It is normal to sweat at night when it’s hot. Trouble is you can get night sweats for all sorts of other reasons and the challenge we face as medics is working out when it could be something serious.

As a haematologist we get asked about people with sweats, because night sweats can be a feature of some blood cancers, especially lymphomas. These tend to be described as “drenching”, people have to change the bed, they sleep on towels, they shower through the night and live with bedside fans – that kind of sweat.

Sweats can be caused by a whole host of other things and I thought the pathologists reading this article would like to think through the sheer range of pathologies that can cause sweats. It can be infection, inflammation, injury, pain, autoimmune, endocrine, malignant, dermatological, hormonal, stress, anxiety, obesity, drugs, sleep apnoea, depression, gastric reflux disease and more. A proper surgical sieve of issues there which should never omit “idiopathic” - it can also be just one of those things with no specific cause.

The issue which mainly faces doctors in primary care is working out when sweating is abnormal and what they should do about it.

It would be so easy to send an armful of blood from every patient, leading to unending worry about borderline results. It would be easy to send everyone for a CT scan, leading to dreadful levels of exposure to radiation, enormous cost and an epidemic of incidentalomas. We have to be careful, methodical and logical in approach. Sweats are a wonderful example of the junction that pathologists sit on, between the clinic and the lab, being able to advise and help interpret a testing strategy.

Sweats on their own are common and on their own rarely indicate anything serious. I don’t think I have met a single patient with lymphoma in whom the only presenting feature was sweats. But I have met a lot of people who were worried they had lymphoma because of sweats and I have certainly met many patients with lymphoma who had sweats and that’s what got them to their doctors.

Sweats fit into my internal “book of normal symptoms” that I want to write one day. We all get them along with headaches, abdominal pain, tiredness, spots, infections, bruises, upset bowels, coughs, sore throats and so on.

Separating these normal symptoms from pathological symptoms is so difficult. Just look up the symptoms of nearly any disease you can think of and several of these will pop up.

You can not apply a formula to symptoms like these, there is no simple flow chart or check box list to follow when someone tells you of them. You have to take a history, listen to the story, take your time understanding what could be going on. We are taught this skill in medical school and you hone it over the years. Never underestimate the power of the medical history. It brings together the science of medicine and the complexity of people. You have a conversation which is thorough, sensitive, empathic, unprejudiced and structured without being ordered. And sometimes you only have a few minutes to do it in. And then you have to write it down, summarise it and act on it too.

It is a sign of experience when you listen more than you act.

I could share many stories of the people I have met with sweats over the years but they are nearly all too personal for me to do that. They go into the other secret book in my head, the one with no name other than “experience”. It is filled with those stories along with my mistakes, revelations, good decisions, bad decisions, accidental success, persistence, the value of my colleagues and the value of waiting, when ignoring things is the right thing to do.

All this from “sweats”. This is why people are interesting. This is why you don’t do every test on every patient.

If you do want to read more about sweats, maybe you are worried about them, I recommend this paper from the BMJ : Patients who report night sweats. BMJ 2023;381:e073982 (Available at http://dx.doi.org/10.1136/bmj-2022-073982)