What Is Septic Arthritis?

Septic arthritis is an infection inside of a joint — the area where two bones meet — like the knee or wrist. “Septic” means infected. “Arthritis” means joint inflammation (pain and swelling in a joint). 

Septic arthritis can affect both children and adults. Bigger joints like the knee, hip, and wrist are more likely to get infected than small joints. One joint is more likely to get infected than many joints. But infection can happen in any joint in the body, or several joints.

Septic arthritis is a medical emergency. This means that it needs medical attention at a hospital as soon as possible. In this article, we discuss the causes, diagnosis, and treatment of septic arthritis.

What causes septic arthritis?

Septic arthritis occurs when germs get inside of a joint. Joints are closed off from the outside world, so how do germs get in there? Germs can enter a joint in three different ways:

1. Directly, such as by puncture wound or needle

2. Through the bloodstream

3. From an infection of the skin or tissues nearby

Different types of germs can cause septic arthritis. Bacteria are the most common cause of septic arthritis and the most severe. Bacterial infections progress quickly and can destroy a joint if not treated.

Other rare germs, like fungus and mycobacteria, can also cause septic arthritis. These infections affect people who are immunocompromised (people with weakened immune systems).

What are the risk factors for septic arthritis?

Fortunately, septic arthritis isn’t very common. In 2012, septic arthritis accounted for only 0.01% of emergency room visits. That’s because septic arthritis tends to happen in people with certain risk factors.

Risk factors include:

• Abnormal joints: Joints that already have arthritis (like rheumatoid arthritis) are at highest risk of infection. Prosthetic joints (joint replacements) are also at risk.

• Conditions that weaken the immune system: For example, diabetes increases the risk of septic arthritis. 

• Medications that weaken the immune system: Examples include medications that treat cancer or autoimmune diseases that affect your immune system.

• Objects breaking the skin: Anything that breaks the skin increases the chance of germs entering the body from the outside. Examples include needles for medical reasons, injection drug use, or bites. 

• Active infections in other parts of the body: Infections of the bloodstream (sepsis) or infections of skin over a joint (cellulitis) can spread into a joint.

What are the symptoms of septic arthritis?

Septic arthritis isn’t subtle. When a joint is infected, it lets the body know! Generally speaking, there’s only infection in one joint at a time. But it may affect more than one joint if the cause of septic arthritis is bacteria that spreads to joints via the bloodstream. When bacteria infects a joint, symptoms develop rapidly — over the course of 24 to 48 hours.

Symptoms of septic arthritis include: 

• Fevers and chills (sometimes)

• Swelling, warmth, redness, and extreme pain of one joint (or sometimes more than one)

• Severe pain with even the slightest movement of the joint

• Inability to bear weight 

Of note, symptoms are less obvious in people who are immunocompromised. These people may not have fevers, and their joints may not be as painful or swollen. If you have a weakened immune system and you’re worried about septic arthritis, talk to your provider. Together you’ll determine the next best steps. 

Is septic arthritis an emergency?

Yes! Septic arthritis may be life-threatening. Bacterial infections can cause rapid and permanent damage to a joint if they don’t quickly get treatment. And if infection spreads to the bloodstream, low blood pressure can cause the heart to stop. If you’re concerned about septic arthritis, seek care in an emergency room. The faster you get treatment for septic arthritis, the better the outcome for you and your joint.

How do you diagnose septic arthritis?

Healthcare providers diagnose septic arthritis based on symptoms, blood tests, and joint fluid tests. Imaging might be helpful, too. They’ll often order X-rays to rule out other issues. Sometimes the diagnosis of septic arthritis can be tricky.

The most important test to diagnose septic arthritis is arthrocentesis. A healthcare provider will insert a needle into your joint and remove joint fluid. Then a specialist examines the fluid under the microscope for white blood cells and bacteria. If there are more than 50,000 white blood cells in the joint fluid, they diagnose septic arthritis. They’ll also check cultures (tests to grow bacteria). If bacteria grow, this also diagnoses septic arthritis. Culture results may take a few days because that’s how long it can take bacteria to grow.  

So what’s the tricky part? Well, the tests available to diagnose septic arthritis aren’t perfect. Sometimes a joint can be infected with fewer than 50,000 white blood cells in the fluid. And about 20% of cultures don’t grow bacteria, even though the joint is infected. The diagnosis at that point depends on “clinical suspicion” — that is, how much a provider suspects septic arthritis based on your risk factors and symptoms.

How do you treat septic arthritis?

Treatment of septic arthritis includes three different pieces:

1. IV antibiotics: If a specific bacteria grows from cultures, providers adjust antibiotics to best kill the infection. If bacteria doesn’t grow, providers use antibiotics that kill many different types of bacteria to be on the safe side. IV antibiotic treatment lasts several weeks.

2. Drainage of infected joint fluid: A procedure to drain infected joint fluid “washes out” the joint and helps antibiotics better penetrate the area. Procedures might include arthrocentesis (removing fluid from the joint with a needle), arthroscopy (minor surgery where small incisions and tools clean out the joint), or surgery (major surgery that opens the joint and cleans out the infection). The specific procedure depends on the severity of infection and how it responds to antibiotics. 

3. Physical therapy (rehab): Septic arthritis can take a pretty big toll on a joint, so rehabilitation is always necessary. Physical therapy strengthens and trains the joint how to move again. You may do physical therapy at home, in a physical therapist’s office, or at a rehab facility. You and your team will work together to decide what’s best for you.

How long does it take to fully recover from septic arthritis?

It depends. Most people start to feel better within days of starting antibiotics. But recovery depends on the severity of your infection and the amount of damage to the joint. It’s possible to feel better within weeks to months. And there are steps you can take to recover fully!

To best recover from septic arthritis: 

• Complete the full course of antibiotics. Stopping antibiotics early can lead to repeat infection.

• Actively participate in your rehab program. This means doing exercises as often as your therapist recommends.

• Follow up with your provider within 1 to 2 weeks of leaving the hospital. Ask the hospital team to help set up an appointment for you before you go home.

Why trust our experts?

Written by:

Samantha C. Shapiro, MD

Samantha Shapiro, MD, is a board-certified rheumatologist and internist with expertise in autoimmune and inflammatory conditions. She founded the division of rheumatology at Dell Medical School at The University of Texas at Austin.

Edited by:

Mandy Armitage, MD

Mandy Armitage, MD, has combined her interests in clinical medicine with her passion for education and content development for many years. She served as medical director for the health technology companies HealthLoop (now Get Well) and Doximity.

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