Septic Arthritis

Q: Is septic arthritis the same condition as reactive arthritis?

No, septic arthritis is not the same as reactive arthritis. In septic arthritis, infection is present within the joint. Reactive arthritis is an autoimmune response triggered by infection, but the infection is not located in the joint. Reactive arthritis generally lasts from a few weeks to a few months and, while its severity may vary, it normally resolves without serious complications. Septic arthritis can lead to disability and, left untreated, may be life-threatening. In septic arthritis, the infection is treated with intravenous antibiotics. In reactive arthritis, oral antibiotics may be used to treat the triggering infection, but this does not shorten the duration of joint symptoms.

Written by Ada’s Medical Knowledge Team

Updated on February 15, 2022 at 8:01 AM UTC

What is septic arthritis?

Septic arthritis, also known as infectious arthritis or pyogenic arthritis, is a very rare condition involving an inflammation of a joint caused by an infection in that joint. It is usually caused by bacteria, though it may be caused by other microorganisms. Incidence is about 6 per 100,000 of the population in any given year. 1

The condition can affect anyone, but those with underlying joint disease or prosthetic joints have an increased risk, as well as older people and those with diabetes or a weakened immune system. Any joint may be affected, but septic arthritis is most common in the knee and hip, sometimes also occurring in the shoulder, elbow and ankle. 2

Symptoms usually develop quickly, over a few hours or days, and include pain, redness, swelling, heat and difficulty moving the affected joint or joints. Many people will develop a fever. If the infection spreads, a person may also develop nausea, dizziness or drowsiness.

Septic arthritis occurs when infectious microorganisms reach the inside of the joints. This can occur indirectly, if the microorganism enters the body via, such as a wound, and travels through the bloodstream to a joint, or directly, by entering a joint as a result of injury, injection or surgery.

A diagnosis is confirmed when a fluid sample from the affected joint shows that bacteria or other infectious agents are present. 2

Septic arthritis is treated with antibiotics delivered into a vein, normally during a hospital stay of two weeks or more, followed by further antibiotics taken orally at home. Fluid may also be drawn from the infected joint using a needle and syringe. 3

Septic arthritis normally occurs in just one joint, but polyarticular arthritis, in which multiple joints throughout the body are infected, is also possible. 4

Good to know: It is very important to seek medical attention immediately if septic arthritis is suspected. The condition can rapidly cause irreversible joint damage and, if left untreated, can be life-threatening. However, with prompt treatment, most people will recover well. 5

Symptoms

The most common symptoms of septic arthritis are localized to the affected joint as a result of inflammation and include: 6 7

Redness of the overlying skin
Swelling
Pain
Stiffness and difficulty moving
Heat
Inability to bear weight

Many people will also feel generally unwell, with symptoms that may include: 8 9

Fever
Nausea
Vomiting
Dizziness
Drowsiness

In most people with septic arthritis, the onset of symptoms occurs very quickly, within a few hours or days.

In about half of cases, the knee is affected. 6 Other commonly affected joints include the hip, shoulder, elbow, ankle and, in the case of intravenous drug users, the sternoclavicular joint between the collarbone and breastbone. The condition may occur as polyarticular arthritis, in which multiple joints are affected.

It is very important to seek medical attention immediately if septic arthritis is suspected. The Ada app can also give you an initial symptom assessment.

Causes of septic arthritis

Septic arthritis is caused by bacteria or other microorganisms entering the joint, most commonly entering the joints via the bloodstream, 10 for instance from a distant site such as a wound infection or a widespread infection, such as gonorrhea. Bacteria may also enter a joint directly, such as through joint replacement surgery, injection or penetrating trauma. 2

Infection causes damage to the joint. This may be a direct result of the infecting microorganism, such as Staphylococcus aureus, or a result of the affected person’s immune response. As soon as bacteria invade the bloodstream, the body begins an immune response. White blood cells and joint lining cells may attempt to surround and digest the bacteria. The chemical signals sent out by these cells contribute to the characteristic inflammation of septic arthritis, which can cause destruction of cartilage. Depending on the infecting organism, the level of inflammatory response and joint destruction varies. For example, infection with Neisseria gonorrhoeae, which is known as gonococcal arthritis and is caused by the bacteria responsible for gonorrhea, induces a relatively mild inflammatory response. 10

Septic arthritis may occur in connection with a prosthetic joint replacement. Prosthetic implant infection may enter the joint during the joint replacement procedure.

Bacteria may also enter the bloodstream at another site and travel to the joint, which may be more susceptible to infection as it heals. 10

Risk factors

Risk factors for developing septic arthritis include: 6 11

Age 80 or above: 45 percent of those affected are older than 65
A prosthetic implant, such as an artificial knee or hip, or other recent joint surgery
Diabetes
Prior joint damage, particularly from rheumatoid arthritis or gout
Liver cirrhosis: liver scarring due to conditions such as hepatitis and chronic alcoholism
HIV and AIDS or otherwise compromised immune system
A recent blood infection
Intravenous drug use
Gonorrhea infection
A bone infection, which is also called osteomyelitis, near to a joint

Good to know: People with diabetes and weakened immune systems should seek prompt treatment for all infections, including urinary tract infections and skin infections.

