Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find the Fever article more useful, or one of our other health articles.
Treatment of almost all medical conditions has been affected by the COVID-19 pandemic. NICE has issued rapid update guidelines in relation to many of these. This guidance is changing frequently. Please visit https://www.nice.org.uk/covid-19 to see if there is temporary guidance issued by NICE in relation to the management of this condition, which may vary from the information given below.
See also the separate Ill and Feverish Child article.
What is a fever?
Fever is a common sign that on its own is usually little help in making a diagnosis. Persistent high fever needs urgent treatment. Fever over 42.2°C (108°F) produces unconsciousness and leads to permanent brain damage if sustained.
Body temperature fluctuates slightly throughout the day. Normal temperature varies depending on the person, the body site where temperature is measured, and the time of day. A body temperature of 38°C or higher is usually considered as a fever. However, a temperature over 37.5°C can often be considered to be a fever in children.
The American College of Critical Care Medicine, the International Statistical Classification of Diseases, and the Infectious Diseases Society of America define fever as a core temperature of 38.3°C or higher, just above the upper limit of a normal human temperature, irrespective of the cause.
Fever may also be described as:
- Remitting - the most common type with daily temperatures fluctuating above the normal range.
- Intermittent - daily temperature drops into the normal range and then rises back above normal. If temperature fluctuates widely causing chills and sweating, it is called a hectic fever.
- Sustained - persistent raised temperature with little fluctuation.
- Relapsing - alternating feverish and afebrile periods.
- Undulant - gradual increase in temperature, which stays high for a few days then gradually reduces.
Fever may also be described in terms of its duration; brief (<3 weeks), or prolonged. The term pyrexia of unknown origin (PUO) is used to describe a condition where no underlying cause can be found.
Night fever or night sweats are common and there is a long list of possible causes, mostly benign but important to diagnose in order to manage effectively. Serious causes of night sweats can usually be excluded by a thorough history, examination and simple investigations if required.
Causes of fever
Most cases of fever are due to self-limiting viral infections, especially upper respiratory tract infections and childhood exanthemas.
Urinary tract infections are also common but more severe infections (eg, meningitis, pneumonia, osteomyelitis, septic arthritis, AIDS) should always be considered. Recent foreign travel should prompt consideration of referral to secondary care for full investigations for tropical infections - eg, malaria.
Non-infection causes of fever include connective tissue disorders, allergy, thyrotoxicosis and malignancy. Disorders of thermoregulation: temperature can suddenly rise up to as high as 41.7°C (107°F) in a life-threatening condition such as heatstroke, thyroid storm, neuroleptic malignant syndrome and malignant hyperthermia and in certain disorders of the central nervous system.
Causes of fever include
- Infection: bacterial, viral or fungal. The origin of infection may be obvious or may require careful assessment for diagnosis - eg, infective endocarditis, tuberculosis or other occult long-term infection.
- Inflammatory: phlebitis, thyroiditis, ulcerative colitis, Crohn's disease, pancreatitis, familial Mediterranean fever, sarcoidosis, pemphigus, severe or exfoliative dermatitis, bullous pemphigoid.
- Connective tissue disorders: rheumatoid arthritis, systemic lupus erythematosus, polyarteritis nodosa, polymyositis, dermatomyositis, giant cell arteritis.
- Malignancy: carcinoma (especially lung cancer, renal cancer), sarcomas, lymphoma.
- Endocrine and metabolic disorders: thyroid disease, gout, porphyria.
- Tissue destruction - eg, surgery, infarction, haemolysis, crush syndrome, rhabdomyolysis.
- Thromboembolic causes, including pulmonary embolism, deep vein thrombosis.
- Blood disorders: aplastic anaemia, agranulocytosis, leukaemias.
- Allergic conditions: allergic reactions, transfusion reactions, Henoch-Schönlein purpura,
- Other conditions causing pyrexia: these include cirrhosis, cerebral haemorrhage, Kawasaki disease.
- Drug reactions: sulfonamides, other antibiotics, barbiturates.
- Factitious pyrexia.
Prolonged fever is used to describe a raised temperature persisting for at least three weeks (persistent, remittent or intermittent). Possible causes of a prolonged fever include:
- Infection - eg, infectious mononucleosis, abscess, chronic pyelonephritis, malaria, Lyme disease, tuberculosis, syphilis, actinomycosis, AIDS, infective endocarditis.
- Prolonged infection associated with underlying disorders - eg, immunodeficiency, bronchiectasis, cystic fibrosis.
- Malignancy - eg, lung cancer, lymphoma, leukaemia.
- Rheumatoid arthritis, systemic lupus erythematosus, polyarteritis nodosa, dermatomyositis, giant cell arteritis, vasculitis.
- Inflammatory bowel disease - Crohn's disease, ulcerative colitis.
- Cirrhosis, aplastic anaemia, agranulocytosis, haemolytic anaemia.
- Drug reactions.
What are night sweats?
- Night sweats are usually defined as episodes of significant night-time sweating that soak the bedclothes or bedding. This is a fairly common symptom.
- Although uncomfortable, night-time sweating typically isn't a sign of a serious underlying medical condition. It may be triggered by something as simple as too warm a room or too many blankets on the bed.
Causes of night sweats
Conditions often associated with night sweats include menopause, mood disorders, gastro-oesophageal reflux disease, hyperthyroidism, and obesity.
