Well, not exactly. It only feels that way because the lab tech sucked seven vials of blood out of you rendering you lightheaded. There are thousands of tests that can be run by various labs and no one’s yearly screening labs or even an extensive workup for a given problem would involve enough tests to check for everything. I don’t bring it up to be harsh or to mock people who say this. There is an important point to be made. This statement points to some misconceptions about medical testing in general. Most people are not aware of the part that it plays in making diagnoses and decisions.
A blood test does not make a diagnosis. For example, many people have been given a screening test for an underactive thyroid gland called a TSH level. The thyroid gland makes thyroid hormone which is like a gas pedal for the body. It tells most systems in the body to keep running at speed. Without it, we end up without the energy to do anything. We sleep for hours more than normal, gain weight and feel terrible. Most of the time, the test serves its purpose, and the result matches the symptoms of the patient. Sometimes, however, the test result is normal. The more specific confirmatory tests are also normal, and the patient still has all of the symptoms of a low thyroid hormone level. This condition even has a name. It is called euthyroid sick syndrome which means that the persons hormone levels are all normal and the person needs more thyroid hormone anyway.
So why do we do blood tests at all? Well, doctors know that the tests are not perfect. They are useful because a lot of diseases end up affecting basic functions of how the body works. These functions can be checked by looking at a few basic blood tests. A lot of conditions cause a high white blood cell count. A lot of conditions cause anemia or changes in kidney function. When we see changes in white blood cell numbers, anemia, or kidney malfunction for example, we can then talk to and examine the patient and order other tests to try to figure out more precisely what is going wrong. In reality, the diagnoses of these problems are not made by a single blood test. Rather, the diagnoses are made through the combination of symptoms, physical exam, and testing.
There is a medical book called the MERCK MANUAL. It lists normal values for many commonly used tests. On that list is a joke for nerds. The MERCK MANUAL lists a normal for serum porcelain level. So according to this book, you can literally be tested for the kitchen sink. I think that is hilarious. I know that I am in a tiny minority.
This sounds like a complicated mess. It is complicated but that is the reason we went to school all those years. It is the reason that we have specialists in areas where the decision-making process gets even more complex.