What makes a medical test diagnostic?

A medical test is a medical procedure that a doctor performs not only to confirm but also, in many cases, to rule out a diagnosis. Medical tests can be thought of as evidence that adds to a doctor's belief about whether a patient contracts a disease. A medical test can be an imaging study, a laboratory study, or an invasive procedure that obtains a specimen for pathology. Several characteristics make a clinical test diagnostic, which will be detailed in the following paragraphs.

Medical tests are different in terms of their sensitivity and specificity. A medical test used for screening should be sensitive to the disease of our concern and is said to be diagnostic if it is specific to the disease. Take lung cancer as an example. On interviewing an elderly with a smoking history, lurking family history, and presenting with hemoptysis and weight loss, the doctor would undoubtedly raise the concern for lung cancer. A low-dose CT study is usually obtained as the next step for screening. The presence of any suspicious lung nodules would prompt tissue proof via biopsy of the nodules. Identifying cancer cells under a microscope is the gold standard for the diagnosis of lung cancer.

In mathematical terms, a medical test is perfectly diagnostic if it has a one-to-one relationship with the disease. Identifying the one and the only etiology of a disease is crucial for the diagnosis of certain genetic disorders because there is a one-to-one relationship between the genetic mutation and the disease. In other cases, clinical manifestations specific to the disease can also be diagnostic. For example, right upper quadrant abdominal pain with a thickened gallbladder wall on ultrasound is pathognomonic of cholecystitis although infiltration of inflammatory cells in the gallbladder is not directly inspected under a microscope.

Last but not the least, a diagnostic test is often more expensive and less accessible than a screening test because of the technique a diagnostic test requires. Not every hospital or clinic has a CT scanner, and pathologists usually serve at teaching hospitals. Due to the accessibility of the diagnostic tests, patients can be referred to hospitals with such facilities and medical experts. In addition, an expensive diagnostic test is usually not performed unless there is enough suspicion for the presence of a disease in consideration of cost-effectiveness.

To sum up, concerning a disease, a medical test is diagnostic if it is specific, and has a nearly one-to-one relationship with the disease. Often, diagnostic tests require more expensive and advanced techniques compared to screening tests. Together, these factors determine whether a medical test is used for screening or diagnosing.