Originally Published: 28 AUG 23 11:08 ET

By Jen Christensen, CNN

(CNN) — Most cancer screenings don’t ultimately give someone extra time beyond their regular lifespan, according to a new review of clinical trials involving more than 2.1 million people who had six kinds of common tests for cancer. But experts say this doesn’t mean you should cancel that colonoscopy or mammogram appointment.

Since long before precision medicine and sophisticated treatments, doctors have encouraged people to get routine cancer screenings for nearly a century. Depending on age, the American Cancer Society recommends regular screenings for breast cancer, cervical and colorectal cancer, and it recommends discussing screening for lung and prostate cancer with a doctor, particularly for people who are at a higher risk of the disease.

The strategy behind these recommendations is to catch cancer early enough — even before symptoms start — so that doctors can take steps to improve a person’s chance of surviving their cancer and prevent premature death.

Early diagnosis has been shown to improve cancer outcomes, which is why the American Cancer Society and the World Health Organization say routine screening is an important public health strategy. Overall cancer mortality worldwide has decreased significantly, falling 33% since 1991, in part due to early detection as well as advances in treatment and declines in smoking.

The latest study, published Monday in the journal JAMA Internal Medicine, found that of the six most common cancer screenings, only colorectal cancer screening with sigmoidoscopy — in which doctors check the lower part of the colon or large intestine for cancer — seemed to make a difference in extending someone’s life. It may extend life by a little more than three months, the research says.

The researchers looked at clinical trials that involved at least nine years of follow-up reporting and found no significant difference in lifetime gain with the other most common cancer screening tests: mammography for breast cancer, colonoscopy, fecal occult blood testing or endoscopy (FOBT), prostate-specific antigen tests, and computed tomography for current or former smokers.

“We do not advocate that all screening should be abandoned,” the researchers wrote. “Screening tests with a positive-benefit-harm balance measured in incidence and mortality of the target cancer compared with harms and burden may well be worthwhile.”

The authors of the new research suggest that rather than emphasize that cancer screenings save lives, doctors should be clearer about their absolute benefits, harms and burdens.

In a related publication in JAMA Internal Medicine on Monday, Dr. Gilbert Welch and Dr. Tanujit Dey of the Center for Surgery and Public Health at the Department of Surgery at Brigham and Women’s Hospital write that with the “growing enthusiasm” for expensive multi-cancer detection blood tests, particularly among policymakers, it will be important to do large randomized clinical trials like the ones in this study to truly understand whether such tests save lives and warrant their costs.

Doctors have long known that while there are significant advantages to cancer screenings, there are also a handful of downsides, studies show.

Some positive screening results are false positives, which can lead to unnecessary anxiety as well as additional screening that can be expensive. Tests can also give a false negative and thus a false sense of security.

Sometimes too, treatment can be unnecessary, resulting in a net harm rather than a net benefit, studies show.

“The critical question is whether the benefits for the few are sufficiently large to warrant the associated harms for many. It is entirely possible that multicancer detection blood tests do save lives and warrant the attendant costs and harms. But we will never know unless we ask,” the authors of the new study write.

Dr. William Dahut, chief scientific officer for the American Cancer Society, said that fully determining whether cancer screenings extended life would require an extremely large clinical trial that would have to follow patients for a very long time. The trials in the newest study weren’t big enough to look at all-cause mortality.

Dahut, who wasn’t involved with the new research, explained that if breast cancer caused 3% of all female deaths and screenings reduced these deaths by 35%, that’s a good result on its own. But screenings may change mortality overall by only about 1%, which sounds less impressive but is still an improvement.

Dahut said that while clinical trials may not pick up on it, cancer screenings seem to have had an effect on cancer deaths. Deaths from things like cervical cancer and prostate cancer have declined over time after health care providers started encouraging people to go in for routine tests.

“Even outside of a randomized trial, you do see evidence of an impact of cancer screening,” he said.

“Cancer screening was never really designed to increase longevity. Screenings are really designed to decrease premature deaths from cancer.”

Explained another way, Dahut said, if a person’s life expectancy at birth was 80, a cancer screening may prevent their premature death at 65, but it wouldn’t necessarily mean they’d live to be 90 instead of the predicted 80.

“No one’s saying ‘if you do your cancer screenings, you’re going to live to be 100 years old,’ ” he said. “But we know that cancer is the second leading cause of death, the leading cause of death before age 85.”

Screenings may not allow people to tack on additional years, but they may catch cancer early and give someone a better chance to survive their cancer and go on to live with many more productive, high-quality years within their normal lifespan.

“Preventing cancer-related symptoms and premature mortality are a meaningful thing,” Dahut said.