Temperature Measurement

A rectal temp of 38.0°C and an oral temp of 38.0°C don't mean the same thing clinically — choosing the wrong route can mask a fever or trigger unnecessary interventions.

Body temperature reflects the balance between heat production and heat loss, with the normal oral range of 36.0–37.5°C (96.8–99.5°F). The route changes the number: rectal reads approximately 0.5–1.0°F (0.3–0.6°C) higher than oral, while axillary reads about 0.5–1.0°F lower than oral. Tympanic closely approximates core temperature when the probe is aimed at the tympanic membrane, not the ear canal wall. Temporal artery is non-invasive and common in pediatrics but can be inaccurate with diaphoresis. Oral measurement requires the thermometer under the posterior sublingual pocket (not the front of the tongue), with the mouth closed for the full reading time. Wait 15–30 minutes after the patient eats, drinks, or smokes before taking an oral temp. Rectal is the most accurate route but is contraindicated in neutropenic patients, those with rectal surgery, significant hemorrhoids or active rectal pathology, or bleeding tendencies, and neonates in some facilities. For infants under 3 months, a rectal temp ≥38.0°C (100.4°F) is a medical emergency requiring immediate notification. Fever (pyrexia) is generally defined as a core temperature ≥38.0°C (100.4°F) oral; hyperthermia above 40.0°C (104°F) signals a dangerous heat-regulation failure.

Key Distinctions

Don't confuse fever (pyrexia, thermostat reset by pyrogens, responds to antipyretics) with hyperthermia (thermoregulatory failure, does NOT respond to antipyretics — requires active cooling). Students often assume axillary and oral temps are interchangeable — axillary reads lower and is the least accurate route in adults. A tympanic reading on the side where the patient has been lying will read falsely high; use the opposite ear or wait several minutes.

Clinical Pearl

Route changes the number: think RAO — Rectal reads Above Oral, Axillary reads below Oral. Always document the route with the value, or the number is meaningless.