Diagnosis of septic arthritis

If a doctor suspects septic arthritis, they will immediately refer a person to the nearest emergency department for an assessment.

The diagnosis is confirmed when a sample of fluid from the joints reveals the presence of bacteria or other microorganisms.

Further diagnostic testing may include blood tests, an ultrasound, an X-ray, a CT scan or magnetic resonance imaging (MRI) of the joints.

Interview and physical exam

It is important that a person describe any joint pain and other symptoms, as well as how long they have been going on for.

A doctor will look for signs of septic arthritis, such as swollen, tender, warm joints with painful or limited movement and fever.

Laboratory tests

After referral to the emergency department, one or more tests may be done to help make a diagnosis. These include: 7 5

A blood test: This may reveal an elevated white blood cell count, which is normally found in cases of septic arthritis. Abnormally high levels of certain markers in the blood would also support a diagnosis that symptoms are caused by an inflammatory condition, such as septic arthritis. Inflammatory markers which may be detected by a blood test include: 6 2

C-reactive protein (CRP)
Erythrocyte sedimentation rate (ESR or sed rate)
Blood cultures to detect bacteria in the blood

However, these tests come back normal in many cases of septic arthritis, so cannot be relied upon to rule out the condition.

Medical imaging

A doctor may also recommend an X-ray, CT scan, ultrasound or MRI scan of the affected joint. This can help with diagnosis of septic arthritis and any complications. However, at an early stage of septic arthritis, there may not be any visible joint damage. 12

Septic arthritis treatment

There are five main aspects of septic arthritis treatment: 2 13

Antibiotics: Septic arthritis infection must be cleared with antibiotics delivered into a vein

Draining the joint: This may be done to clear any infected fluid from the joint. This will either be done using a needle and syringe or by arthroscopy, in which a thin, metal tube is inserted through a small cut made near the affected joint.

Pain relief: NSAIDs may be given for pain relief and to reduce inflammation.

Physiotherapy: Most people will require a lot of physiotherapy to maximize functioning of the joint after infection has cleared, to promote healing and prevent permanent muscle shortening.

In some cases, surgery: Occasionally, this may be done to clean the joints. If the septic arthritis followed a joint replacement surgery, the joint replacement itself may need removal.

Good to know: Treatment normally involves a hospital stay of two weeks or more, followed by further antibiotics taken orally at home, normally for at least four more weeks.

Complications

If septic arthritis is left untreated, serious complications may arise. These may include: 15

Osteomyelitis, a painful bone infection and inflammation
Bone erosions, the loss of bone, caused by destructive joint inflammation
Fibrous ankylosis, decreased range of motion, which may be caused by destruction of the joint lining
Sepsis, also known as blood poisoning, in which the body attacks its own organs and tissues in response to infection. Sepsis can be life-threatening.

Good to know: It is important to seek medical attention immediately if septic arthritis is suspected, to avoid serious complications. The condition can rapidly cause irreversible bone and joint damage and, left untreated, can be life-threatening. However, with prompt treatment, most people will recover well.

Septic arthritis in children

Septic arthritis in children, also known as pediatric septic arthritis, is a very rare but serious emergency that must be treated promptly.

Signs and symptoms include:

Pain
Inability to bear weight on the affected joint
Fever

Young children with septic arthritis are likely to be irritable and may cry if the infected joint is moved. They may also try to avoid using or putting any weight on an affected joint.

If a doctor suspects pediatric septic arthritis, they will immediately refer a child to the nearest emergency department for an assessment.

Treatment involves draining the joint, followed by intravenous antibiotics. Ongoing follow-up with an orthopedic surgeon is recommended to monitor and treat long-term effects, such as cartilage damage and growth disturbance. While these effects are rare, early diagnosis and intervention may improve long-term outcomes. 16

Other names for septic arthritis

Infectious arthritis
Pyogenic arthritis
Suppurative arthritis
Bacterial arthritis
Infectious arthritis
Fungal joint infection
Bacterial joint inflammation
A joint infection
A septic joint

FAQs

Q: Is septic arthritis the same condition as reactive arthritis?
A: No, septic arthritis is not the same as reactive arthritis. In septic arthritis, infection is present within the joint. Reactive arthritis is an autoimmune response triggered by infection, but the infection is not located in the joint. Reactive arthritis generally lasts from a few weeks to a few months and, while its severity may vary, it normally resolves without serious complications. Septic arthritis can lead to disability and, left untreated, may be life-threatening. In septic arthritis, the infection is treated with intravenous antibiotics. In reactive arthritis, oral antibiotics may be used to treat the triggering infection, but this does not shorten the duration of joint symptoms.