Septic arthritis

Septic arthritis means infection inside a joint, most often with bacteria that have reached it through a wound or infected cut, through the bloodstream from an infection somewhere else in the body, or occasionally after joint surgery. It is sometimes called bacterial or infectious arthritis.

It is rare, and a hot, swollen joint is far more often something else: gout, pseudogout and flares of existing arthritis are all more common and can look strikingly similar. The reason septic arthritis still deserves its own guide is what the rare real case can do: an infected joint can be permanently damaged within days, and an untreated infection can spread to the bloodstream and cause blood poisoning (septicaemia). That combination, rare but time-sensitive, is why doctors take every hot, swollen joint seriously, and why the question should be answered promptly rather than watched. The encouraging side of the same coin is that treatment works: with prompt antibiotics, most people make a full recovery.

Septic arthritis usually affects a single joint, and the symptoms usually develop quickly, over a few days:

• Severe pain in one joint that started suddenly
• Swelling around the joint
• Skin over the joint that is hot and has changed colour
• Difficulty moving the joint or putting weight on it
• Fever, or feeling hot, cold, shivery or generally unwell

Not every case shows the full picture. After joint replacement surgery the symptoms can develop more slowly, and a fever is not always present at the start. The practical rule of thumb: a hot, swollen, unusually painful joint deserves a prompt answer in every case, and it becomes a same-day emergency when a fever or feeling unwell comes with it.

There are two routes, and the split is simple.

If the joint comes with a fever, shivering or feeling unwell in yourself, go to A&E or call NHS 111 the same day. That combination is what makes an infected joint urgent: it is never something to watch overnight, and hospital is where the treatment for a genuinely infected joint lives. Tell whoever assesses you which medicines you take, especially any that quieten the immune system. The banner at the top of this page lists the signs of sepsis, the rare situation where infection starts to overwhelm the body; those signs mean 999, not 111.

If you feel well in yourself, the joint still deserves a prompt answer, within days rather than weeks, and a specialist clinic is a perfectly good place to get it, often the most direct one, since the assessment that settles the question is exactly what a rheumatologist does: an examination, ultrasound and, where needed, testing fluid from the joint. It helps to know how doctors approach this: a hot, swollen, tender joint is checked for infection rather than assumed innocent, even though infection usually turns out not to be the cause, because the cost of missing an infected joint is so much higher than the cost of checking one. When the assessment ends with reassurance and a different diagnosis, that is the system working exactly as intended, and it is by far the more common outcome.

Anyone can get septic arthritis, but it is more likely in some situations:

• Rheumatoid arthritis or another inflammatory joint condition. Inflamed joints are more vulnerable to infection, which is one reason people with rheumatoid arthritis are asked to take a hot, swollen joint seriously rather than assuming a flare.
• Medicines that quieten the immune system, including methotrexate, biologics and JAK inhibitors and longer courses of steroids. Our guide to infections and arthritis medicines explains the practical precautions that go with them.
• A weakened immune system from illness, for example diabetes, cancer or its treatment, or HIV.
• An artificial joint or recent joint surgery, such as a knee or hip replacement.
• An injection into or near a joint. This is an uncommon cause, and the risk after a properly performed injection is small, but it is why aftercare advice always includes watching for a joint that worsens rather than settles.
• Injecting non-prescription drugs, and some sexually transmitted infections, such as gonorrhoea, can also reach a joint through the bloodstream.

Being in one of these groups does not mean septic arthritis is likely; it remains uncommon. It means the warning signs deserve a lower threshold for acting.

The key test is a sample of fluid from the joint, taken with a needle, a procedure called joint aspiration. The fluid is examined for germs, and wherever possible this is done before antibiotics are started so the germ responsible can be identified and the treatment matched to it. The same fluid is also examined for crystals, because gout and pseudogout can look identical to an infected joint, and a flare of existing arthritis can too. Blood tests help, and an X-ray or MRI scan may be used to see what is happening inside the joint.

This is worth knowing, because it is what makes the assessment worth having rather than something to dread: the tests that would rule septic arthritis in are the same ones that rule it out, and far more often than not they deliver better news, a crystal flare rather than an infection, together with the diagnosis you actually needed. Our guide to managing a flare explains why that question is not one to settle at home.

Septic arthritis is treated in hospital. Antibiotics are usually given directly into a vein, and fluid is often drained from the joint with a needle and syringe, which both helps clear the infection and eases the pain. A hospital stay of around two weeks is typical, and most people start to feel better quickly once antibiotics begin.

Treatment does not end at discharge: antibiotic tablets usually continue for several weeks at home, and it is important to keep taking them for as long as advised, even when you feel completely well, because stopping early can let the infection return.

