Septic arthritis

Septic arthritis is an infection inside a joint, most often by bacteria that have entered through a wound or an infected cut, travelled in the bloodstream from an infection elsewhere in the body or, occasionally, got in during joint surgery. Bacterial or infectious arthritis are other names for it.

It is rare. Far more often a hot, swollen joint is gout, pseudogout or a flare of existing arthritis, and all three can look strikingly like an infection. What gives septic arthritis a guide of its own is what the rare real case can do: an infected joint can quickly be damaged for good, and an infection left untreated can spread into the bloodstream and cause blood poisoning (septicaemia). Because the rare infected joint is so urgent, doctors check every hot, swollen joint rather than watch and wait. The reassuring part 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 whole picture. Symptoms can build more gradually after a joint replacement, and there is not always a fever to begin with. The working rule: a joint that has become hot, swollen and unusually painful over a few days or less needs an answer the same day, whether or not there is a fever, and a fever or feeling unwell alongside it makes that a hospital matter; the next section sets out the routes for each. A joint that will not take your weight at all, or that you cannot move, is a 999 or A&E emergency in its own right, whatever the cause.

Two routes, and a simple rule for choosing between them, with the timing set by how the joint arrived.

Fever, shivering or feeling unwell in yourself alongside it means assessment the same day, either at A&E or by calling NHS 111. It is those features that turn an infected joint into an urgent one, never a thing to sleep on, and a joint that really is infected is treated in hospital. Make sure whoever sees you knows about every medicine you take, and above all about any that quieten the immune system. The signs of sepsis, the situation in which the body's own response to an infection starts to harm the body itself, are listed in the banner at the top of this page; those signs mean 999 or A&E, not NHS 111, as does a joint that takes no weight at all or cannot be moved.

If you feel well in yourself, the place to go changes, but an answer is still needed the same day. A joint that has become hot, swollen and painful suddenly, over a few days or less, needs that answer the same day, because the fever is not always there to begin with and the damage an infection does starts early: the right places to get it are an urgent GP appointment, NHS 111 or a specialist clinic that can see you that day, and pain that is getting worse rather than settling is a reason to press for that same-day slot. Symptoms that are mixed and have grumbled on for a while without a fever can wait a few days, though not weeks. A specialist clinic is as good a place as any to find the answer, and often the most direct, because what settles the question, an examination, ultrasound and, where needed, testing of fluid from the joint, is exactly what a rheumatologist's assessment consists of. It helps to know why: a hot, swollen, tender joint is checked for infection rather than assumed innocent, because although infection is unlikely, an infected joint that is missed does far more harm than a healthy one that was checked. Most assessments end in reassurance and a different diagnosis.

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, which can carry bacteria into the bloodstream, and some sexually transmitted infections, such as gonorrhoea, which can reach a joint the same way.

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.

That is worth knowing: the tests that could confirm septic arthritis are the same tests that rule it out, and far more often than not the answer is a crystal flare or a flare of existing arthritis rather than an infection, which is better news and the diagnosis that was needed. Why that is not a question to settle at home is explained in our guide to managing a flare.

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 or, for some joints, by washing the joint out in a keyhole or open operation, 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. Lips, mouth, throat or tongue that suddenly swell, a throat that feels tight, very fast breathing or a struggle to breathe or to swallow, skin, tongue or lips turning blue, grey or pale (on black or brown skin this may be easier to see on the palms of the hands or the soles of the feet), becoming suddenly very confused, drowsy or dizzy, or fainting and not being able to be woken is a 999 or A&E emergency, since this can be a severe allergic reaction, anaphylaxis, which can follow a dose of any medicine, the antibiotics among them. Call 999 and do not drive yourself to A&E.

A confirmed infection is dealt with in hospital, and hospital is the direct destination for the feverish, unwell picture. Everything around that, though, belongs largely to rheumatology. Ahead of any episode, for people living with inflammatory arthritis or taking medicines that quieten the immune system, the task is to make sure the warning signs are known and the threshold for acting on them is low.

Most of the territory, in fact, is the middle ground, which is also where the question most often arises. It may be a swollen, sore joint in someone who feels perfectly well; a mix of symptoms that has grumbled along for a while with no fever; or simply something you have read about septic arthritis that you would like put to rest. A specialist appointment exists for exactly this. Point-of-care ultrasound is part of the consultation with Dr Borukhson, so as she examines the joint she can also see what is going on inside it; where a fluid sample would decide the matter, she can carry out 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. For most visits that is the end of it: a clear diagnosis, and the treatment it calls for. Now and then something points to infection; when it does, hospital care is arranged that same day, with the diagnosis already made. The same-visit approach is described 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.

