Bacterial Arthritis

Does this patient have bacterial arthritis?

Bacterial arthritis is typically divided into gonococcal and non-gonococcal arthritis (NGA). Non-gonococcal arthritis is of greatest concern, because it is the most damaging; therefore, early diagnosis and treatment is critical. The duration of untreated infection is the most important determinant of joint damage. An infected joint may be destroyed within a few days.

Staphylococcus aureus is the most common organism. This has not changed for decades, however, methicillin resistant staph aureus (MRSA) is clearly increasing in frequency. This should be kept in mind when determining empiric antibiotic treatment in anticipation of culture and sensitivity results.

Non-gonococcal arthritis typically presents as a single red, hot and swollen joint. The knee is most commonly affected. Other cardinal signs of infection may be present. The onset is relatively rapid.

Other joints may be involved, and polyarticular septic arthritis may occur. However, NGA is usually monoarticular. When joints such as the sternoclavicular, acromioclavicular, sacroiliac joints are involved, consider atypical organisms like gram negative bacteria, including pseudomonas; particularly if the patient is an intravenous drug abuser or is otherwise immunosuppressed.

History and physical may lead to the source of the infection. Skin infections are a common cause of Staphylococcus aureusbacterial arthritis. In adults, hematogenous spread is the most common route of infection. Previously injured or inflamed joints are more commonly infected. Transient or persistent bacteremia underlies the importance of blood cultures as part of the evaluation.

Differential diagnoses

The most common differential diagnosis for acute inflammatory monoarticular arthritis is crystalline arthritis. Gout and pseudogout commonly affect the knee. The presentation can be identical to that of a NGA septic arthritis. The history and physical and labs may provide clues for differentiation. However, non-gonococcal septic arthritis and crystalline arthritis are not mutually exclusive. Therefore, the more damaging septic arthritis needs to be addressed, first and foremost, until it has been ruled out.

Other conditions may also cause monoarthritis, but usually the joint is not as inflamed. One can be lulled into thinking that a joint is not infected with a damaging organism, based upon this presentation. Likewise, one may attribute the subacute presentation to a patient's underlying disease (such as rheumatoid arthritis). However, it is important to recall that previously injured or inflamed joints are at higher risk of becoming infected, and uncommon presentations of septic arthritis occur.

The other differential diagnoses for nongonococcal septic arthritis include: Lyme arthritis (particularly in endemic areas), viral arthritis, and other bacterial arthritis (including GCA). In tuberculous and fungal arthritis, the affected joint is usually cooler and does not present acutely.

Culture- and crystal-negative acute inflammatory arthritis may represent an atypical presentation of an autoimmune arthritis, such as RA or SLE; alternatively, a spondyloarthropathy, such as reactive, psoriatic, enteropathic arthritis, or ankylosing spondylitis, may present this way. Acute rheumatic fever or post-streptococcal arthritis may also produce a culture- and crystal-negative inflamed joint.

What tests to perform?

Laboratory testing

Synovial fluid culture

The most critical laboratory test for the diagnosis of a NG septic arthritis is the synovial fluid culture. If you get only a drop of fluid, send it for this test. If you have another drop, perform a Gram stain. The remaining fluid can be sent for crystal analysis and cell counts.

Cell counts of greater than 50,000 WBC/mm3 should raise suspicion for NG septic arthritis, although counts of this magnitude can be seen with crystalline arthritis. Moreover, lower cell counts are not helpful in excluding septic arthritis. Hence, culturing the synovial fluid is the most important test.

Gram stain

Gram stains may help provide guidance for empiric antibiotic therapy if positive. Although a negative Gram stain, in a setting of high suspicion for septic arthritis, does not preclude the initiation of empiric antibiotic therapy. Likewise, the identification of crystals does not exclude infection, and empiric antibiotic therapy may be appropriate, based upon the clinical circumstances.

False positive Gram stains may be reported by inexperienced microscopists; as mucin may be mistaken for gram positive cocci. Typically, these pseudo-cocci are much larger than bacterial organisms, when viewed at the same optical power.

Blood cultures

It is important to draw blood cultures and to culture any area found on the PE that may have placed the patient at risk for hematogenous spread. Genital, anal and pharyngeal cultures are needed to rule out gonococcal arthritis in patients in whom that differential diagnosis is being considered. Urine probes for GC may also be helpful.


CBC, ESR, CRP are useful, but not always helpful, and may erroneously lead you away from the diagnosis of NG septic arthritis.



