What makes a doctor want to take a blood test?

It depends entirely on the problem at hand.

It depends a bit on what the test is for. There are a lot of different blood tests around, and they can reveal a huge amount of information about what's going on in the body- they can reveal if something is there that shouldn't be there (such as an infection), they can reveal if chemical levels and cell numbers in the body are normal or abnormal, and they can look for markers and signs of all kinds of diseases.

As with any test, though, for it to be useful to the doctor, the doctor usually needs to know what kind of test to order and what information they are going to get from it (and how that's going to affect their diagnosis). There are also some risks from drawing blood, plus the cost of the test. The decision to take the test will be based on the symptoms you describe to the doctor, any further questions they ask, and what the doctor thinks is wrong.

For instance, suppose you show up to the doctor's office with symptoms of rheumatoid arthritis. There are some markers for that in the blood, and there are other conditions that cause similar symptoms which don't have those markers. So in that case, the doctor might do a test to see if they are there- this would help confirm the diagnosis and rule out other conditions, so that's useful information. So that's a case where a blood test is useful- the doctor has a suspected diagnosis, and the test gives him information to help find out more.

Some things, though, blood tests aren't that useful. The doctor might already be sufficiently confident in the diagnosis based on other symptoms that there's no reason to order a blood test. In other cases, there might not be any test that reveals information useful to that case. Some conditions are diagnosed based solely on the symptoms the patient describes.

Medicine is too complicated to identify one thing that decides if you do or don't have a test- it's very situation specific. But the general concept is that you order a test if you need further information for your diagnosis, and a blood test will give you useufl information.

There is also some variation between doctors as well on what tests they do/don't like to use. It's also not unheard of for doctors to be completely sure what's wrong without a test, but order a test just to be absolutely sure, and also to reassure the patient as well.

There are hundreds of blood tests, so we have to have a pretty good idea of what we are looking for to start with.

Some blood tests are for screening - to look for warning signs of common problems. Examples of that are the PSA test for prostate cancer, and the blood tests ordered when a woman is pregnant.

Some blood tests are done to look for early side effects from prescribed medications. If we know that a drug might hurt your liver, we check liver function tests and stop the drug if the tests show early liver problems. That's much better than waiting for all out liver failure, when it's too late.

Some blood tests monitor the specific levels of drugs, drug effects or hormones in the body. Examples would be digoxin levels for people on that drug, coagulation times for people on blood thinners, and thyroid levels in hypothyroid people who take medication to replace the lost hormone.

Most blood tests are done to rule out something or confirm a diagnosis. For example, if you have symptoms of diabetes, a blood sugar and HbA1c would let me know if that's right or not. Sometimes we order tests to make sure the diagnosis isn't something horrible, so we might look at blood cells to make sure a lingering illness isn't really leukemia.

Some blood tests narrow down the diagnosis to something more specific so that treatment can target the right thing. If you are anemic, we can do tests to see if that's likely due to blood loss, cancer, vitamin deficiency, an inherited condition, or something else.

Blood tests are just one piece of the puzzle when making or confirming a diagnosis, and can also be used to guide treatment. It all depends on what you've got.

Doctors take blood tests for many many many reasons and It's nothing bad or serious and It's a normal thing..There is nothing to worry about.Blood tests say so much about your health so It's pretty normal for a doctor to take it

The blood contains a huge amount of information that is useful to support a diagnosis.

Well, in my case, it typically means he wants to check for drugs. I hate when that happens. Be careful!