In The Time of Coronavirus What Should Be Considered A Normal Temperature And What Is A Fever?

As a medical student, I was taught that the normal temperature of humans is 37.0°C (98.6°F) and that the upper limit of normal temperature is 38.0°C (100.4°F). The criterion for fever during this current SARS-CoV-2 epidemic is still considered to be a temperature over 38.0°C (100.4°F) despite strong evidence supporting a lower temperature cut-point.

Multiple studies in the last 30 years have clearly established that the average human temperature has dropped progressively in the last 150 years and is more accurately considered to be 36.8°C (98.2°F).

Moreover, normal temperature varies between individuals and multiple other variables including according to what time of day it is taken.

For example, when I take my temperature first thing in the morning it is 96.4°F. Later in the day, it rises to 97.4°F. After exercise, it reaches 98.0°F. My overall average is 97.0, significantly lower than the canonical average.

This change in the average normal temperature and the upper limits of normal has significant implications for temperature screening for the determination of fever and consequently who may or may not have a coronavirus infection.

Origins Of 98.6 As Normal

The concept of 98.6 as normal can be traced back 150 years to work from the German scientist Carl Reinhold August Wunderlich who made multiple measurements of axillary temperature in 25000 subjects. Wunderlich reported a range of 36.2°C (97.2°F) to 37.5°C (99.5°F). Although he also noted a significant diurnal variation in temperature, with a nadir in early AM and peak in later afternoon,  he would have considered me hypothermic.

Wunderlich also felt that temperatures >100.4 were “probably febrile.”

Change in Temperature Over Time

Studies since Wunderlich have shown a progressive decline in the average human temperature. Mackowiak, et al, (PDF available here) published  data showing a substantially lower normal human temperature such that  that 98.6 “should be abandoned as a concept relevant to clinical thermometry.”

Mackowiak, et al found in 148 individuals the average temperate was 98.2 (36.8). These individuals were monitored over a 48 hour period and the average and range varied considerably

Screen Shot 2020-03-28 at 9.29.19 AM

The average temperature varied according to time of day.

Screen Shot 2020-03-28 at 9.29.35 AM

In the graph above if we look at 6 AM temperature measurement, we see the average is actually 97.5°F. My 6 AM recordings of 96.4°F puts me in the bottom fifth percentile. More importantly, 99% of subjects had a temperature of <98.9°F.

If one is recording a temperature on a patient at 6 AM and it is >98.9 that should probable be considered abnormal. On the other hand, if the temperature is being measured between noon and 6 PM, >99.8 should be considered abnormal.

A publication from earlier this year in eLife confirmed that the average temperature in humans is progressively dropping.

The researchers used measurements from the Union Army Veterans of the Civil War (N = 23,710; measurement years 1860–1940), the National Health and Nutrition Examination Survey I (N = 15,301; 1971–1975), and the Stanford Translational Research Integrated Database Environment (N = 150,280; 2007–2017)

They found:

mean body temperature in men and women, after adjusting for age, height, weight and, in some models date and time of day, has decreased monotonically by 0.03°C per birth decade..

Screen Shot 2020-03-28 at 8.12.20 AM

As you can see from the graph above, the average temperature has dropped from the 1970s to the 2000s. The most recent temperatures from 2007 to 2017 in orange lie consistently below the green temperatures from 1971 to 1975. This is irrespective of gender (men in top two graphs, women in bottom two) and race (white in left boxes, black in right boxes.)

Additionally, in all categories (except perhaps civil war men) average temperature declines significantly with age.

Choosing A Cut-point Temperature To Decide Fever

All of this temperature variation data would be simply of academic interest but given the COVID-19 pandemic, the definition of fever has become incredibly important.

If we choose a cut-point that is too high we risk missing cases. If we choose one too low we risk overwhelming the limited testing resources and unnecessarily alarming patients.

What I have decided to do is to establish a norm for myself based on time of day. If I record a temperature >1.8 °F over my norm I will consider myself as having a fever.

