When to call

Call 999 or go to A&E now if your child:

has a rash that does not fade when you press a glass against it
has a stiff neck, or is bothered by light
is drowsy and hard to wake, or is not responding like they normally do
is extremely agitated, does not stop crying, or is confused
has difficulty breathing — grunting noises, the stomach sucking in under the ribcage, breathlessness or very fast breathing
has changes to their skin colour: blue, grey, paler than usual or blotchy skin, or blue, grey or paler than usual lips or tongue
has unusually cold hands and feet
has a fit (febrile seizure) for the first time

That list is the NHS's, and NICE's guideline on fever in under-5s (NG143) contains the same features in its high-risk column. On brown and black skin, colour changes may be easier to see on the soles of the feet, palms, lips, tongue and inside the eyelids. Do not drive to A&E yourself; ask someone else to drive, or call an ambulance.

Call 111 or your GP surgery now if your child:

is under 3 months old and has a temperature of 38C (100.4F) or higher, or you think they have a high temperature
is 3 to 6 months old and has a temperature of 39C (102.2F) or higher
has other signs of illness, such as a rash, as well as a high temperature
has a high temperature that has lasted for 5 days or more
does not want to eat, or is not their usual self and you are worried
is dehydrated: nappies that are not very wet, sunken eyes, no tears when crying

Outside the UK and Ireland, use your local urgent-care line and emergency number (911, 000 or 111).

Why a small baby with fever is a different problem

This is the reason fever advice cannot be written as a single rule.

NICE recommends: "recognise that children younger than 3 months with a temperature of 38C or higher are in a high-risk group for serious illness", and separately that "children aged 3 to 6 months with a temperature of 39C or higher are in at least an intermediate-risk group." A fever in a newborn is treated as a red flag on the number alone, regardless of how content the baby looks.

For an older baby or toddler, NICE says the opposite: "in children older than 6 months do not use height of body temperature alone to identify those with serious illness." A well-looking eighteen-month-old at 39.5C (103.1F) is a less worrying picture than a listless one at 38.2C (100.8F). NICE also says not to use duration to predict serious illness, except that a fever lasting five days or longer should prompt assessment for Kawasaki disease.

The NICE traffic light system

NG143's table 2 is what a clinician runs through when they look at your child, and it is worth knowing because it tells you what to describe on the phone. Any red feature means high risk; any amber with no red means intermediate; only green features means low risk.

Colour

Green: normal colour. Amber: pallor reported by parent or carer. Red: pale, mottled, ashen or blue.

Activity

Green: responds normally to social cues; content or smiles; stays awake or awakens quickly; strong normal cry or not crying. Amber: not responding normally to social cues; no smile; wakes only with prolonged stimulation; decreased activity. Red: no response to social cues; appears ill to a healthcare professional; does not wake, or if roused does not stay awake; weak, high-pitched or continuous cry.

NICE defines social cues as a child's response to social interaction, "such as response to their name, smiling and/or giggling". Whether your feverish toddler still turns when you say their name is more informative than the thermometer.

Respiratory

Amber: nasal flaring; a respiratory rate over 50 breaths per minute at 6 to 12 months or over 40 at more than 12 months; oxygen saturation of 95% or less in air; crackles in the chest. Red: grunting; a respiratory rate of more than 60 breaths per minute; moderate or severe chest indrawing.

Circulation and hydration

Green: normal skin and eyes; moist mucous membranes. Amber: tachycardia — over 160 beats per minute under 12 months, over 150 at 12 to 24 months, over 140 at 2 to 5 years; capillary refill time of 3 seconds or more; dry mucous membranes; poor feeding in infants; reduced urine output. Red: reduced skin turgor.

Other

Amber: age 3 to 6 months with a temperature of 39C (102.2F) or more; fever for 5 days or more; rigors; swelling of a limb or joint; a non-weight-bearing limb or not using an extremity. Red: age under 3 months with a temperature of 38C (100.4F) or more; a non-blanching rash; a bulging fontanelle; neck stiffness; status epilepticus; focal neurological signs; focal seizures.

NICE notes that some vaccinations can induce fever in children under 3 months.

Taking the temperature

NICE says not to use the oral or rectal route routinely in children aged 0 to 5. Under 4 weeks, use an electronic thermometer in the armpit; from 4 weeks to 5 years, an electronic or chemical dot thermometer in the armpit, or an infra-red tympanic thermometer. Forehead chemical thermometers "are unreliable and should not be used".

The NHS's method: place the thermometer in the top of the armpit, close the arm over it, and leave it as long as the instructions say. If your child has just had a bath or been wrapped up, wait a few minutes and try again. A normal temperature varies slightly between children; the HSE gives the normal range as 36 to 37.4C (96.8 to 99.3F).

What to do at home

NICE is unusually direct about what does not help. "Tepid sponging is not recommended for the treatment of fever." "Children with fever should not be underdressed or over-wrapped." And, critically: "antipyretic agents do not prevent febrile convulsions and should not be used specifically for this purpose."

On medicines, NICE says to "consider using either paracetamol or ibuprofen in children with fever who appear distressed", and to "not use antipyretic agents with the sole aim of reducing body temperature." When you do use them: continue only as long as the child appears distressed; consider switching to the other agent if the distress is not relieved; "do not give both agents simultaneously"; and only consider alternating them if distress persists or recurs before the next dose is due.

The NHS's practical cautions: no aspirin under 16; no paracetamol under 2 months; no ibuprofen for a child under 3 months, under 5kg (about 11lb), with chickenpox, or dehydrated; and no ibuprofen for children with asthma unless a doctor has recommended it.

Otherwise: offer fluids regularly (breast milk if breastfed), offer food if they want it, check on them during the night, and keep them off nursery while the fever persists but tell the nursery about it. NICE's dehydration signs for parents are a sunken fontanelle, dry mouth, sunken eyes, absence of tears and poor overall appearance.

Two things that mislead people

First, response to medicine. NICE: "when a child has been given antipyretics, do not rely on a decrease or lack of decrease in temperature to differentiate between serious and non-serious illness." A temperature that comes down with paracetamol does not rule anything out.

Second, the absence of a temperature. The NHS's rash guidance warns that a child "might not have a high temperature but still be seriously unwell."

Finally, febrile seizures. The NHS says they usually last 2 to 3 minutes, rarely more than 10, and affect children between 6 months and 6 years. A first seizure always needs emergency assessment. During one, protect their head, note the start and end time, do not restrain them or put anything in their mouth, and use the recovery position afterwards.

Get help again if things change

NICE says to seek further advice if the child has a fit, develops a non-blanching rash, seems less well than when you last sought advice, if the fever lasts five days or longer, or simply if you "are more worried than when they previously sought advice". It also tells professionals that "reported parental perception of a fever should be considered valid and taken seriously." Being more worried than you were an hour ago is, in the guideline itself, a reason to ring back.

Toddler Fever: What Is Normal and When to Worry