Most convenience stores or pharmacies have an aisle dedicated to “feminine hygiene” filled with shelves of various menstrual products such as tampons, pads, cups and discs. On the side of each package, you’ll find a small “absorbency” label proclaiming how much liquid blood the product is designed to hold.

But that number can be misleading, according to a recent paper published on August 7 in BMJ Sexual & Reproductive Health. A team of researchers at Oregon Health & Science University found that many menstrual products had a much lower or higher liquid capacity than advertised when real blood was used instead of a saline solution—a mixture of water, salt and bicarbonate that is more commonly employed in the product development process. This is the first known study to test the absorbency of period products with blood, the researchers say. The results suggest that doctors may be underdiagnosing heavy menstrual bleeding, which can be a sign of certain medical conditions.

Study co-author Bethany Samuelson Bannow, a clinician and an assistant professor of hematology, started to investigate menstrual product capacity after she noticed an increase in the number of her patients who opted for reusable menstrual discs and cups—insertable products shaped like shallow bowls and narrower cups, respectively, that rest below the cervix and collect blood—in place of disposable pads and tampons. These patients switched partly to reduce waste, but Samuelson Bannow suspects they also did so to better deal with heavy menstrual flow. “We realized that there wasn’t really a metric for diagnosing heavy menstrual bleeding in folks who use those products,” she says.

Estimates of the capacities of many products were based on decades-old tampon research. In the 1980s, a tampon task force was convened to help develop safer menstrual products after a particular brand of tampons was found to slightly increase the risk of toxic shock syndrome, a serious medical condition that occurs when certain strains of bacteria from outside the circulatory system enter the bloodstream and release toxins that can cause severe organ damage or death. This research set the standard for modern tampon absorbency. Additionally, many of those initial studies used a saline solution rather than menstrual blood. Saline solutions are still predominantly employed to test menstrual products today.

Blood and saline solution have very different viscosities, meaning they are absorbed at different rates. “The saying ‘blood is thicker than water’ is technically true,” Samuelson Bannow says. “We really felt that [the absorbency metric] needed to be updated.”

Saline solution is a homogenous mixture that is only slightly more viscous than distilled water. Blood, however, is not homogenous. It is full of cells and platelets, as well as proteins and other molecules that move around and vary in concentration depending on a myriad of factors, such as how hydrated a person is. Menstrual blood is even more variable than circulating blood because it also contains vaginal secretions and tissue that has been shed from the uterine wall. Its viscosity can change from person to person and even hour to hour.

These characteristics make it hard for manufacturers to accurately approximate the amount of blood a given period product will absorb based on tests that primarily use saline. In the new study, Samuelson Bannow and her team tested 21 different menstrual products, including discs, tampons, pads, period underwear and cups. They found that menstrual underwear products were the least absorbent ones on the market, soaking up one to three milliliters of blood, depending on size. “Light” pads were slightly more absorbent, maintaining 3 to 4 ml. Tampons held between 20 and 34 ml, depending on the brand and flow rating, and “heavy” pads, which advertised 10- to 20-ml capacity, could hold up to 52 ml. Menstrual discs held the most blood—61ml on average—with one brand holding up to 80 ml.

The results startled researchers. Doctors typically diagnose heavy bleeding based on either a visual chart—called the pictorial blood loss assessment chart (PBAC)—or the frequency with which an individual needs to change their period product. Using two or more tampons that absorb 20 ml of blood within a two-hour time frame is considered a heavy flow. But a heavy bleeding metric based on menstrual discs—which can hold about four times as much as a tampon and are used by many people with intense periods—hasn’t been established.

This could mean “we’re probably missing a lot of heavy menstrual bleeding,” Samuelson Bannow says. It’s important to catch this type of excessive bleeding, she adds, because this can be a sign of more serious conditions, including bleeding disorders, certain types of cancer and fibroids—smooth muscle tumors in the lining of the uterus. It can also lead to anemia, which manifests as extreme fatigue. Heavy bleeding during menstruation is also “really a mental health and quality of life issue,” says Candace Tingen, a researcher at the Gynecologic Health and Disease Branch at the National Institute of Child Health and Development, who was not involved in the study.

So why don’t period product manufacturers test absorbency with blood for better accuracy?

Blood is a precious commodity. Approximately five million people in the U.S. need a blood transfusion every year, according to the National Institutes of Health. Even though about 6.8 million individuals donate blood annually, not all of those donations are compatible with the blood types of those who need them, and donations have a short shelf life (six weeks for red blood cells and five days for platelets). Frequent blood shortages in hospitals make it difficult to justify using donations for anything other than transfusions. The fluid is also considered a potential biohazard. Researchers who handle blood in the lab must undergo training and take special precautions to work with it safely. In contrast, “you don’t have to have any special training to work with saline,” Tingen says. Additionally, blood—and artificial blood—is expensive. A 10-ml vial of research-quality human blood costs around $100. Meanwhile 200 ml of high-quality synthetic blood—a lab-made mixture of amino acids and hemoglobin—can run upward of $150. In contrast, labs can buy a full liter of saline solution for $45.

These barriers don’t mean that period product companies shouldn’t update their standards, however—or that doctors shouldn’t update their charts for diagnosing heavy bleeding. “I think these results are very good justification for revisiting the PBAC and making it more applicable,” Tingen says. “The science isn’t settled.”

Tingen and Samuelson Bannow say these types of studies can help break down the stigma around menstruation. Discussing menstrual health can lead to better care, the development of better products and improved gender equity in health care, Samuelson Bannow says. “The main thing is just to get people talking about their periods,” she adds.