Clearly, this doesn’t work for widespread screening of the public which is occurring now at multiple sites as most individuals have not established what is their normal average temperature, let alone their average temperature by time of day.

At a minimum, though, it seems to me that public health authorities should recognize the lower normal average temperature and the corresponding lower cut-point fever that has been known since 1997.

Adjusting the cut-point to >99.8°F for abnormal would align more closely with what is currently known about the range of normal human temperatures.

Thermoregulatorily Yours,

-ACP

20 thoughts on “In The Time of Coronavirus What Should Be Considered A Normal Temperature And What Is A Fever?”

  1. Exactly what I was wondering since my average temperature is 97.4 also. I had a couple of days of not feeling my best about 10 days ago. My temperature hit 100.2, I had diarrhea, chills and I just felt pooped. Little bit of pain in my chest. Diarrhea only lasted the first day, chills and tiredness lasted about 3 days and I woke up drenched in sweat two nights in a row. Have no idea if that was regular flu or Corona virus but I’ve kept myself home since then.

    1. I’m hearing lots of reports of temperatures right around 100 F with mild symptoms. We will likely never know what the total infection rate is of coronavirus due to outdated temperature cut-off

      1. For me I wasn’t allowed to get a test because my temp wasn’t high enough, but for the flu for example they test everyone who walks in the door regardless of symptoms (at least places I’ve been) because they have plenty of tests.

        With the serological tests for antibodies for COVID-19, they may be able to do more testing and see the full scope of it. There are 15 minute POC tests you can do for COVID-19 that test for the IgM and IgG antibodies. I’m not sure how long the antibodies stay elevated so I don’t know how much retrospective testing can be done. I’ve been trying to get one of the tests for myself. Up until last week they were for sale to consumers, but the FDA put out a statement spooking the companies and now they’re only for sale to physicians and I don’t know of any places near me using them yet. I have Type 2 diabetes and can easily draw my own capillary blood for this type of cartridge test—I draw blood all the time, and these work with capillary blood. Wish I had ordered the test before the FDA spooked the companies.

    2. Indeed, I have a low baseline temperature which is rarely over 96. Mornings are lower. A temperature of 99 means I’m sick. Since I have a suppressed immune system, this is important and physicians other than my regular internist and neurologist don’t get it. Thanks so much for discussing these variations during these times. I appreciate this well reasoned topic!

  2. So, I should probably call my doc/hospital if I go over 100ºF.

    Do you have any idea what temperature heights hospitalized covid-19 patients achieve?

      1. That was “if”. I’m fine so far.

        I’ve read that hospitals are “layering” NSAIDs and acetaminophen to bring down fevers – which makes the patient more comfortable.
        But it’s been the conventional wisdom that fevers do harm to the invading microbe, which makes lowering seem counterproductive. No?

        Is there a temperature threshold that’s too dangerously high in itself?

        1. Jeff,
          I have also wondered if the fever is something that should be suppressed or whether it should be allowed to run its course unfettered. Are we hindering the body’s defenses by lowering the temperature with medications?
          Joe, who is a pediatrician and deals with fevers much more than I may have insightful comments on this topic and what is too dangerously high.
          My sense is that in adults a high fever in and of itself is rarely dangerous. But in children fevers over 104-5 are considered capable of causing seizures so you see lots of recommendations to cool the child down.

          1. I maintained 104º for two weeks 20+ years ago in my early 50s. NSAIDs notwithstanding.
            Typhus. Yes, “epidemic typhus”. I passed a crisis and survived before any diagnosis. Believe me, fever was the least of it!

  3. It depends also on how the temperature is taken. I live in a CCRC, and, for the past few weeks, anyone coming through the gate has had his temperature taken by a member of our safety team holding the thermometer to our foreheads . One of my friends has had hers recorded from anywhere between 95 degrees F. to over 101F, depending upon whether she had the heat or air conditioning on in her car (we’re in CA). She learned to open her window and turn on the air when returning from shopping or a medical appointment to avoid an issue at the gate.

