Fever (high temperature) in adults: causes, red flags and when to seek help

The NHS explains that normal body temperature varies between people and through the day. A temperature of 38C or above is usually considered a high temperature. But the NHS points out that you may have one without a thermometer: your chest or back may feel hotter than usual, you may feel hot or cold, shivery or sweaty, and your skin may be warm and red, which can be harder to see on black or brown skin.

You do not need a thermometer, but if you use one the NHS gives the method. Clean the tip of a digital thermometer with cold water and soap, put it under the tongue towards the back of the mouth with the lips closed, and wait for the beep. An armpit reading is possible with the arm held tight against the body. The NHS says not to use glass thermometers because they can be dangerous, infrared thermometers because some do not meet UK regulations, or forehead strips because they are not accurate.

This page is about adults. For children and babies the NHS has separate guidance, and the thresholds and warning signs differ.

The NHS says many things can cause a high temperature and that it is often the body fighting an infection. The table sets out the usual suspects, with the features that tend to go with each.

The NHS says for sepsis, in an adult, to call 999 or go to A&E if the person is breathing very fast, is confused, has slurred speech or is not making sense, has blue, pale or blotchy skin, lips or tongue, has a very high or very low temperature or feels hot or cold to the touch or shivery, has a rash that does not fade when pressed, or has symptoms you fear might be sepsis. It says to trust your instincts.

For meningitis, the NHS lists a high temperature with very cold hands and feet, a stiff neck, joint and muscle pain, a severe and worsening headache, eye pain in bright light, seizures, vomiting, confusion, being very sleepy or difficult to wake, and spots or a rash that does not fade when pressed with a glass. It says not everyone gets the rash.

A fever with urinary symptoms needs particular care if you are 65 or over, are a man, have diabetes, are pregnant, use a catheter, have a weakened immune system or have pain under the ribs at the back, because the NHS says this may be a kidney infection, which can be serious and cause sepsis. It also asks for an urgent GP appointment or 111 if symptoms worsen quickly or do not improve within 48 hours.

• Confusion, slurred speech or unusual sleepiness.

• Very fast breathing or difficulty breathing.

• A rash that does not fade when a glass is pressed against it.

• Severe headache with a stiff neck or dislike of light.

• Blue, grey, pale or mottled skin, lips or tongue.

• Passing very little or no urine.

• A fever that is not improving, or is getting worse, despite home treatment.

The clinician asks how long the fever has lasted, what other symptoms are present (cough, urinary symptoms, rash, headache, diarrhoea, pain), recent travel, contact with infection, medicines, long-term conditions that weaken the immune system, and whether the person has recently had surgery or has a catheter or other device. Examination includes temperature, pulse, breathing rate, blood pressure, hydration, throat, chest, abdomen, skin and neck.

The NHS UTI page says a GP may do a urine test, though it is not always needed. For chest infections it says a blood test or a sample of mucus may be needed to find what is causing the infection. If sepsis is suspected, hospital treatment begins quickly: the NHS says it is treated in hospital with antibiotics, oxygen and fluids into a vein.

The NHS advice is to get plenty of rest, drink plenty of fluids, with water best, enough so that urine is light yellow and clear, and take paracetamol or ibuprofen if uncomfortable. It suggests staying at home and avoiding other people until the temperature has gone.

Dehydration is more likely with a fever, particularly in older adults and anyone with diabetes. The NHS dehydration page says to start with small sips if feeling sick, to avoid caffeine and alcohol, and that a pharmacist can suggest oral rehydration solutions if you have been sick or have diarrhoea.

Most fevers do not need a doctor. The NHS says to ask for an urgent GP appointment or call 111 if a high temperature is not getting better or is getting worse. Anyone with warning signs listed above should go to A&E or call 999 rather than wait for an appointment.

People who are frail, over 65, pregnant, have diabetes or have weakened immunity are better to seek advice earlier, as the NHS lists these groups among those who need urgent advice for urinary infections and chest infections.