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Clinical controversies: Negative CRP and ESR mean a patient… : Emergency Medicine News

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C-reactive protein, CRP, erythrocyte sedimentation rate, ESR, septic arthritis

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A pervasive myth perpetuated in ED is the false power given to inflammatory markers. C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are routinely prescribed as part of septic arthritis assessments in adults, but do they really change management?

The absence of septic arthritis is detrimental in terms of morbidity and mortality. The mortality rate has been quoted at 3%, while another study put it at 50% for polyarticular septic arthritis. (Medicine [Baltimore]. 1993;72[5]: 296.) A study of 121 cases of septic arthritis found poor joint outcome such as amputation, prosthetic surgery, and severe deterioration in one-third of patients. (Rheumatic arthritis. 1997;40[5]:884.)

This is clearly a diagnosis not to be missed. The gold standard is arthrocentesis, which carries a risk of iatrogenic infection, however small. The measured risk of iatrogenic infections by arthrocentesis is estimated between 0.01% and 0.037%. (Clinical Procedures of Roberts and Hedges in Emergency Medicine. 6th ed. Philadelphia: Elsevier Health Sciences; 2013.)

ESR and CRP are acute phase reactive inflammatory markers. Typical practice in most emergency departments is to order them as part of a soft tissue infection workup, but minimal evidence supports that they are changing practice. Indeed, the admission services and our surgeon colleagues regularly request them even if we are not convinced to order these tests.

Highly variable sensitivity

An excellent 2011 systematic review by Carpenter, et al., examined the validity of history, test results, and CRP/ESR to assess patients with septic arthritis. The study isolated 32 primary studies, all of which found a huge spread in the sensitivity and specificity of CRP and ESR. Accuracy also changed based on marker elevation. The sensitivity of an ESR greater than 50 mm/h varied from 42% to 92%! They concluded that no single CRP or ESR value significantly increased or decreased the post-test probability of septic arthritis. (Acad Emergency Med. 2011;18[8]:781; http://bit.ly/37Pudsw.)

Another study found ESR did not differ significantly between positive and negative aspirations in 458 patients who underwent knee arthrocentesis for evaluation of septic arthritis. (Orthopedics. 2016;39[4]:e657.) Another retrospective study in 2010 confirmed that ESR was not predictive. (Southern Mediterranean J. 2010;103[6]:522.)

CRP and ESR do not affect our post-test probability. In other words, they do not systematically change direction. Unfortunately, the story and the physical aren’t great interpreters either. This study by Carpenter et al. also found that the only story items that had a high odds ratio for septic arthritis were recent joint surgery and a skin infection covering a prosthetic joint. With little to help us, we have to keep it simple.

Any patient with a history of acute joint pain and effusion should give you a low threshold to perform arthrocentesis, which is the only reliable method to rule out septic arthritis. You can use ultrasound if you are unsure whether an effusion is present; it has been found to be more sensitive and detects effusions earlier than X-rays. (medical [Bucur]. 2021;16[1]:22; https://bit.ly/3ySA3ua.)

Arthrocentesis has a low risk of iatrogenic infection and potential patient discomfort, and it could be a fruitless attempt as some are dry taps. We often exaggerate these negatives, tricking us into ordering a CRP/ESR and hoping they are negative in order to avoid sticking a needle in a joint. No lab value or piece of history and physicality completely rules out septic arthritis, but stop giving undeserved power to CRP and ESR, and start tapping more joints when you’re concerned about it. septic arthritis.

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Dr. Briggsis an assistant professor of emergency medicine at the University of South Alabama in Mobile. He is the founder, podcast co-host and editor of EM Board Bombs (https://www.emboardbombs.com), a cross-platform educational tool designed to prepare for counseling and focus on what you need to know to practice emergency medicine. Follow him on Twitter@blakebriggsmd.