Maintain a High Index of Suspicion for Septic Arthritis

Dr Louise Warburton Reviews Essential Guidance on Septic Arthritis, Covering Diagnosis and Management from the Perspective of Primary Care

Septic arthritis is a serious condition in which a pathogen enters a joint and causes acute infection and inflammation. If not treated promptly and appropriately, septic arthritis can cause long-term joint damage, with cartilage loss and deformity of the articular surfaces.^1,2 The condition has a high case-fatality rate of 11%,^3,4 so it should be considered in any case of monoarthritis, and whenever a single joint is affected with pain and swelling.⁴

Septic arthritis is estimated to have an incidence of 4–29 cases per 100,000 person-years, depending on population variables.¹ The risk of septic arthritis increases with age and lower socioeconomic status.^2,5

For primary care practitioners in the UK, the most relevant current guidelines on the diagnosis and management of septic arthritis are:

• a 2006 guideline⁴ (updated in 2017⁶) from the British Society for Rheumatology (BSR) and other British organisations, reviewed in 2017—BSR & BHPR, BOA, RCGP and BSAC guidelines for management of the hot swollen joint in adults
• a 2021 commentary in American Family Physician²—Septic arthritis: diagnosis and treatment
• a 2023 guideline specifically concerning the secondary care management of septic arthritis of a native joint⁷—Guideline for management of septic arthritis in native joints (SANJO).

Suspecting Septic Arthritis 

Because septic arthritis has such a high case-fatality rate, all patients with a short history of a red, hot, swollen, and painful joint with restricted movement should be considered as having septic arthritis until proven otherwise.⁴ In the classical presentation, the joint will be extremely painful, and the patient will not be able to bear weight on an affected lower limb.^1,2,4 Purulent drainage, for example around a prosthetic joint, with redness and cellulitis should certainly prompt further investigation.⁷

Septic arthritis may also be a presenting feature of generalised sepsis, in which case the seat of infection will be elsewhere—such as in the lungs (pneumonia) or urinary tract. The constitutional symptoms of infection, such as fever, may therefore be present in septic arthritis. 

However, constitutional symptoms are not particularly specific or sensitive to septic arthritis.^1,8 For example, fever is present in about 40–60% of people with the condition.^1,8 In a 2007 meta-analysis, it was determined that the sensitivities of constitutional symptoms for septic arthritis are as follows:⁹ 

• fever—57%
• chills—27%
• rigors—19%.

Atypical Presentations

When considering a diagnosis of septic arthritis, it is worth remembering that the classical signs of infection may not be as obvious in elderly people and in those who are immunosuppressed, so a high index of suspicion would be particularly sensible in these groups. 

In my personal practice, I recently saw an elderly gentleman who had developed a painful, red, hot wrist over the course of a morning. When I visited him at lunchtime, he was slightly confused but apyrexial, and other observations were normal. Because of the new onset of confusion and the acutely painful wrist, I admitted him to hospital. He did actually have septic arthritis of the wrist, and made a good recovery.

Risk Factors

The key risk factors for developing septic arthritis are listed in Box 1.^1,2,10 Notably, the use of intravenous drugs is a double risk for septic arthritis: injecting nonsterile substances is associated with a risk of septic arthritis, and this risk is increased by the situation of needle puncture sites near to joints, such as in the groin.¹¹

Differential Diagnoses

As part of any assessment for possible septic arthritis, it is important that clinicians consider the differential diagnoses of a red, hot, and swollen joint.^2,4 One of the most common causes presenting in primary care will be gout,⁴ and the clinical history is diagnostic if the person has either tophi or a rapid onset of pain, with redness or swelling in the first metatarsophalangeal joint.¹² With gout, the swelling will usually develop rapidly over 12–24 hours, often overnight. Gout can also become polyarticular and affect other joints, such as the ankle or the knee—this is where the differential relationship with septic arthritis arises.

Useful guidelines for diagnosing gout have been produced by NICE¹² and the BSR,¹³ and the diagnosis and differential diagnoses of gout are also discussed in my previous Guidelines in Practice article, Gout: treat to target to prevent irreparable joint damage.¹⁴ 

Other differential diagnoses include acute inflammatory arthritis—such as rheumatoid arthritis—and reactive arthritis, both of which can sometimes develop as monoarthritis.² With reactive arthritis, there will most likely be a recent history of sexually transmitted infection, such as chlamydia, or bowel infection with pathogens such as Salmonella.¹⁵ In these cases, it would be important to isolate and treat the ongoing infection in the bowel or genital tract.¹⁵

Diagnosis and Referral 

Figure 1 outlines the pathway for the diagnosis of septic arthritis recommended in the BSR guidance.⁴ It is essential that any suspected septic arthritis is referred to an emergency or specialist department for urgent assessment,⁴ but certain investigations—particularly joint aspiration—may potentially be done in a primary care setting.