- Any cause of fever.
- Diabetes insipidus.
- Nocturnal hypoglycaemia (in people with diabetes)
- Medications - eg, over-the-counter antipyretics, antihypertensives, antipsychotics (can cause rebound temperature symptoms).
- Drug misuse or alcohol misuse.
- Obstructive sleep apnoea.
- Gastro-oesophageal reflux disease.
- Night terrors.
- Serotonin syndrome.
Assessment of night sweats
- Immediate assessment includes measurement of temperature, assessment of the likely underlying cause, well-being of the patient and signs of dehydration.
- It is important to know the complete medical history, including immunosuppressive treatments or disorders, infection, trauma, surgery, any medication.
- Recent travel may suggest more exotic causes of fevers.
- Infrared ear thermometers or thermometers placed in the axilla should be used.
- Oral measurements are affected by mouth breathing, liquids and respiratory rate.
- Forehead chemical thermometers are unreliable and should not be used by healthcare professionals.
- There are diurnal, menstrual and exercise-induced variations in normal body temperature.
Investigations are often unnecessary in primary care when the cause of an infection is clear from the history and examination. Possible fever investigations may include:
- FBC: raised white cell count in infection, inflammatory conditions and malignancy; tests for infectious mononucleosis.
- Erythrocyte sedimentation rate (ESR), C-reactive protein (CRP): nonspecific and again raised in a wide range of conditions, including infection, inflammation and malignancy.
- Urinalysis: may provide clear evidence of a urinary tract infection.
- Cultures: rarely indicated in primary care apart from sending an MSU.
The patient admitted to hospital will often require a much more extensive list of investigations when exploring the underlying cause of fever, including:
- Full infection screen, including lumbar puncture for cerebrospinal fluid and also stool and blood cultures.
- Renal function tests, electrolytes, LFTs and blood gases may also be required.
- Autoimmune antibody screen may be required when considering a possible underlying connective tissue disorder.
- Tuberculin test for possible tuberculosis.
- CXR may be indicated to identify pneumonia, tuberculosis or malignancy.
- Further investigation for infection - eg, syphilis, HIV, malaria and other tropical diseases.
- Further radiology - eg, ultrasound, isotope scans, CT or MRI, depending on specific presentation of the patient.
Fever treatment and management
- The most important aspect of management is the identification and appropriate management of the underlying cause. However, in the case of self-limiting viral infections, the only management required is advice and reassurance.
- Do not prescribe oral antibiotics to a child with fever without apparent source - see Further Reading, below.
- If meningococcal disease is suspected, give parenteral antibiotics at the earliest opportunity (either benzylpenicillin or a third-generation cephalosporin) - see Further Reading, below.
Immediate hospital treatment of a child with a very high fever
- Children with shock: give immediate intravenous fluid bolus of 0.9% sodium chloride (20 ml/kg). Give further boluses as necessary.
- Give oxygen if there are signs of shock, oxygen saturation of less than 92%, or as clinically indicated.
Simple explanations for patients and their relatives
- Drink lots of fluid.
- Do not wear too many clothes (do not overdress or underdress) or use too many blankets.
- Keep the room at a comfortable temperature but make sure that fresh air is circulating (use a fan if available).
- A damp vest and a fan can be effective in lowering temperature.
- Don't wipe the sweat off immediately as this helps to cool the body.
- Cool baths and tepid sponging are not recommended.
- There is evidence that host defence mechanisms are enhanced by a raised temperature.
- Fever is the normal response of the body to infection and unless the child becomes distressed or symptomatic, fever alone should not be routinely treated.
- Antipyretics (eg, paracetamol and ibuprofen) should therefore not be used routinely but can be of value, especially for patients with systemic disease (particularly heart failure or respiratory failure) and when fever causes acute confusion.
- Consider either paracetamol or ibuprofen as an option if a child appears distressed or is unwell.
- Do not administer paracetamol and ibuprofen at the same time but consider using the alternative agent if there is insufficient response to the first drug - see Further Reading, below.
- There is some evidence that both alternating and combined antipyretic therapy may be more effective at reducing temperatures than monotherapy alone. However, there is insufficient evidence to know which of combined or alternating therapy might be more beneficial.
- Antipyretic agents do not prevent febrile convulsions in young children and should not be used specifically for this purpose - see Further Reading, below.
Further reading and references
Fever in under 5s: assessment and initial management; NICE Guidance (last updated November 2021)
Feverish child - risk assessment; NICE CKS, July 2020 (UK access only).
Feverish child - management; NICE CKS, December 2018 (UK access only)
Attard L, Tadolini M, De Rose DU, et al; Overview of fever of unknown origin in adult and paediatric patients. Clin Exp Rheumatol. 2018 Jan-Feb36 Suppl 110(1):10-24. Epub 2018 May 3.
Beresford RW, Gosbell IB; Pyrexia of unknown origin: causes, investigation and management. Intern Med J. 2016 Sep46(9):1011-6. doi: 10.1111/imj.13180.
Bryce C; Persistent Night Sweats: Diagnostic Evaluation. Am Fam Physician. 2020 Oct 1102(7):427-433.
Moriarty C, Carroll W; Ibuprofen in paediatrics: pharmacology, prescribing and controversies. Arch Dis Child Educ Pract Ed. 2016 Jul 25. pii: edpract-2014-307288. doi: 10.1136/archdischild-2014-307288.