Staten Island Corner: Septic Joint - The Original Kings of County

Welcome back to this month’s Staten Island Corner. Please come up with ideas or topics that you are interested in and I will be happy to try to find some useful information and articles on the topic. I think we will all get more out of the blog that way.

So this month’s topic is septic arthritis in adults. I know it is not a common disease, but it is something that we all need to be comfortable identifying and managing as EM docs. The inspiration for this topic comes from a comment that was made by one of our ortho colleagues recently. I was coming on to a shift recently and I overheard the ortho resident (who is actually a really good resident and nice guy) say to the EM attending that there is no reason to tap this particular febrile patient’s joint because the wbc and crp (or esr I forget which one) were normal. I turned around and laughed because I thought the resident was making a joke. When I quickly realized that wasn’t the case, I couldn’t keep my mouth shut (no surprise to everyone I am sure). We then had a “friendly disagreement.” Ortho insisted that “their literature” proves that a normal wbc and esr rules out septic arthritis. I knew this wasn’t true, but I quickly became embarrassed and had to shut up because I realized I didn’t have the data or articles to back it up. So hopefully after reading this, you will all have the facts to present if a similar situation were to arise. I will discuss septic arthritis only in adults.

Who gets Septic Arthritis and why is it important?

The annual incidence in the United States is 10 per 100,000. It is more common in patients with rheumatoid arthritis or a prosthetic joint with an annual incidence of 70 per 100,000. Risk factors for septic arthritis include diabetes, age over 80, rheumatoid arthritis, HIV, prosthetic joint, and overlying skin infection. The reason why this disease is so important is that it carries a significant morbidity and mortality. Untreated it can destroy a joint within a few days. Also, the in hospital mortality rate is 7-15% (which is pretty significant and scary when you think about it!!!). Any bacteria can cause a septic joint, but the most common are staph and strep. It is most common in the knee (about 50% of cases), but also commonly affects the hip, shoulder, and elbow.

What aspects of history and physical predict septic arthritis or rule out septic arthritis?

To answer this question quickly, history and physical are unacceptable to rule out septic arthritis. As EM docs, we care about sensitivity in a disease process like this. We cannot afford to miss this diagnosis. Just to throw some numbers out there: sensitivity of fever is only 57%, pain 85%, and swelling is 78%. Obviously, you cannot rule anything out with these types of numbers. Obviously, if a patient has a fever and a red, hot, swollen knee, our pretest probability of a septic knee is high, but the absence of these signs does not rule out the disease.

What about the lab work?

As you all probably assume, lab work is also somewhat useless when trying to rule out septic arthritis. I looked a few recent articles that were review articles of the literature. The articles continue to mention how the literature on this topic is not good and is lacking. Here are the numbers. For the most part, the studies used a positive synovial fluid culture, positive gram stain, or positive intra op findings as the gold standard of septic arthritis because synovial fluid culture alone is only 75-95% sensitive. Regardless of the threshold selected, no study in the meta analysis demonstrated an acceptable sensitivity or overall diagnostic accuracy of peripheral WBC count for septic arthritis (sensitivities seen between 23% and 90%). No cutoff for ESR or C-reactive protein (CRP) significantly increases or decreases the posttest probability of septic arthritis. Procalcitonin, tumor necrosis factor, and various cytokines including interleukin (IL)-6 and IL-β were generally specific with very poor sensitivity. A small retrospective study at Jacobi Medical Center looked at the sensitivity of three tests (WBC, ESR, and joint WBC) for septic arthritis. The sensitivities of WBC >11, ESR>20 and synovial WBC >50,000 were 75%, 75%, and 50%.

So where does this leave us?…..the patient needs a tap.

We have already stated that history, physical, and lab work do not help us rule out septic joint. Therefore, an arthrocentesis is ALWAYS indicated when the diagnosis of septic joint is considered. Arthocentesis is not completely risk free with the major risk being that of infection. This risk has been estimated to be 0.01% in the general population and 0.05% in the immunocompromised.