X-rays will give you a sense of structural damage and perhaps urgency for intervention. They may also alert you to the more unusual (adult) occurrence of joint infection from a contiguous structure, such as osteomyelitis. Serial x-rays may be needed to evaluate structural integrity over time. Structural damage is suggested by extensive erosions and loss of joint space. In advanced settings the articulations may be indistinguishable. Cortical or periosteal irregularities may alert you to underlying bone infection. However, adults are more likely to develop primary septic arthritis and secondary osteomyelitis.


MRI may provide important information about the possibility of infection in contiguous areas. It may also determine if fluid collections are septated or loculated, which require more aggressive techniques to access and drain. Ultrasound by an experienced user may provide similar information and may guide needle placement for aspiration.


For NG septic arthritis, synovial biopsy is not indicated. Certainly, if NG septic arthritis is ruled out and conditions such as tuberculous arthritis, fungal arthritis or atypical Lyme arthritis are still in the differential diagnosis, biopsy with special staining, culture and PCR may be useful.

Interpretation of test results

If the Gram stain is negative, empiric antibiotic therapy is still warranted in situations where there is a reasonable suspicion for septic arthritis. Positive Gram stains will help guide empiric therapy until culture results return (see Table I). It is reasonable to consider Gram-positive cocci in clusters to be MRSA until proven otherwise.

Besides the importance of obtaining culture of the synovial fluid, there is little uncontroverted evidence that history, physical examination, laboratory studies and x-rays are adequate or even helpful to rule out NG septic arthritis. The experienced clinician must make judgment calls.

Certainly, not every joint effusion requires an aspiration and culture. However, when there is potential for septic arthritis, the experienced clinician is not dissuaded from culturing synovial fluid and treating with empiric antibiotics in the presence of mixed or unsupportive clinical findings or diagnostic tests.

The consequences of a missed diagnosis can be severe joint destruction.

How should patients with bacterial arthritis be managed?

Empiric treatment may be guided by the Gram stain results, if they are available. A negative Gram stain or insufficient fluid to perform a Gram stain would require empiric therapy based upon the clinical setting. Modifications in treatment may be made after the culture results have returned (see Table I).

Table I.

Initial Treatment Options for Non-Gonococcal Septic Arthritis Based Upon Clinical Settings

If no Gram stain results are available and empiric therapy is warranted, coverage for MRSA is advised. This approach will also cover most streptococcal species. In special populations, such as the immunosuppressed or patients with chronic infections (such as UTI), or colon inflammation/infections or cancer, the addition of coverage for Gram negative organisms is needed.

If unusual joints, such as sternoclavicular, acromioclavicular and sacroiliac joints are involved, antibiotic coverage should be broadened to include coverage for pseudomonas. In younger, sexually active individuals coverage for gonococcal arthritis, in addition to MRSA, should be considered.

If the Gram stain shows gram positive cocci in clusters, coverage for MRSA is wise until culture results return. Streptococcal infections will be empirically covered by most antibiotics for MRSA.

Gram negative rods require suitable coverage (see Table I), but it may be wise to also cover for Staphylococcus aureusuntil the culture results have returned. Gram negative rod coverage should include treatment for pseudomonas, particularly if there is a history of intravenous drug abuse and if unusual joints (sternoclavicular, sacroiliac) are involved. Pseudomonas may also be a concern when septic joints develop in the hospital setting.

The culture and sensitivity results will allow refinement of antibiotic therapy.

Duration and route of therapy has been anecdotally defined with reasonable agreement. There is a paucity of high quality studies in the adult literature that address this. Meta-analyses are hampered by small studies of poor or inconsistent design. Despite this, there is reasonable agreement on duration and route of therapy.

With variation based upon the clinical setting, a total of 6 weeks of antibiotic therapy is usually adequate to eradicate the infection. Typically, the first 2-3 weeks is given intravenously.

Although high quality studies are lacking, there is strong agreement that drainage of joints with high WBC counts is warranted to prevent damage and destruction to cartilage and other tissues. There appears to be a subtle preference for open drainage (arthrotomy), however, it is perfectly reasonable to serially aspirate joints that are easily accessible. Open drainage or arthroscopy may be needed for septated or highly loculated fluid, or for fluid that is difficult to access by joint aspiration.

Again, there is little evidence, but much agreement that drainage should continue until the WBC count consistently fall below 50,000 WBC/mm3.

Simple ROM exercise is wise (within the limits of the setting). However, more aggressive physical therapy should be reserved until after the acute inflammation has resolved. The extent of weight bearing can be determined largely by the patient, as long as the joint is stable.

What happens to patients with bacterial arthritis?

