasingly frequent causes in IVDA and immunocompromised hosts. Gonococcal arthritis affects women 2-3 times often and is often preceeded by a migratory tendinitis or arthritis (6). Though usually chronic arthritides, tuberculosis (7), atypical mycobacterial (8), fungal (9), herpes (10) and other viruses (11,12) have been described in the literature as presenting acutely.
Doxycline is a good choice for associated Chlamydia infections. Given the single organism seen on the gram stain, one is obligated to treat with IV ceftriaxone. However, one could not fault some one for empirically treating for the often associated Chlamydia infection. One would need to do a pelvic exam and GC/Chlamydia probe to lend more support to just treating this patient with IV ceftriaxone. One could also use a fluoroquinolone to treat Neisseria. Bactrim has variably poor effective killing power against Neisseria and would not be a good choice. Though NSAID's, heating packs and steroids may help the inflammatory process these would not treat the underlying infectious cause and actually might masquerade ongoing joint cartilage damage.
There are some classical findings for gout and calcium pyrophosphate disease that you should always look for in a patient presenting with arthritis. You should first start by looking for the classics signs of tophi:
You also want to be able to review aspirated synovial fluid for crystal analysis. I strongly encourage you to review these slides yourself. The laboratory technicians have a poor sensitivity to finding crystals. You want to look specifically for crystals inside neutrophils. Using polarized light, crystal analysis is important in making the diagnosis of gout (yellow=parallel):
And for the diagnosis of calcium pyrophosphate disease:
Also helpful in making the diagnosis of crystal disease are some classical radiographic findings. Which include chondrocalcinosis in CPPD:
and "rat bites" in gout: