A diagnostic test is a medical test or intervention used to establish whether a particular condition is or is not present in a person suspected of having a condition because they show symptoms or have an abnormality/anomaly of some kind. The results of a diagnostic test nearly always lead to a diagnosis or the exclusion of a diagnosis.
Deeper dive
Does a patient have a particular condition? Most of the time, at least part of the answer rests in giving that patient a test, whether it’s a questionnaire to identify a likely mental illness, an imaging test to see inside the body, analyzing a biological specimen (such as urine or a piece of a skin) or some other test. All these examples fall into one of two categories of medical tests, and it’s very important that journalists (and patients) do not confuse the two: screening tests and diagnostic tests.
Screening tests look at presumably healthy people to determine whether there is any indication that it’s possible a disease might exist. They are designed to offer probability of a disease’s existence and are used to determine whether further exploration — with additional screening tests or, more often, with diagnostic tests — is warranted. A diagnostic test, on the other hand, is given to establish whether a condition is or is not present in a person suspected of having a condition because they show symptoms or have an abnormality/anomaly of some kind.
For example, a clinical breast exam is a screening test: a health care provider feels the breast of a presumably healthy woman and looks for lumps, discoloration, unusual discharge or anything unusual that might suggest a problem. A biopsy, on the other hand, is a diagnostic test: a piece of a tissue mass is tested in a lab to determine what it is and whether it’s benign or malignant.
Screening tests always involve probability. They cannot predict with 100 percent certainty that a person has a condition, but they can suggest what a person’s likelihood is of having a condition within a certain range. Diagnostic tests may not always be definitive either, but the doctor is basically making a decision about the presence of a condition so that the next step in treatment can be decided. (Some conditions prevent total certainty because they are based only on symptoms identified through a clinical exam and don’t have a biological way to test for them, such as the majority of mental illnesses and many autoimmune conditions and seizure disorders.)
All tests have the potential to be wrong, whether incorrectly pointing toward having a condition when the patient does not (false positive) or whether neglecting to identify a problem the patient has (false negative). But when a screening test is “wrong,” it really means that a person has ended up in the minority side of the probability curve. For example, if a screening test suggests a 90% likelihood that a woman has a child with a chromosomal disorder, and then a diagnostic test determines through an amniocentesis that no chromosomal disorder exists, then the screening test was not really “wrong.” Rather, the woman exists into that 10 percent of lesser probability. Where it gets admittedly a bit more confusing is that a diagnosis can certainly take into account the results of one or more screening tests. However, a screening test alone cannot make a diagnosis.
Another key difference that can exist between screening and diagnostic tests — most of the time — is how insurance companies and other payers treat them. As a (usually) preventive intervention, screening tests are nearly always covered 100 percent by insurance companies. Diagnostic tests, on the other hand, often require a copay and are more frequently more expensive than screening tests. Screening tests also tend to be simpler and noninvasive whereas diagnostic tests may be invasive or otherwise more involved.
Two key questions can help a reporter determine whether a test is screening or diagnostic (aside from asking a clinician, which is the best idea when possible): Is the person receiving the test presumed to be healthy and non-symptomatic at the start? If so, it’s a screening test. Is the test designed to lead to a definite diagnosis? If so, then it’s a diagnostic test. This link offers a helpful chart to the differences as well.