Every branch of science has its constants, and if there are any in medicine, normal body temperature – 37° Celsius – might just be one of them, writes Dr Perry Wilson of the Yale School of Medicine in a commentary in Medscape.
He writes:
While even my seven-year-old knows that “normal body temperature is 37°”, it turns out that this isn’t normal at all.
How did we arrive at 37.0°C for normal body temperature? We got it from German physician Carl Reinhold August Wunderlich, who was the first to realise that fever was not itself a disease but a symptom of one.
In 1851, Wunderlich released his measurements of more than 1m body temperatures taken from 25 000 Germans – a painstaking process at the time, which employed a foot-long thermometer and took 20 minutes to obtain a measurement.
The average temperature measured, of course, was 37°C.
But we’re more than 150 years post-Wunderlich right now, and the average person might be quite different from the average German in 1850.
Moreover, we can do a lot better than just measuring a ton of people and taking the average, because we have statistics.
However, the problem with measuring a bunch of people and taking the average temperature as normal is that you can’t be sure that the people you are measuring are normal.
There are obvious causes of elevated temperature that you could exclude. Let’s not take people with a respiratory infection or who are taking Tylenol, for example. But as highlighted in this paper in JAMA Internal Medicine, we can do a lot better than that.
The study leverages the fact that body temperature is typically measured during all medical office visits and recorded in the ever-present electronic medical record.
Researchers from Stanford University identified 724 199 patient encounters with outpatient temperature data. They excluded extreme temperatures – less than 34° C or greater than 40° C – excluded patients under 20 or above 80, and excluded those with extremes of height, weight, or BMI.
You end up with a distribution where the peak is clearly lower than 37°C.
But we’re still not at “normal”. Some people would be seeing their doctor for conditions that affect body temperature, like infection.
You could use diagnosis codes to flag these individuals and drop them, but that feels a bit arbitrary.
The researchers used data to fix this problem – a technique called LIMIT (Laboratory Information Mining for Individualised Thresholds). It works like this:
Take all temperature measurements and then identify the outliers – the very tails of the distribution.
Look at all diagnosis codes in those distributions. Determine which diagnosis codes are over-represented in those distributions. Now you have a data-driven way to say that yes, these diagnoses are associated with weird temperatures.
Next, eliminate everyone with those diagnoses from the dataset.
What you are left with is a normal population, or at least a population that doesn’t have a condition that seems to meaningfully affect temperature.
So, who was dropped? Well, a lot of people, actually. It turned out that diabetes was way over-represented in the outlier group.
Although 9.2% of the population had diabetes, 26% of people with very low temperatures did, so everyone with diabetes is removed from the dataset.
While 5% of the population had a cough at their encounter, 7% of the people with very high temperature and 7% of those with very low temperature had a cough, so everyone with a cough gets thrown out.
The algorithm excluded people on antibiotics or with sinusitis, urinary tract infections, pneumonia, and, yes, a diagnosis of “fever”.
What do we have left? What is the real normal temperature?
Ready?
It’s 36.64° C.
Of course, normal temperature varied depending on the time of day it was measured – higher in the afternoon.
The normal temperature in women tended to be higher than in men. The normal temperature declined with age as well.
In fact, the researchers built a nice online calculator where you can enter your own, or your patient’s, parameters and calculate a normal body temperature for them.
Here’s mine. My normal temperature at around 2pm should be 36.7° C.
So, we’re all more cold-blooded than we thought. Is this just due to better methods? Maybe.
But studies have actually shown that body temperature may be decreasing over time in humans, possibly due to the lower levels of inflammation we face in modern life (thanks to improvements in hygiene and antibiotics).
Of course, some of you are asking yourselves whether any of this really matters. Is 37° C close enough?
Sure, this may be sort of puttering around the edges of physical diagnosis, but I think the methodology is really interesting and can obviously be applied to other broadly collected data points.
