The difference between screening and diagnostic tests


Hello again, dear readers, and welcome back to our monthly letters column. We’re in the heart of summer now, and with extreme heat affecting so many parts of the nation, we hope you are doing what you can to stay safe. The heat has generated a lot of mail, and we’ll have a column next week to address your concerns. This includes precautions and strategies, as well as warning signs of heat-related illness.

Meanwhile, onward to your letters and comments.

-- In a recent column about pancreatic cancer, we talked about the lack of a screening test for the disease. Two of our readers have responded with a logical question. “You mention that there are no screening tests for this disease,” they wrote. “Yet the column states that it was diagnosed in this patient with an MRI. Is that not a screening test?”

Your letter highlights an important distinction between a screening test and a diagnostic test. While the same tools may be used in each, the tests differ in their intent. A screening test looks for evidence of disease in someone who has no symptoms. A good example of this is a mammogram, which can detect breast cancer before there are any signs or symptoms of the disease. It is preventative. A diagnostic test is used when symptoms are present, in order to find the reason that they are occurring.

In the case of the person in the column who was found to have pancreatic cancer, the MRI was diagnostic. Screening tests are used when the method can discover a disease when it is still curable. At this time, the scans that are used to diagnose pancreatic cancer have not been shown to lower the risk of dying from this disease.

Researchers are pursuing new avenues in search of a pancreatic cancer screening test. Among the most promising is to identify biomarkers, which are unique organic molecules, that are associated with early onset of the disease.

-- Many of you, including this reader, have questions about COVID-19 boosters. “More people, especially seniors, are getting COVID-19 even after being twice vaccinated and boosted,” she wrote. “If you get COVID-19 after your first booster, should you still get a second one?”

The Centers for Disease Control and Prevention counsels that yes, those who are eligible for a second booster should receive one, even after having COVID-19. You can get it once symptoms are gone, you’re testing negative and any required isolation period has ended. However, some immunologists recommend waiting a bit longer -- several weeks or more -- to allow the immune system a bit of time to recover. While infection does confer some natural immunity, the data show that it wanes after about three months.

Meanwhile, as the parade of ever-more infectious variants continues, it’s important to continue to protect yourself. High-quality masks -- look for N95 and KN-95 -- do a very good job. Yes, the virus is microscopic, but it’s inert. It hitches a ride on tiny droplets. These masks, when they are fitted properly, are very good at blocking them.

As always, thank you for your letters and sharing your thoughts. Some of you took the time to reach out with kind words and success stories. Please know that they mean a lot to us.

(Send your questions to [email protected], or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)