Understanding the Testing Process
Many patients find themselves feeling somewhat confused when the doctor talks about “performing tests” on blood, urine, or tissue biopsy samples. They hope the tests will provide a diagnosis but don’t really understand the process doctors use to move from tests to other tests and then to a diagnosis.
Before a doctor orders a test, the doctor usually has a suspicion about what might be wrong based on the patient’s symptoms, characteristics, history, and the results of a physical examination. For example, if a relatively young patient who lives in an area with a high incidence of Lyme presents with a swollen, painful knee, the doctor may strongly suspect it is Lyme disease, but it could also be a torn meniscus, septic bursitis, rheumatoid arthritis, or even gout. Thus, the doctor will order a blood test for Lyme disease first and other blood abnormalities that might suggest infection, gout, or an autoimmune disease.
Accuracy of the Test
No diagnostic test is perfect; they all have a certain rate of errors. In addition, many blood test results are interpreted by comparing them to the expected “normal” level but what is “normal” may vary significantly depending on diet, typical activities, age, and so forth. For example, blood tests for gout measure the level of uric acid in the blood, but blood uric acid levels naturally vary. If our patient with the painful knee has a test that indicates a slightly elevated level of blood uric acid and a negative Lyme disease test, before proceeding the doctor needs to consider the accuracy of these tests. At the stage where Lyme disease causes knee pain, the blood test for it is considered to be 97% accurate. The slightly elevated uric acid level is not very accurate for gout at all.
Doctors can calculate the post-test probability of a disease by using Bayes’s theorem, but in actual practice it is easier to eyeball a nomogram. In our knee pain patient’s case, the doctor thought she had a 75% chance of having Lyme disease and a 10% chance of gout, so after the testing the doctor concludes there is a 20% likelihood of Lyme and still only a 10% chance of gout. The doctor now considers that there is a good chance the problem is a torn meniscus, so the next step is to get an MRI to evaluate this possibility.