Objective:Core temperature measurement using a pulmonary artery (PA) catheter is considered the gold standard for measuring temperatures in critically ill patients. The objective of this study was to compare oral and tympanic temperature measurements (in both the oral and core equivalence modes) against PA core temperature measurements to determine which method was the most accurate and reliable in the absence of a PA catheter.Design:Prospective, descriptive comparative analysis.Patients:Convenience sample of 102 critically ill orally intubated patients with a PA catheter in place.Setting:A 24-bed medical/surgical/trauma intensive care unit in a university-affiliated medical center.Interventions:Four experienced intensive care unit nurses were trained in the use of temperature measurement with the oral, tympanic (both core and oral equivalence modes were used), and PA core methods. Simultaneous temperature measurements were then taken once in each subject using each method. The potential covariates that were analyzed were mean blood pressure, patient acuity using the Simplified Acute Physiology Score II, age, sex, ambient room temperature, and ventilator circuit temperature.Measurements and Main Results:The training period indicated that it took more time to train experienced nurses in the use of tympanic thermometry than oral thermometry. Descriptive statistics were the following: core, x = 37.33 (SD = 0.89); oral, x = 37.18 (SD = 0.92); tympanic oral, x = 36.80 (SD = 0.93); and tympanic core, x = 37.12 (SD = 1.0). Bias averages were calculated and were significantly different from 0 for all three methods (oral-PA core, −0.15 [SD = 0.36]; tympanic core-PA core, −0.11 [SD = 0.57], tympanic oral-PA core, −0.52 [SD = 0.53]), indicating that there is some degree of decreased accuracy associated with each method when compared with PA core. However, scatter plots using the Bland and Altman methodology (Altman DG, Bland JM: Practical Statistics for Medical Research. London, Chapman and Hall, 1991) illustrate that the greatest variability is associated with the tympanic method.Conclusions:Temperature measurement is an important piece of clinical data in a critically ill patient population. We found oral thermometry to be the most accurate and reproducible method when a PA core measurement was not available.