So what does the meta analysis say about the numbers for synovial fluid. The summary sensitivity for synovial WBC > 25 is 73%, >50 is 56%, and >100 is 19%. The summary positive likelihood ratio for a sWBC count of >50 × 10⁹/L is 4.7 (95% CI = 2.5 to 85), and the negative likelihood ratio is 0.52 (95% CI = 0.38 to 0.72), while for a sWBC count of >100 × 10⁹/L the positive likelihood ratio is 13.2 (95% CI = 3.6 to 51) and the negative likelihood ratio is 0.83 (95% CI = 0.80 to 0.89). Synovial polymorphonuclear cells greater than 90% and synovial glucose and protein levels do not significantly increase or decrease the probability of septic arthritis. Based on a single trial, a synovial lactate dehydrogenase (LDH) of <250 U/L may be sufficient to exclude the diagnosis of septic arthritis. Synovial lactate seems to be the best point of care test that we have. Depending on the cut off used (10, 11, 15) sensitivities ranged from 55-100%. This is obviously an unacceptable range, but the specificities ranged from 95-100%. Also, the positive likelihood ratios ranged from 19-infinity. The interval LR for synovial WBC range of 0 to 25 × 10⁹/L is 0.33, 25 × 10⁹–50 × 10⁹/L is 1.06, 50 × 10⁹ to 100 × 10⁹/L is 3.59, and >100 × 10⁹/L is infinity. However, the synovial wbc much be looked at in the context of the entire picture (history, physical, pretest probability, other synovial findings, etc.) A negative Gram stain does not even come close to ruling out septic arthritis since there are 45% to 71% false-negative rates.

Conclusion:

So back to the ortho resident and “their literature.” The literature that he was referring to is a pediatric study (Kocher criteria which can be found on Wheeless orthopedics online) on septic hips and has four criteria: non-weight bearing, ESR >40, WBC > 12, and fever. When 4/4, there is a 99% chance, when ¾, there is a 93% chance, when 2/4, there is a 40% chance, and when ¼, there is a 3% chance. There are no clinical decision rules, such as the Kocher criteria, for adult septic arthritis. These criteria have not been validated in adults and also only looked at hips and therefore are not really relevant to our everyday adult practice. In pediatrics, it is a different story. There is a recent study that looked at children aged 3 months to 15 years with signs and symptoms suggesting acute osteoarticular infection. The sensitivity of elevated ESR >20 was 94% (95% CI, 90%–96%) and CRP > 20 was 95% (95% CI, 91%–97%). The sensitivity of combining elevated CRP or ESR was 98% (95% CI, 96%–99%). Also, an elevated ESR or CRP within the first 3 days was seen in all patients (100% sensitivity; 95% CI, 99%–100%).

The take home point is that in an adult patient who presents with joint pain that you have determined has the possibility of being septic arthritis, you have an obligation to do the tap regardless of what the orthopedic states. Currently, none of the peripheral laboratory tests are at the point where a septic joint can be ruled out. Our best test is synovial WBC and it seems that synovial lactate will also play more of a role in the future.

References:

Carpenter CR, Schuur JD, Everett WW, Pines JM. Evidence-based diagnostics: adult septic

arthritis. Acad Emerg Med. 2011 Aug;18(8):781-96.

Li SF, Cassidy C, Chang C, Gharib S, Torres J. Diagnostic utility of laboratory tests in septic

arthritis. Emerg Med J. 2007 Feb;24(2):75-7.

Margaretten ME, Kohlwes J, Moore D, Bent S. Does this adult patient have septic arthritis?

JAMA. 2007;297:1478–1488.

Pääkkönen M, Kallio MJ, Kallio PE, Peltola H. Sensitivity of erythrocyte sedimentation rate and

C reactive protein in childhood bone and joint infections. Clin Orthop Relat Res. 2010 Mar;468(3):861-6.

The views expressed on this blog are the author's own and do not reflect the views of their employer. Please read our full disclaimer here. Any references to clinical cases refer to patients treated at a virtual hospital, Janus General Hospital.