Methods of Core Temperature Measurement
Gold Standard: Central Temperature Monitoring
Central temperature monitoring methods—including pulmonary artery catheter thermistors, bladder catheter thermistors, and esophageal thermistors—are the gold standard for measuring core body temperature and should be used when these devices are already in place or when accurate temperature measurement is critical to diagnosis and management. 1
Hierarchy of Central Methods (Most to Least Accurate):
Pulmonary artery catheter thermistors provide the reference standard against which all other methods are compared, with excellent accuracy (bias of only -0.15°C with precision of ±0.13°C) 1, 2
Bladder catheter thermistors show essentially identical readings to pulmonary artery thermistors (bias of only -0.04°C), provide continuous monitoring, and maintain stable measurements regardless of urine flow rate 1, 3, 4
Esophageal thermistors (placed in distal third of esophagus) provide readings comparable to intravascular sites and bladder catheters, with clinically acceptable limits of agreement 1
Acceptable Alternatives When Central Monitoring Unavailable
For patients without central monitoring devices, use oral or rectal temperatures as they are significantly more reliable than peripheral methods, though both have important limitations. 1
Oral Temperature:
- Safe and convenient for alert, cooperative patients 1
- Shows bias of only -0.15°C compared to pulmonary artery core temperature, making it the most accurate peripheral method 5
- Critical limitation: Distorted by mouth breathing, hot or cold fluids/gases near the mouth, and impractical in intubated or uncooperative patients 1
Rectal Temperature:
- Traditionally used but reads a few tenths of a degree higher than core temperature and is not predictably consistent 1
- Major drawbacks: Perceived as unpleasant/intrusive, access limited by patient position, small risk of trauma/perforation (particularly problematic in neutropenic, coagulopathic patients, or those with recent rectal surgery), and potential for spreading enteric pathogens like C. difficile 1
- Often impractical in ICU settings despite being more accurate than peripheral methods 1
Unreliable Methods to Avoid
Do not rely on tympanic membrane infrared thermometers, temporal artery thermometers, axillary measurements, or chemical dot thermometers for clinical decision-making, as these methods show unacceptable variability and can miss fever or hypothermia by 1-2 degrees. 1, 6
Why These Methods Fail:
Tympanic membrane infrared thermometers: Show consistently poor agreement with pulmonary artery/esophageal thermistors (bias of -0.38°C with wide variability), inaccurate with auditory canal inflammation/blockage, and require perfect operator technique to engage the tympanic membrane 1, 4
Temporal artery (no-touch) thermometers: Unreliable estimates influenced by environmental temperature and sweating, with 25% of measurements showing clinically significant differences (>0.9°F) from core temperature 1, 6, 7
Axillary measurements: Consistently underestimate core temperature by 1.5-1.9°C with variability up to almost 1°C 6
Chemical dot thermometers: Show inconsistent agreement with pulmonary artery catheter thermistors and should not be used in critically ill patients 1
Clinical Decision Algorithm
Step 1: Assess if central monitoring is present or needed
- If pulmonary artery catheter, bladder catheter with thermistor, or esophageal probe already in place → use these exclusively 1
- If accurate temperature is critical for diagnosis/management (suspected sepsis, heat-related illness, therapeutic hypothermia) → consider placing bladder thermistor catheter 1, 6
Step 2: If no central monitoring available
- For alert, cooperative, non-intubated patients → use oral temperature (most accurate peripheral method with bias of only -0.15°C) 5
- Ensure patient has not consumed hot/cold fluids for 15-30 minutes and can maintain mouth closure 1
Step 3: If oral measurement not feasible
- Consider rectal temperature despite practical limitations, recognizing it reads slightly higher than core and has infection control concerns 1
Step 4: If only unreliable methods available
- Do not base clinical decisions on the temperature value itself—any reading from tympanic, temporal artery, or axillary methods is unreliable 6
- Instead, rely on other clinical signs of infection (hemodynamic instability, altered mental status, leukocytosis, elevated lactate) rather than the temperature measurement 6
Critical Pitfalls to Avoid
Missing life-threatening infections: Temperature discrepancies of 1-2 degrees can lead to missed diagnoses of fever or hypothermia, which has mortality implications since patients with serious infections may be euthermic or hypothermic 6
Over-reliance on convenient but inaccurate methods: The ease of no-touch infrared thermometers does not justify their use when they can miss critical temperature abnormalities 6
Assuming all thermistors are equal: Even pulmonary artery catheter thermistors vary in technical performance and should be calibrated according to manufacturer specifications 1