Every route measures a different thing, so the route belongs in the chart beside the number. Pick the right route for each patient, convert a reading to its oral equivalent and classify it, then decide whether what you are looking at is a fever or a hyperthermia — because only one of the two answers to an antipyretic.
Which route, for this patient?
Choose a patient, then choose a route. Every route will answer — some of them will tell you why they are the wrong answer.
Convert it, then classify it
Routes are not interchangeable and the conversions between them are approximations. Enter what you measured and how you measured it.
Unit
Route offsets, from the chapter. Rectal runs about 0.5 °C (0.9 °F) higher than oral. Axillary runs about 0.5 °C (0.9 °F) lower than oral. Tympanic sits close to rectal, roughly 0.3–0.6 °C above oral. Temporal artery runs about 0.4 °C above oral. These are approximations, which is exactly why you compare like with like across a shift instead of converting.
Fever, or hyperthermia?
In fever, pyrogens raise the hypothalamic set point and the body works to reach the new, higher target. In hyperthermia the set point is normal and heat gain simply overwhelms heat loss. The distinction decides the treatment.
Route notes
Pick a patient, then press a route to see whether it is the best choice, an acceptable one, or contraindicated — and why.
Oral · 36.5–37.5 °C (97.7–99.5 °F)
Probe in the posterior sublingual pocket, lips closed, mouth breathing avoided. Wait 15 to 30 minutes after hot or cold intake, smoking or chewing gum. Avoid in confusion, seizure risk, inability to follow directions, oral surgery or trauma, and in children under about 4 years.
Rectal · 0.5 °C above oral, 37.0–38.0 °C
Lubricated probe, side-lying, insert 3.5 cm (1.5 in) in an adult or 1.5–2.5 cm (0.5–1 in) in an infant, held in place the entire time. Avoid in neutropenia, thrombocytopenia or anticoagulation, rectal or perineal surgery, diarrhoea, imperforate anus, spinal cord injury with autonomic dysreflexia risk, and cardiac patients (vagal stimulation).
Tympanic · close to rectal, about 0.3–0.6 °C above oral
Pull the pinna up and back in adults and children over 3, down and back under 3; aim at the tympanic membrane and use a clean cover. Avoid with otitis, ear drainage, impacted cerumen, ear surgery or a hearing aid in place.
Temporal artery · about 0.4 °C above oral
Slide across the forehead to the hairline, then behind the ear lobe if the forehead is diaphoretic. Avoid with diaphoresis, a forehead scar or dressing, or a recent cold pack.
Axillary · 0.5 °C below oral, 36.0–37.0 °C
Probe deep in a dry axilla, arm held firmly against the chest for the full dwell time. Avoid any time accuracy matters — it is the least reliable route, for use only when the others are contraindicated.
Pearl. A patient on oxygen by nasal cannula may still have an accurate oral temperature. A patient breathing through the mouth may not — the sublingual pocket cools with every breath and the reading drifts low.
Infants under three months. Rectal is the reference standard and the number the decision rules are built on. A rectal 38.0 °C (100.4 °F) or higher in an infant under 3 months means emergency evaluation and a full sepsis workup — the infant cannot localise infection, so there is no reassuring examination to fall back on.
Interpretation
—oral equivalentEnter a value
Enter a temperature, its route and the patient, then press Interpret. The default is a genuine 38.6 °C oral — change the route and watch what the same number becomes.
Convert by hand
°C to °F: multiply by 1.8 and add 32. °F to °C: subtract 32 and divide by 1.8. Adult fever is 38.0 °C = 100.4 °F; hypothermia is below 35.0 °C = 95.0 °F.
Normal temperature moves on a clock. It is lowest between 0400 and 0600 and highest between 1600 and 1800, swinging 0.5 to 1.0 °C across the day. It rises 0.3 to 0.6 °C after ovulation, and with exercise, heavy clothing and a hot environment. If you are comparing a morning temperature with an evening one, part of the difference is the time of day.
Set point, or heat load?
Choose a vignette, then decide what you are looking at.
Hyperthermia does not respond to antipyretics. There is no raised set point to lower — paracetamol and ibuprofen work by resetting the hypothalamus, and in heat stroke, malignant hyperthermia, neuroleptic malignant syndrome and anticholinergic toxicity the hypothalamus is already set correctly. It requires active cooling and removal of the cause. A temperature above 40 °C (104 °F) with altered mental status and hot dry skin is heat stroke until proven otherwise, and it is an emergency.
The three phases of a fever.Chill phase — the set point has risen, so the patient vasoconstricts, feels cold and shivers while the temperature climbs; this is the phase in which students pile on blankets that the body is already fighting. Plateau — the new set point is reached; the patient is hot and vasodilated. Defervescence — the set point resets downward and the patient diaphoreses and vasodilates as the fever breaks.
The plateau flush is not a colour finding on every patient. On lighter skin, vasodilation reads as redness. On brown and black skin it is far easier to feel as warmth with the dorsum of your hand and to see in the conjunctivae and oral mucosa than to read as redness in the face. Do not let “she isn’t flushed” stand in for a measured temperature, and do not let it stand in for a pulse and a respiratory rate either.