Confirmed septic arthritis is treated in hospital, and the feverish, unwell picture should head there directly. Around that, though, most of the territory belongs to rheumatology. Beforehand, for people living with inflammatory arthritis or taking immune-quieting medicines, it is making sure the warning signs are known and the threshold for acting is low.

Most of it, in fact, is the middle ground, and it is the commonest way this comes up. A joint is swollen and sore but you feel well in yourself; the symptoms are mixed, or have grumbled on for a while without a fever; or you have read about septic arthritis and would simply like the question settled. That is exactly what a specialist appointment is for. Dr Borukhson examines the joint and uses point-of-care ultrasound in the same visit to see what is happening inside it, and where taking fluid would answer the question, she can perform an ultrasound-guided joint aspiration there and then and send the sample to the laboratory to be tested for infection and for the crystals of gout and pseudogout. Usually that ends the matter with a clear diagnosis and the treatment to go with it. In the uncommon event that something points towards infection, she will arrange hospital care the same day, so you arrive with the answer already in hand rather than waiting for it. You can read more about the same-visit approach on the ultrasound clinic page.

Afterwards, it is picking up the threads: reviewing how the joint is recovering, adjusting arthritis treatment that may have been interrupted, and, where the tests found gout, pseudogout or a flare rather than infection, sorting out that diagnosis properly so the episode is less likely to repeat itself.

The one situation that skips the clinic is the feverish one: a hot, swollen joint when you also feel unwell goes to A&E or NHS 111 the same day, and the clinic picks things up afterwards.

Septic arthritis is rare, and most hot, swollen joints have a better explanation and a good outcome. The reason for promptness is the rare exception: treated early, an infected joint usually clears completely; treated late, it can be left permanently damaged. So the message of this guide is a calibrated one rather than an alarming one: a hot, swollen joint deserves an answer within days, not weeks, and the same assessment that rules infection out usually rules the real cause in, most often something a rheumatology clinic treats every week. With a fever or feeling unwell alongside it, make it the same day, through A&E or NHS 111.

Is a hot, swollen joint an emergency?

Usually not, and usually it is not an infection either: gout, pseudogout and flares of existing arthritis are far more common and can look very similar. The combination that does need emergency care is a hot, swollen joint together with a fever or feeling unwell, and that means A&E or NHS 111 the same day. Without those features the right move is still a prompt assessment rather than watching and waiting, partly because the rare infected joint is not something to sit on, and partly because whatever is causing the swelling deserves a diagnosis and treatment of its own.

How do I know whether it is septic arthritis, gout or a flare of my arthritis?

You cannot tell these apart safely at home, and doctors do not rely on looks alone either: the usual test is taking a small sample of fluid from the joint, which is examined for germs and for the crystals of gout and pseudogout. Doctors check a hot, swollen joint for infection rather than assuming it innocent, so wherever you are assessed, expect that question to be settled properly. Far more often than not the news is good and the cause turns out to be a crystal flare instead.

Can I see Dr Borukhson to rule out septic arthritis?

Yes, and it is a common reason to come. If a joint is swollen and sore but you feel well in yourself, or the symptoms are mixed and have grumbled on without a fever, a specialist assessment can settle the question: Dr Borukhson examines the joint, uses point-of-care ultrasound in the same visit to see what is happening inside it, and where taking fluid would help, she can do so under ultrasound guidance and send it to the laboratory to be tested for infection and for crystals. Usually that ends with a clear diagnosis and the right treatment started; in the uncommon event that something points to infection, she arranges hospital care the same day. The exception to all this is the feverish picture: a hot, swollen joint when you also feel unwell should go to A&E or NHS 111 the same day.

Do arthritis medicines make septic arthritis more likely?

Medicines that quieten the immune system, such as methotrexate, biologics and JAK inhibitors and steroids, do make infections somewhat more likely, and rheumatoid arthritis itself makes joints more vulnerable to infection. That is a reason for awareness, not alarm: septic arthritis remains uncommon, and the practical response is knowing the warning signs and acting on them the same day. Our guide to infections and arthritis medicines covers the sensible precautions.

Can a steroid injection give me septic arthritis?

It is possible but very uncommon. The skin is cleaned carefully before any joint injection or aspiration, and infection afterwards is rare. Even so, if a joint becomes increasingly red, hot, swollen or painful in the days after an injection, or you feel feverish or unwell, seek advice the same day rather than waiting for it to settle.

How is septic arthritis treated?

When a joint really is infected, treatment is given in hospital: usually antibiotics into a vein, and often draining fluid from the joint to clear the infection and ease the pain. A hospital stay of around two weeks is typical, followed by antibiotic tablets for several weeks at home, and it is important to finish the whole course even once you feel well. Most people start to feel better quickly once antibiotics begin, and make a full recovery.