Robert Shmerling, M.D., is associate physician and clinical chief of rheumatology at Beth Israel Deaconess Medical Center and an associate professor in medicine at Harvard Medical School. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program and has been a practicing rheumatologist for over 25 years.

Question:

If we can bring our cars in for diagnostics testing, why can't we take a tube or two of blood from a patient and do the same? Doctors do not have a crystal ball to tell them what's wrong. They have to guess from what they've learned in medical school. I don't believe there is such a test, is there?

Answer:

That's a great question! I've often had patients say, "Just do all the tests and tell me why I feel bad."

But, you are right. There is no single test or set of tests that will routinely identify the cause of all illnesses in humans.

There are many parallels between taking your car to the mechanic and seeing your doctor. In fact, I often use automobile repair as a way to talk about matters of health and disease with my patients. Routine physical examinations and screening tests are much like the routine maintenance and tune-ups we provide to our cars.

But that analogy breaks down when it comes to diagnostic testing because of some important differences between people and cars:

• People are much more complicated than cars.

• People are not assembled with on-board computers specifically designed to search for malfunctions.

Doctors use the things they learn in medical school along with everything they've learned since (including, importantly, experience) when evaluating a patient. Then, this information is integrated to come up with a set of possible explanations for the problem. This information includes:

• A detailed description of symptoms by the patient (the history)

• Risk factors for disease, including family history

• Past medical problems

• Medication use

• Social history, such as smoking or other health-related habits

• A survey of symptoms from all parts of the body (called the Review of Systems)

• The physical examination

After collecting this information, it may be clear what the problem is and you may not need any tests. When tests are needed, your doctor will choose the ones that are most likely to provide an answer.

There are hundreds of blood tests, imaging tests (such as x-rays or MRIs), and other tests that could be requested. It's simply not possible to perform them all. Even if it were possible to perform every available test, the cost would be prohibitive. And there might still be no answer about the illness! That's because many common conditions, such as migraine or back pain, don't show up on tests.

Perhaps in the future, we will have simple blood tests to screen for hundreds or thousands of diseases at once. If it does happen, my guess is that it will not be soon.