    1. Excellent point. I wanted to get out the information about the normal range change and nocturnal variation but there are a tone of other factors involved. I’ve noticed a higher temperature if I take it right after eating. Timing after exercise is huge. Ambient temperature has also shown to influence temperature. Clearly rectal temperature is higher than oral which is a little higher than skin temperature. More reasons why standardized conditions for determining one’s own normal temperature are warranted.

  4. there are a lot of variables involved here. age plays into it as well with pediatric patients having a different thermostat and hence a different “normal” range. unfortunately, trying to introduce a new “normal” during a pandemic during which testing for the virus is problematic is unlikely to be helpful.

  5. In pediatrics we often see temperatures in the 103-05 temperature range. roseola is the one the comes to mind: frightening temps of 105 and the children seem fine! and are. febrile seizures, frightening to the parents but, for the most part harmless, cannot be prevented by attempts to keep a temperature down. though that’s advice that is often given in the er. I have also been taught that the body’s first line of defense against invaders is to raise the temperature. too high a temp can cause the body to work harder increasing oxygen requirements, water requirements,etc. brain and organ changes are seen at 106 but heat exhaustion and heat stroke can occur at lower temperatures. Anthony, all of this is raising my temperature and giving me a headache. now I need to get tested!

  6. It is not merely when we are the grip of a pandemic that the medical community needs to reexamine what is considered “normal.” Several months ago I started experiencing subclinical fevers of 100.3 degrees or less with accompanying chills and fatigue. It wasn’t until my temperature hit the “magic” 100.4 degrees that I went to see my doctor.After a misdiagnosis of a UTI, and a round of antibiotics with little effect on the elevated temperatures, I soon started the medical odyssey that ultimately lead to a diagnosis of rheumatoid arthritis. My body was telling me that something was amiss, but because the cut-point is currently set at 100.4, I was thinking maybe I was imagining things. It’s time to relook at what’s considered “normal”, and listen to the patient when he/she says something is not right.

  7. I think I had it: cough, what felt like a fever (highest was 100.4), drenching night sweats multiple times each night, achiness, profound fatigue, some shortness of breath, and only very mild stomach things (a bit looser stool and flatulence–may or may not have been part of same thing). But my temperature was too low to be tested. Lasted about a week and a half. I know I was sick, but I can’t say with what. The timing certainly increased my anxiety. But now as strange as it is to say, I hope that this was the coronavirus as it would mean my body was able to clear it. Maybe it was a flu virus and I haven’t gotten the coronavirus (yet).

  8. Thank you for a most helpful and interesting post – will now include waking temperature to my monitoring diary, as well as changes over the day.

    I was raised on a farm according to Spartan principles, so an interest in Ancient Greek Stoicism was kindled. A very tough upbringing, intended to build a tough young man.

    The Stoics’ philosophy PREMEDITATIO MALORUM was explained to me as “hope for the best but plan for the worst”, which is exemplified by your suggested monitoring of one’s own daily temperature fluctuations.

    Or as Louis Pasteur said, “fortune favours the prepared mind”.

    I have also ordered a pulse oximeter from Amazon, just in case.

    Also monitoring the price of cemetery plots, and would not be surprised to see signs of panic buying. 😉

  9. My temp range is 35.somethingC (taken 7:00am but as low as 32.9) up to 36.4C tops (taken 7:00pm). Assuming this my normal range. If so what would constitute a fever temp for me?

  10. (I left a comment, but don’t know where it went ?) This is an important discussion. I have a low baseline temperature, 95 in a.m. to at most 97 in afternoon. If I’m running 99, I’m sick. I have a suppressed immune system, but this is lifelong. No physicians get this issue, except my internist and neurologist. I’m wondering if I would be tested if I had virus symptoms, but a “low” body temperature? Thank you for addressing this.

    1. Hi smdesq, as you can see from the next post up, I’ve a similar issue to you. A comment from the good doctor would be welcome.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.