About 40 percent of women between the ages of 40 and 74 years old have dense breasts and many states require providers to tell women who have mammograms if that shows up on their scan. But a new study in the Annals of Internal Medicine has found that information isn’t always disclosed.

“The likelihood of a woman being told she has dense breast varies substantially according to which radiologist interprets her mammogram,” according to an abstract of the study.

The information is important because it could be a factor in developing breast cancer, along with family history of breast cancer, a personal history of breast abnormalities, age of first period and age when first giving birth. That is likely the reason about half of states in the U.S. require providers to tell women if they have dense breasts.

It’s also important because dense breasts can make it harder for radiologists to see any abnormalities on the mammogram and often refer patients for additional screening like ultrasound.

With the objective of determining variation in breast density assessment among radiology practices, the researchers looked at 30 radiology facilities within the three breast cancer screening research centers of the Population-based Research Optimizing Screening through Personalized Regiments (PROSPR) consortium. Radiologists who interpreted at least 500 screening mammograms between 2011 and 2013 were included in the study, resulting in data from 216,783 mammograms from 145,123 women between the ages of 40 and 89, according to the abstract.

The survey found 36.9 percent of the mammograms were rated as showing dense breasts but the “proportion of mammograms assigned to those two categories by individual radiologists ranged from 6.3 percent to 84.5 percent,” according to the NPR. The participants in the study were 83 radiologists in Pennsylvania, Vermont, New Hampshire and Massachusetts.

“Part of the message is that the assessment of dense breasts is subjective,” lead author Brian Sprague of the Office of Health Promotion Research at the University of Vermont in Burlington told Reuters. “The paper highlights the disconnect between density laws and breast density measurement, it was never intended to be something that really dictated screening decisions.”

Doctors not involved in the study say women should not be overly concerned with the findings because there is usually a lot of variation, particularly in the middle, according to Dr. Priscilla Slanetz, a radiologist at Beth Israel Deaconess Medical Center.

She told NPR.org that women should “use the density report as a jumping-off point for a broader discussion” about their risk for breast cancer.

Dr. Norman Boyd of the University of Toronto in Canada, who was also not involved in the analysis, agreed. He told Reuters dense breast tissue is “not a diagnosis, just a description.” He noted that automated methods are being developed now for use with digital mammograms that will probably be much more reliable in the future.

Slanetz agreed those women with dense breasts might want to get digital mammography as well. Other screening options for women with dense breasts include ultrasound, digital breast tomosynthesis and MR, according to the NPR report.

One of the conclusions of the study was that the wide variation in density assessment should be “carefully considered by providers and policymakers when considering supplemental screening strategies.”