Why is 37°C considered the threshold to say you have fever?



Why is 37°C considered the threshold to say you have fever?

In: Biology

37 degrees Celsius is the average temperature of the human body (give or take a few decimals of a degree). Recent studies have actually shown that this has been going down in modern times. Regardless, it’s the best mark we have to figure out if someone is “normal” or has a higher-than-normal temperature.

Typically your body will be about 37°C (98.6°F) day in day out no matter what. Body temperature higher than that is a sign something is wrong.

[Mayo Clinic : Feaver](https://www.mayoclinic.org/diseases-conditions/fever/symptoms-causes/syc-20352759)

37-37.9 would not be considered febrile in my field (paramedic). Most people read 36.7 if measured oral but things like exercise, having a bath or just the time of day could mean you jump to 37-something. 38° and up we would consider definitely a fever because it is hard to happen in daily fluctuations.

It’s not. Most sources refer to either 38 or 38.5 C as febrile. 37 is roughly average. 38 to 38.5 are just arbitrary cut offs. The vast majority of the population will not be walking around at this temperature unless something is wrong. It’s like any other “reference range” for determining if some value is normal or abnormal.

A lot of people saying its arbitrary…
It’s not.
This is a bit above ELIA5, but anyway…
Like all physiological references ranges, they’re based on healthy individuals.
You measure lots of and lots healthy individuals for something e.g. how much haemoglobin they have in their blood. A very few people will have a low haemoglobin, and that’s normal for them. The vast majority will have a haemoglobin thats middle of the pack. And a very few will have a naturally very high haemoglobin – and that’s normal for then.
We then dictate the “normal range” as being the range that the middle ~90% of people (the vast majority) sit inside. So at the extreme highs and low, we know that up to 5% of people have those levels naturally, but the vast majority of people who we test suspecting that their haemoglobin will be abnormal who return results that are very high/low have extreme results because there is something wrong with them causing this result thats so far off the average.
(Google “Normal Distribution”for nice pictures of this)
Replies here have been debating “well in my job as an x, its not a temperature unless its over y”. This is because individual laboratories and equipment use slightly different methods, and tests and equipment has different error margins etc etc. So your “normal ranges” will often vary slightly from place to place.
In addition to this, different diagnostic pathways kick in at different disease severities. We may treat a temp of 38.5 with paracetamol (tylenol…) because the risk of being seriously ill is low and youre more likely to come to harm because of aggressive treatment than to benefit.

But we know a temperature of 40 is much more serious and you’re more likely to come to harm if we do nothing. This is why sometimes healthcare workers give different amounts of shits at different disease (or in this case, fevers) severeties.