All that is needed to make a diagnosis of hypothermia is a low temperature recorded on an accurate thermometer.

The diagnosis can be easily missed by not obtaining a full set of vital signs on the patient, or by being misled by an inaccurate tympanic or oral thermometer. If hypothermia is suspected, a core temperature should be performed to confirm the diagnosis.

The diagnosis of hypothermia may also be suspected by Osborn J waves on ECG. The upward deflection of the terminal S wave (at the junction of the QRS and the ST segment) occurs at or near 32°C. It is first seen in leads II and V6.