Two pictures go past the clinic: a hot, swollen joint in someone who also feels unwell is assessed the same day by the hospital route given earlier, and a joint that will take no weight at all, or that cannot be moved, means 999 or A&E, with the clinic resuming its part afterwards in either case.

Septic arthritis is rare. For most hot, swollen joints the explanation is kinder and the outcome good. Speed matters for the sake of the rare exception, because an infected joint usually clears completely if it is found early and can be permanently damaged if it is found late. So the message is a measured one: a joint that has become hot and swollen suddenly deserves an answer the same day, from an urgent GP appointment, NHS 111 or a specialist clinic that can see you that day; one that has grumbled on deserves it within days, from a GP or a specialist clinic, not weeks; and the assessment that clears infection usually names the real cause, most often something a rheumatology clinic sees every week. Add a fever or feeling unwell, and hospital is where that answer is due: the same day, either at A&E or by calling NHS 111, and the signs of sepsis in the banner at the top of this page are a 999 or A&E emergency. A joint that will take no weight at all, or that cannot be moved, is a 999 or A&E emergency, whether or not there is a fever.

Is A&E the right place for a hot, swollen joint?

Usually not, and usually it is not infected either. Gout, pseudogout and a flare of existing arthritis are far likelier, and any of them can look much the same. A&E is for two pictures. A joint that cannot take any weight, or cannot be moved, is a 999 or A&E emergency whether or not there is a fever. The signs of sepsis, listed in the banner at the top of this page, mean 999 or A&E too, not NHS 111. Call 999 and do not drive yourself to A&E. A hot, swollen joint in someone who also has a fever or feels unwell is assessed the same day, either at A&E or by calling NHS 111. With neither picture and no sign of sepsis, the answer is still due the same day if the joint came up suddenly, from an urgent GP appointment, NHS 111 or a specialist clinic that can see you that day, and it should not be left to see what happens: the rare infected joint will not keep, and whatever is behind the swelling deserves its own diagnosis and treatment.

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

Not safely at home, and doctors do not go by looks alone either: the usual test is a small sample of fluid drawn from the joint, examined for germs and for the crystals of gout and pseudogout. No hot, swollen joint is assumed innocent; it is checked for infection, and wherever you are seen you can expect that check to be done properly. Far more often than not the news is good, and a crystal flare or a flare of existing arthritis turns out to be the cause.

Can I see Dr Borukhson to rule out septic arthritis?

Yes, and people often come for exactly that. Where there is no fever, you feel well in yourself and the joint still takes weight and moves, a specialist assessment is the place for the question, and it runs like this: point-of-care ultrasound is part of the consultation with Dr Borukhson, so while she examines the joint she can also look inside it, and, where a fluid sample would help, take one under ultrasound guidance and send it to the laboratory to be tested for infection and for crystals. Most people leave with a diagnosis made and treatment under way; in the uncommon case where infection is suspected, she gets hospital care organised for that same day. Timing follows how the joint behaved: one that came up suddenly needs its answer the same day, so if the clinic cannot see you that day, an urgent GP appointment or NHS 111 takes its place, while symptoms that are mixed and have dragged on can wait a few days, not weeks. The feverish picture is different: a hot, swollen joint in someone who also feels unwell needs assessment the same day, either at A&E or by calling NHS 111, and the signs of sepsis in the banner at the top of this page are a 999 or A&E emergency.

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. The risk is still small: septic arthritis remains uncommon, and the sensible response is to know the warning signs and act on them the same day, by the routes the banner at the top of this page sets out. 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, get it seen the same day rather than waiting for it to settle. With a fever or feeling unwell that means A&E or NHS 111, and the signs of sepsis in the banner at the top of this page are a 999 or A&E emergency; a joint that will take no weight at all, or that cannot be moved, is a 999 or A&E emergency, whether or not there is a fever; and with neither, if the joint still takes weight and moves, it means an urgent GP appointment, NHS 111 or a clinic that can see you that day.

How is septic arthritis treated?

Treatment for a genuinely infected joint happens in hospital: antibiotics, usually into a vein, and often the fluid drained from the joint to help clear the infection and ease the pain. A stay of around two weeks is typical, then antibiotic tablets for several weeks at home, and it is important to finish the whole course even once you feel well. Improvement usually starts soon after the antibiotics do, and most people recover fully.