But these data show us that thin, older individuals really do run cooler, and that we may need to pay more attention to a low-grade fever in that population than we otherwise would.
In any case, it’s time for a little re-education. If someone asks you what normal body temperature is, just say 36.6° C. For his work in this area, I suggest we call it Wunderlich’s constant.
Dr Perry Wilson is an associate professor of medicine and public health and director of Yale’s Clinical and Translational Research Accelerator.
Study details
Defining Usual Oral Temperature Ranges in Outpatients Using an Unsupervised Learning Algorithm
Catherine Ley, Frederik Heath, Trevor Hastie, et al.
Published in JAMA Internal Medicine on 5 September 2023
Key Points
Question Can personalised temperature norms be defined to improve the clinical utility of oral temperature measurements?
Findings In this cross-sectional study, machine learning was applied to 618 306 adult outpatient encounters to define the usual or mean “normal” temperature as 36.64 °C. Using individual and temporal characteristics, the range of mean temperatures for the coolest to the warmest individuals was 36.24 °C to 36.89 °C.
Meaning These findings suggest that age, sex, height, weight, and time of day are factors that contribute to variations in individualised normal temperature ranges.
Abstract
Importance
Although oral temperature is commonly assessed in medical examinations, the range of usual or “normal” temperature is poorly defined.
Objective
To determine normal oral temperature ranges by age, sex, height, weight, and time of day.
Design, Setting, and Participants
This cross-sectional study used clinical visit information from the divisions of Internal Medicine and Family Medicine in a single large medical care system. All adult outpatient encounters that included temperature measurements from April 28, 2008, through June 4, 2017, were eligible for inclusion. The LIMIT (Laboratory Information Mining for Individualised Thresholds) filtering algorithm was applied to iteratively remove encounters with primary diagnoses overrepresented in the tails of the temperature distribution, leaving only those diagnoses unrelated to temperature. Mixed-effects modelling was applied to the remaining temperature measurements to identify independent factors associated with normal oral temperature and to generate individualised normal temperature ranges. Data were analysed from July 5, 2017, to June 23, 2023.
Exposures
Primary diagnoses and medications, age, sex, height, weight, time of day, and month, abstracted from each outpatient encounter.
Main Outcomes and Measures
Normal temperature ranges by age, sex, height, weight, and time of day.
Results
Of 618 306 patient encounters, 35.92% were removed by LIMIT because they included diagnoses or medications that fell disproportionately in the tails of the temperature distribution. The encounters removed due to overrepresentation in the upper tail were primarily linked to infectious diseases (76.81% of all removed encounters); type 2 diabetes was the only diagnosis removed for overrepresentation in the lower tail (15.71% of all removed encounters). The 396 195 encounters included in the analysis set consisted of 126 705 patients (57.35% women; mean [SD] age, 52.7 [15.9] years). Prior to running LIMIT, the mean (SD) overall oral temperature was 36.71 °C (0.43 °C); following LIMIT, the mean (SD) temperature was 36.64 °C (0.35 °C). Using mixed-effects modelling, age, sex, height, weight, and time of day accounted for 6.86% (overall) and up to 25.52% (per patient) of the observed variability in temperature. Mean normal oral temperature did not reach 37 °C for any subgroup; the upper 99th percentile ranged from 36.81 °C (a tall man with underweight aged 80 at 8:00am) to 37.88 °C (a short woman with obesity aged 20 at 2:00pm).
Conclusions and Relevance
The findings of this cross-sectional study suggest that normal oral temperature varies in an expected manner based on sex, age, height, weight, and time of day, allowing individualised normal temperature ranges to be established. The clinical significance of a value outside of the usual range is an area for future study.
Medscape article – The New Normal in Body Temperature (Open access)
See more from MedicalBrief archives:
Parents too quick to medicate children’s fevers – US poll
Average body temperature has decreased over time
Flexible temperature screening options for challenging times