The Systematic Bias of Ingestible Core Temperature Sensors Requires a Correction by Linear Regression

Abstract

An accurate measure of core body temperature is critical for monitoring individuals, groups and teams undertaking physical activity in situations of high heat stress or prolonged cold exposure. This study examined the range in systematic bias of ingestible temperature sensors compared to a certified and traceable reference thermometer. A total of 119 ingestible temperature sensors were immersed in a circulated water bath at five water temperatures (TEMP A: 35.12 ± 0.60°C, TEMP B: 37.33 ± 0.56°C, TEMP C: 39.48 ± 0.73°C, TEMP D: 41.58 ± 0.97°C, and TEMP E: 43.47 ± 1.07°C) along with a certified traceable reference thermometer. Thirteen sensors (10.9%) demonstrated a systematic bias > ±0.1°C, of which 4 (3.3%) were > ± 0.5°C. Limits of agreement (95%) indicated that systematic bias would likely fall in the range of −0.14 to 0.26°C, highlighting that it is possible for temperatures measured between sensors to differ by more than 0.4°C. The proportion of sensors with systematic bias > ±0.1°C (10.9%) confirms that ingestible temperature sensors require correction to ensure their accuracy. An individualized linear correction achieved a mean systematic bias of 0.00°C, and limits of agreement (95%) to 0.00–0.00°C, with 100% of sensors achieving ±0.1°C accuracy. Alternatively, a generalized linear function (Corrected Temperature (°C) = 1.00375 × Sensor Temperature (°C) − 0.205549), produced as the average slope and intercept of a sub-set of 51 sensors and excluding sensors with accuracy outside ±0.5°C, reduced the systematic bias to < ±0.1°C in 98.4% of the remaining sensors (n = 64). In conclusion, these data show that using an uncalibrated ingestible temperature sensor may provide inaccurate data that still appears to be statistically, physiologically, and clinically meaningful. Correction of sensor temperature to a reference thermometer by linear function eliminates this systematic bias (individualized functions) or ensures systematic bias is within ±0.1°C in 98% of the sensors (generalized function).

The Effects of Heat Exposure During Intermittent Exercise on Physical and Cognitive Performance Among Team Sport Athletes

“…We then recorded T C 1-minute into each 2-minute exercise block throughout the CISP. As per Hunt et al (2017), we corrected sensor readings using the following equation: corrected temperature ( C) ¼ 1.00375Âsensor temperature ( C) À 0.205549, in order to help restrict systematic bias to within aþ0.1C accuracy range. We averaged and analyzed this measure per quarter of the CISP.…”

Independent or simultaneous lowering of core and skin temperature has no impact on self-paced intermittent running performance in hot conditions

“…A safety stop criteria was set at 39.5 °C but this was not exceeded by any participants in any conditions. Based on the recommendations by Hunt et al (2017) all telemetric pills were checked against a reference mercury thermometer in a water bath and where appropriate corrections were applied to reduce any systematic bias. T sk was measured at eight sites (forehead, chest, upper back, upper arm, forearm, hand, thigh, and calf) using iButtons ® (Maxim Integrated Products, Inc., Sunnyvale, California, USA).…”

Estimating Resting Core Temperature Using Heart Rate

“…Advances in micro-electronics enabled the development of ingestible temperature sensing via a pill that telemeters data of continuous measures from a near-infrared measure from the location (swallowed or placed rectally) (O'Brien, Hoyt, Buller, Castellani, & Young, 1998). Improvements to these pills have been made continuously (Hunt, Bach, Borg, Costello, & Stewart, 2017) and these telemetry-based pills are a great improvement over the use of rectal probes (less invasive, easier to use, fairly scalable).…”