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TABLE 2 Prediction Diagnostics for Recurrent Exertional Heat following a season of environmental heat stress without a re-
Illness Based on Heat Tolerance and Patient History currence of EHI. The HTT is used during the initial diagnostic
HTT, def 1 HTT, def 2 Patient history and evaluation phase to assist in categorizing Servicemembers
Participant status, no. as heat tolerant or intolerant. 15
True positive 2 1 1
True negative 33 36 23 Conclusion
False positive 8 5 18 The HTT has shown utility as a clinical tool to help determine
False negative 0 1 1 RTD decisions and timelines for the Israel Defense Force and
Prediction diagnostics (95% CI) U.S. SOF but is not widely regarded as a standard in diagnos-
Sensitivity 100.0 50.0 50.0 tics for EHI among clinicians and researchers. The purpose of
(15.8, 100.0) (1.3, 98.7) (1.3, 98.7) this study was to evaluate follow-up data on Servicemembers
Specificity 80.5 87.8 56.1 who had undergone an HTT after an EHI and then determine
(65.1, 91.2) (73.8, 95.9) (39.7, 71.5) whether its predictive validity for recurrent EHI would sup-
Negative 100.0 97.3 95.8 port it being used as a standard clinical diagnostic tool follow-
predictive value (89.4, 100.0) (85.8, 99.9) (78.9, 99.9) ing an EHI event. Our high NPV reflects the low recurrence
Positive 20.0 16.7 5.3 rate among heat-tolerant individuals, whereas the high sensi-
predictive value (2.5, 55.6) (0.4, 64.1) (0.1, 26.0) tivity indicates that recurrent EHIs are likely to occur in heat-
For HTT, heat intolerance (HIt) either included borderline cases (def intolerant individuals. In addition, borderline cases should be
1) or excluded borderline cases (def 2). Patient history is based on re- classified as heat intolerant, given that they did not definitively
current/complex (RC) vs. single/simple (SS), as defined in the methods.
Positive test results are based on Hit for HTT definitions or RC for meet heat tolerant criteria. This evidence demonstrates that
patient history. True positive = Positive test result (HiT or RC) + re- an HTT can provide objective data to inform RTD decisions
current event; true negative = negative test result + no recurrent event; and timelines. Because of the limitations of a small sample size
false positive = heat intolerant + no recurrent event; false negative = and few recurrent cases, additional studies are needed to vali-
heat tolerant + recurrent event.
HTT = heat tolerance testing. date HTT as a clinical method for EHI cases. Overall, we have
shown the utility of HTT in predicting EHI recurrence and
to be collected and analyzed prior to considering widespread believe it should be considered as a diagnostic tool in the eval-
adoption of HTT as a diagnostic tool. Our results indicate that uation of RTD following an EHI event.
heat-tolerant Servicemembers with a history of EHI may have
a low risk of EHI recurrence. In contrast, the risk of recurrent Author Contributions
EHI for heat-intolerant or borderline people was more difficult FO, JL, PD, and RK conceived the study concept. FO obtained
to quantify because only two Servicemembers had a recurrent funding. PA and JK analyzed the data. RK collected the data.
EHI. We believe that borderline cases should be considered RK wrote the first draft, and all authors read and approved the
heat intolerant to err on the side of caution. A Servicemember final manuscript.
must be dichotomized for the best clinical outcome. However,
this may need to be evaluated with additional HTT sessions Disclaimer
or clinical assessments, given the increased relative risk of The opinions and assertions expressed herein are those of the
recurrence. The extremely high NPV reflects the low rate of authors and do not reflect the official policy or position of the
recurrent EHI among heat-tolerant Servicemembers. The high Uniformed Services University or the Department of Defense.
sensitivity is due to the fact that both Servicemembers with
recurrent EHI were previously classified as heat intolerant on The contents of this publication are the sole responsibility of
the HTT. In contrast, if we only use EHI history to gauge fu- the authors and do not necessarily reflect the views, opinions,
ture risk (i.e., RC or SS), most prediction diagnostics are lower, or policies of The Henry M. Jackson Foundation for the Ad-
which supports the benefit of using HTT in addition to medical vancement of Military Medicine, Inc. Mention of trade names,
history. These statistics demonstrate that HTT results can assist commercial products, or organizations does not imply en-
with RTD decisions and timelines following EHI and provide dorsement by the U.S. Government.
critical data points informing the risk of recurrent EHI.
Disclosures
Our study does have some limitations, including the small The authors have no financial interests or relationships to
sample size of participants tested and the low number of cases disclose.
of recurrent EHI. Uncertainty in the results is reflected in the
wide confidence intervals. In addition, the initial HTT results Funding
largely impact any longitudinal findings as the test results may This project was funded by: USUHS Intramural funding PPG
affect the Servicemember’s military career (e.g., medical board, ANE 80 -337. An Interdisciplinary Approach to Investigating
separation). In turn, these career changes affect our ability to Exertional Injuries in Service Members.
capture recurrent EHI numbers with our study parameters. Per
AR 40-501, a Servicemember who experiences an isolated EHI References
may need to be evaluated by a physician prior to returning 1. Epstein Y, Yanovich R. Heatstroke. N Engl J Med. 2019;380(25):
to limited duty with eventual return to full duty. A Service- 2449–2459. doi: 10.1056/NEJMra1810762.
member who experiences more than three EHIs in 24 months 2. Alele FO, Malau-Aduli BS, Malau-Aduli AEO, J Crowe M. Epi-
automatically enters the medical evaluation board process. A demiology of exertional heat illness in the military: A systematic
review of observational studies. Int J Environ Res Public Health.
Servicemember who experiences an EHS must be evaluated 2020 Sep 25;17(19):7037. doi: 10.3390/ijerph17197037.
by a physician, followed by a referral to the medical evalu- 3. DeGroot D, O’Connor F. Update: Heat illness, active component,
ation board. Subsequently, the Servicemember can only RTD U.S. Armed Forces, 2019. MSMR. 2020;27(04):4–9.

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