On CORE's accuracy

In September 2021, researchers published a study evaluating the reliability and validity of the CORE sensor. At first glance, The Verdel et al study does not look positive, but it actually confirms the high level of accuracy of the CORE sensor. The conclusion in the study suggests that the “CORE sensor is not a valid measure of core body temperature” and the authors claim that the accuracy does not support the manufacturer's claims.

A closer look at the test data reveals that the CORE sensor is the most accurate wearable solution for athletes when e-pills or rectal thermometers are not a suitable options

Even though the study appears to discredit the CORE sensor, when the actual test data is analysed, the opposite is confirmed. There are a few reasons for this apparent contradiction.

For further details on the science and methods used in the study, please visit greenTEG's website.

How the CORE sensor was Tested

For the study, the authors undertook testing in a laboratory environment and measured the CORE sensor against rectal thermometer data. This is a highly accurate reference, though the study ignored a crucial detail – the CORE sensor calculations are validated against e-pill data and it is well documented that e-pill and rectal thermometer data have divergences.

In communication with the author, the staff at CORE outlined how the CORE sensor data is calculated, how it was tested and how it is validated, as this is fundamental. As an emerging technology, we are proactive in sharing details on the clinical testing and studies we have engaged in, the validation, and the limitations. This helps athletes, coaches and researchers to decide whether the CORE sensor is the most suitable solution for their needs.

The ‘machine learning’ approach we use allows us to continuously improve the accuracy of the CORE sensor over time. Thus, for any detailed studies, it is necessary to document details such as the firmware version of the CORE sensor and the divergence when different testing approaches are used, as well as disclose that detailed input was provided on the validation. All of this is relevant for evaluation. The study is incomplete without appropriate disclosure or sufficient documentation that would impact the authors' conclusion.

 How the accuracy of the CORE sensor was calculated

The authors took the liberty to use their own preferred calculation method and select a much narrower accuracy threshold when evaluating the test data for accuracy, even though CORE has published and communicated our calculation method and cut-off threshold.

It is unsurprising that the authors' conclusions would be influenced by this different calculation method. Nevertheless, we are pleased to confirm that the test data does indeed confirm with the expected accuracy.

This context is omitted and the authors' conclusion is subsequently misleading. Furthermore, if the authors' preferred calculation and threshold were applied to certified medical grade thermometers that are commonly used in hospitals and recognised as suitable measure of core body temperature, many of these devices would also be regarded as inaccurate.

The suitability of the CORE sensor

Rectal thermometers and electronic pills are popular options for research, laboratory testing and some sports performance testing. However, due to the costs, the invasive format, and the limited monitoring time frame, these are not as accessible to most users.
The CORE sensor is not presented as a replacement for rectal thermometers or e-pills. It is, however, a lower cost solution that is wearable, non-invasive, easy to use and highly accurate. This makes it an ideal solution for monitoring core body temperature in a number of applications.

In guidance for using the CORE sensor, we consistently recommend seeking medical advice and professional coaching. In contrast to suggestions of the authors, the CORE sensor is not intended nor is marketed as a solution to diagnose medical emergencies.
For sports use, athletes can benefit from understanding their own individual thermoregulation, undertaking safe heat training, identifying effective cooling approaches, and monitoring core body temperature to avoid overheating.
The broad adoption by elite and amateur sports people along with the international sporting success of athletes who use the CORE sensor demonstrate an interest in a convenient, non-invasive and continuous monitoring solution for core body temperature. Many elite coaches and athletes are familiar with e-pills and rectal thermometers. Their broad adoption of the CORE sensor  for field use for training and racing further demonstrates that it is suitable for these applications and provides a suitable level of accuracy to benefit sporting performance.

Real-world applicability

The level of accuracy that is possible with the CORE sensor is a result of seven years of hard science by our R&D team alongside partners in medical fields, sports and other professions. But the gold-standard evaluations come from our users. In short, how does it benefit athletes?

One of the CORE sensor's earlier adopters is Olav Aleksander Bu, the performance coach of the Norwegian Triathlon team. He shared his insights.


Dan Lorang, head coach of BORA–hansgrohe UCI World cycling team, also evaluated the CORE sensor.

We mainly used the 2021 CORE sensors to prepare for the Tour de France, the Olympic Games and the Vuelta. The main focus was on the individual differences in acclimatization to heat under exertion, both to monitor and to control the intensity of exertion. We mainly used the sensors in training. The riders then carefully looked at the numbers. In the race we recorded the data, but the riders didn't look at it.
The CORE sensors showed us for the first time how big the individual differences are, especially regarding the temperature threshold. This means the point from which the stress on the rider becomes uncomfortable or no longer tolerable. There were differences of up to 0.8 °C/ 1.4 °F, which is a huge difference in body temperature. At the same time, we could really measure that with training we managed to get higher performance at the same core body temperature after a good heat adaptation.
By measuring with CORE, we were also able to measure the influence of cooling and, above all, to train the athletes to pay attention to the topic. The topics of cooling and heat were perceived more consciously and also led to a calibration between body awareness and measured values. So far, we have not worked with temperature zones as much, as this is not so easy out on the road. What we did was, on the intense intervals, we made sure that we didn't go too far beyond individual temperature limits, regardless of what the wattage was. Intensity was adjusted to core body temperature for these intervals.

In closing

At CORE, we recognise our responsibility in supplying equipment that can be used safely. We have maintained transparency regarding capabilities of the CORE sensor, including in the dialogue with the authors of the study when they were testing.

After reviewing the test data we could confirm that it conforms with claims we have made regarding the accuracy range of the CORE sensor. Furthermore, we were able to identify the discrepancies and omissions in the study.
The resounding support from many researchers, coaches and athletes following the publication of the study provided affirmation of the interest and suitability of the CORE sensor for sports use. Our research and development is continuing and CORE sensor owners will benefit from continuing updates and accuracy improvements.