Clinicians shouldn’t aim to nail an immediate diagnosis. Instead, we create a mental list of common possibilities, called the “Differential Diagnosis.” Then we systematically rule them out, especially deadly ones, while searching for a most-likely cause.
We have lots of short-cuts. Often by clarifying a time-frame, we can rule out conditions that don’t fit. For example, a headache that developed gradually won’t be the stroke or hemorrhage that begins abruptly. Another headache that lasted 2 weeks, disappeared for a month, & has now returned, won’t be cancer, because if cancer has grown enough to cause pain, it won’t go away.
Our main tool is the Medical History. I’ve had friends complain, “my doctor hardly examined me.” Honestly, in most cases we get very limited information from physical exam. It’s important for some conditions, but not many. That may seem weird, but it’s really the case. The more questions we ask, the better it is for everyone.
Laboratory test, x-rays, etc., don’t help that much either. They usually serve to reinforce or confirm a diagnosis that was already likely from the history. Tests may be necessary to rule out possibilities we don’t want to miss, but can easily give misleading information.
For example, an EKG can be normal in the middle of a heart attack. On the other hand, studies have done MRIs on people without back pain, & found lots of “slipped disks.” So if you have back pain, & the MRI is abnormal, it doesn’t necessarily mean that the abnormality has anything to do with your pain (in which case, the last thing you’d want would be surgery).
If the chance of having a certain disease is low, and a test turns up positive, it’s most likely a false-positive. That can do harm, if it leads to other more invasive (dangerous) testing as we try to be sure. Click for an explanation why to not test for unlikely diseases.
Acute symptoms (those that began recently) are often more concerning to us than ongoing symptoms, because we don’t know which direction they may go. Once they’ve been present for a while, the pattern is much clearer. However, chronic symptoms that are getting worse are equally a cause for concern.
We consider some patients high-risk for serious illness. It may be because of diseases that run strongly in the family. Or it may be because they are immunocompromised to some degree (weakened immune systems). We do lots more tests on them, because serious infections are not only more likely, but may also cause fewer symptoms (for example, it’s our immune system that produces fever). Such conditions include:
- Inject street drugs
- Kidney failure
- Liver failure
- Active cancer
- Medicines that weaken immune system
- Sickle cell disease
- Spleen was removed
- Certain other chronic diseases
We also worry about the Severely Mentally Ill (mainly people with untreated psychosis). They tend to tolerate pain and other symptoms, so when they do seek care, something bad may be going on. Unfortunately, they may also be unable to describe their symptoms well.
It may be interesting to read our hints on Describing Symptoms to Your Medical Provider. But the best way to get an idea of medical diagnosis is to delve in to a symptom or two from our list.