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 the

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.