Figure 1: Algorithm for Patients Presenting With Acute Increase in Pain and/or Swelling in One or More Joints⁴ 

Joint Aspiration

Aspiration of joint fluid is the investigation of choice for the diagnosis of septic arthritis.^1,2,4 In most cases, this will be done in specialised or emergency care⁴—under either Orthopaedics or Rheumatology, depending on the local pathway. When possible, aspiration should always be undertaken before antibiotics are given, as antibiotic therapy is guided by organism.^1,2,4 

Occasionally, a GP or primary care clinician will aspirate joint fluid in the community, such as when considering a steroid injection into the joint. It is best practice to send a sample for microbial investigation if there is any suggestion that the joint fluid might be infected, such as increasing turbidity of the sample or an aspiration of frank pus. 

If a person’s joint fluid appears turbid or cloudy when aspirated, this is a sign of septic arthritis.⁴ In this situation, GPs should refer their patient urgently to secondary care with the sample.⁴ Any steroid injection should be deferred until a culture has been performed and infection has been excluded.⁴

Even if Gram staining and culture reveal no organisms in a person’s joint aspirate, there may still be infection in the joint; therefore, any person with associated symptoms and signs but negative results should still be regarded as having septic arthritis.⁴ 

Other Investigations 

When investigating suspected septic arthritis, blood cultures should also be arranged,⁴ as the person may have sepsis with a seat of infection elsewhere in the body. Other recommended investigations include tests for white cell count, erythrocyte sedimentation rate, and C-reactive protein, which are all usually raised in septic arthritis, and are useful for monitoring a patient’s response to treatment.⁴ In addition, tests of urea and electrolytes, liver function tests, and bone screening may provide useful information.⁴

As part of the management of septic arthritis, these investigations would usually be done in secondary care.^1,2,4 However, these tests are all regularly carried out in primary care, and routine testing may provide useful contextual information for secondary care management. For example, pre-existing knowledge of poor liver or renal function may influence antibiotic choice, and plain imaging—although it is unlikely to show abnormalities in a septic joint—can indicate the loosening of a prosthetic joint or aid in differential diagnosis.^1,4,16

The Diagnostic Potential of Serum Procalcitonin

As identified in the 2017 update to the BSR guideline, serum procalcitonin (PCT)—a peptide precursor of the hormone calcitonin—has shown potential as a marker to use in the diagnosis of septic arthritis.⁶ This is because PCT has a low concentration in healthy individuals (less than 0.1 ng/ml), but rises sharply in the presence of bacterial endotoxin.⁶

Studies of systemic and respiratory infections have demonstrated that PCT can be used to discriminate between bacterial and nonbacterial inflammation, and some smaller studies have also indicated the possible use of serum PCT for diagnosing musculoskeletal infection and guiding its management.^6,17,18 Thus, there may be a place in the future for using PCT levels to discriminate between joint inflammation caused by septic arthritis and that caused by nonbacterial inflammatory arthritis, such as rheumatoid arthritis. 

Management in Secondary Care

In general, septic arthritis is treated with joint aspiration, antibiotic therapy, and surgery when required.^1,4,7 In most cases, septic joints should be aspirated to dryness as often as required, and joint fluid sent for culture to assess clinical response.^1,4 Intravenous antibiotics are usually used for 2–4 weeks or until the infection has settled.^1,4,7 The specific protocol depends on the organism that has been identified.^4,7

Prosthetic Joints

The management of pain, swelling, and redness in prosthetic joints is different from in native joints,^1,19 and it is imperative that clinicians refer any patient with a potential prosthetic joint infection (PJI) to Orthopaedics for further investigation and treatment.⁴ The prosthetic joint is a foreign body, and infection can be introduced at the time of surgery or may settle in the joint during episodes of bacteraemia.^19,20 

PJI is relatively rare, but it is a devastating complication of hip and knee arthroplasties, with an approximate incidence of 1–2% following primary arthroplasties and 4% following revisions.²¹ A 2017 study based on data from multiple national registries found that 0.97% of total hip arthroplasties and 1.03% of total knee arthroplasties had to be revised because of infection.²²

Usually, the investigation and treatment of PJI involves aspiration of the joint in theatre under aseptic conditions, washing out of the joint, blood and joint fluid cultures, and treatment with intravenous antibiotics.^19,20 Sometimes, the whole prosthesis will have to be removed and the infection treated before a replacement prosthetic joint can be introduced.^19,20 During this treatment, the patient usually cannot mobilise normally,^19,20 and other complications such as pneumonia and thrombosis can develop.^23,24 

Summary

Septic arthritis can be a life-threatening condition and should be diagnosed and treated as rapidly as possible. The condition is more commonly seen in patients who are elderly or immunosuppressed, and infection is more likely to affect prosthetic joints.