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Comment by tuukkah | Hacker News Reader

As CRP testing is easy and cheap, why do we still cling to heuristics such as measuring fever instead of measuring the infection?

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CRP has a three day delay - it gives you an idea of what was going on. Fevers are induced my macrophages encountering the antigen and are quite immediate.

I was replying to a case where there was no fever but CRP was high when measured.

I have been in a similar situation where after several days of illness my condition didn't appear bad enough and the doctor sent me home. This was some 15 years ago and they didn't have quick tests back then, but luckily they did prescribe a lab CRP test and called me back in for treatment immediately after they got the lab result later that day.

Yes, all these tests have different characteristics. I think I was making the point that there isn't an "infection" test. Temperature's are useful because they are an immediate response by your body and they can be checked instantly in remote locations. CRP is a fairly standard test, but it is raised in infection/inflammation/surgery etc, and the delay can be unhelpful. Other tests include blood cultures (low sensitivity), white cell count (again quite none specific) and then more location-specific tests, like xrays, CTs, PET-CTs, urinalysis.

A lot depends on your index of suspicion, how well the person is, what the risks are of getting it wrong (in both directions), what safety nets you have available to you (i.e. are they home with someone sensible who can keep an eye on them).

In summary - there is no perfect test or perfect heuristic to apply to everyone.

My wife did receive a CRP test that confirmed her infection in the hospital(it was over 200) -- that was among the first things the emergency department checked, then an ultrasound revealed the burst apendice, but I don't think there's any easy way to check it at home.

She already had a lot of burst veins from the hospital, so we decided to just do what her surgeon said, basically take amoxicillin for another 7 days and just about it.

CRP can’t really confirm an infection. It’s a marker of inflammation. Inflammation often correlates with infection but not always. Can be lots of other things too, including a lot of the things that are false positives on these sepsis algos.

Ultimately doctors have to take the whole clinical picture into account instead of relying on a couple markers or vital signs.

You’re right that a big problem is the inability of our system to do testing at home, which might greatly expand the scope of the clinical picture. But it still wouldn’t be a cure-all

You have to compare to the current situation where we are overreliant on fever as a marker. If after an operation, your CRP levels shoot high unexpectedly, wouldn't you want to know and start investigating what's going on?

CRP normally does shoot up post operatively, and white cell count behaves similarly. Generally if there is a post op problem people know- maybe not a fever but people feel generally unwell. That is a good reason to investigate further. Sometimes for higher risk operations we do routine blood tests as an inpatient, but if people are will enough to go home, it’s reasonable to rely on people not feeling right.

I think what you are assuming is that early intervention in infection is good. Very likely to be true. And that we can reliably detect early infection. Much less likely to be true.

The biggest issue is the costs of false positives, which with any early intervention protocol are likely to be high and, questionably, on balance a net negative.

You should instead be doing a WBC count with a breakdown of neutrophils and other leukocytes, not a CRP which is highly non-specific

You should be doing both. WCC and neutrophils can also be misleading, and tend to fluctuate. Procalcitonin can be helpful in some situations but isn’t used as often.

A nurse can take a CRP quick test. Home test kits are available to order as well, but I haven't tried them.

Would it maybe mean that CRP means nothing in isolation? Like a majority of what we test for?

As it's easy and cheap, you have no reason to use a measurement in isolation. For instance, you can track it every day to see if the infection is growing or shrinking. Here's a recent example regarding COVID-19: "We realized that whereas a single CRP lab value from hospital admission wasn't very practical as a predictor of who might get sicker, tracking the rate of change from Day 1 to Day 2 or 3 was a very powerful and very clinically predictive test" https://www.sciencedaily.com/releases/2020/11/201105112947.h...

Wow, incredible!

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