The duration of untreated infection is the most important determinant of outcome in NG septic arthritis. Early diagnosis and treatment improves outcomes. An untreated NG septic joint can be destroyed in a matter of days. Certain joints rendered terminal by septic arthritis may be amenable to total joint arthroplasty, once the infection has been eradicated.

How to utilize team care?

Questions regarding choice of antibiotic therapy, particularly in complicated settings, may require the assistance of an infectious disease consultant. A rheumatologist may be needed to access more difficult joints. Orthopedists may be needed to access deep joints or joints that are septated or where fluid is loculated or sequestered. Orthopedists are also needed for open drainage (arthrotomy), arthroscopy or rarely bone biopsy of contiguous areas. Orthopedists may perform total joint arthroplasty after the infection has been eradicated.

Close attention to wound drainage equipment is required.

Physical and/or occupational therapy is limited to ROM during the acute phase. Afterwards, attention is given toward optimizing ROM and function; with strengthening of muscles surrounding the affected joint. PT and OT play important roles in preparing the patient for upcoming total arthroplasty as well as during the rehabilitation period.

Are there clinical practice guidelines to inform decision making?

There are no clinical practical guidelines.

Other considerations

  • 711.0 Pyogenic arthritis

  • 716.6 Unspecified monoarthritis

  • 716.5 Unspecified polyarthropathy or polyarthritis

  • 711.9 Unspecified infective arthritis

  • 711.4 Arthropathy associated with other bacteria diseases (not gonococcus or menningococcus)

  • 81.91 Arthrocentesis

For most uncomplicated cases of septic arthritis, the first few days of intravenous antibiotic therapy is given in the hospital setting. The patient is usually discharged with arrangements for home IV therapy as soon as they are clinically improved, and the WBC count in the affected joint falls below 50,000 WBC/mm3.

What is the evidence?

Becker, J, Winthrop, KL. "Update of rheumatic manifestations of infectious diseases". Curr Opin Rheumatol. vol. 1. 2010. pp. 72-7.

(This recent update on pathogens (some emerging) addresses clinical presentation; identifying features and patterns.)

Kim, H, Kim, J, Ihm, C. "The usefulness of multiplex PCR for the identification of bacteria in joint infection". J Clin Lab Anal. vol. 3. 2010. pp. 175-81.

(This emerging technology has the potential to shorten the interval between diagnosis and treatment, and the potential for improved outcomes resulting from early and specifically directed intervention.)

Chander, S, Coakley, G. "What’s new in the management of bacterial septic arthritis?". Curr Infect Dis Rep. vol. 5. 2011. pp. 478-84.

(The current condition and future management trends are covered in this review of epidemiology, diagnosis, and treatment.)

Butt, U, Amissah-Arthur, M, Khattack, F, Elsworth, CF. "What are we doing about septic arthritis? A survey of UK-based rheumatologists and orthopedic surgeons". Clin Rheumatol. vol. 5. 2011. pp. 707-10.

(This paper compares and contrasts opinions about closed versus open drainage of septic joints in the UK. It is not generalizable and reaches no clear conclusions, but is an example of one of several areas in septic arthritis where opinion dominates evidence in decision making due to a lack of well conducted trials.)

Garcia-De La Torre, I, Nava-Zavala, A. "Gonococcal and nongonococcal arthritis". Rheum Dis Clin North Am. vol. 1. 2009. pp. 63-73.

(A broad review that allows the reader to compare and contrast these two forms of bacterial arthritis.)

Al-Nammari, SS, Bobak, P, Venkatesh, R. "Methicillin resistant Staphylococcus aureus versus methicillin sensitive Staphylococcus aureus adult haematogenous septic arthritis". Arch Orthop Trauma Surg. vol. 7. 2007. pp. 537-42.

(This retrospective study compares methicillin sensitive and resistant strains of staphylococcus aureus. The article addresses differences in comorbid conditions, presentations, treatments and outcomes.)

Barcia-Arias, M, Balsa, AS, Mola, EM. "Septic arthritis". Best Pract Res Clin Rheumatol. vol. 3. 2011. pp. 407-21.

(This paper is a recent review of diagnostics and therapeutics for septic arthritis.)

Mathews, CJ, Coakley, G. "Septic arthritis: current diagnostic and therapeutic algorithm". Curr Opin Rheumatol. vol. 457. 2008. pp. 62.

(This review highlights the lack of high-quality evidence based medicine to guide clinicians in the management of the inflamed joint.
You must be a registered member of Endocrinology Advisor to post a comment.

Sign Up for Free e-Newsletters

CME Focus