Menstrual Product Absorbency Has Been Misunderstood for Decades—This Study Changes That

Research reveals shocking discrepancies in how period products are tested—and what that means for women's health.

For decades, women and people who menstruate have been told to rely on absorbency labels found on pads, tampons, menstrual cups, discs, and period underwear. These labels are meant to guide consumers in managing their flow—but they’ve been based on a major scientific flaw: the use of saline, not blood, during testing.

Now, for the first time, researchers have tested these products using actual blood. The results, published in BMJ Sexual & Reproductive Health, reveal that most period products absorb significantly more or less than advertised—and the implications for menstrual health diagnosis are profound.

Dr. Bethany Samuelson Bannow, a hematologist and assistant professor at Oregon Health & Science University, co-led the groundbreaking study. She was driven by a growing trend in her practice: patients with heavy menstrual bleeding were increasingly turning to reusable menstrual cups and discs, reporting that traditional products just didn’t cut it. But despite the popularity of these newer options, there was no accurate medical metric for diagnosing heavy bleeding based on their usage.

“Patients were switching to discs and cups because they could hold more,” said Samuelson Bannow. “But our diagnostic standards didn’t account for that—and we realised the entire system for measuring menstrual flow was flawed.”

Why Saline Doesn’t Cut It

Historically, absorbency testing has relied on saline solution, a watery, uniform liquid far removed from the reality of menstrual blood. Menstrual blood is thicker and more variable, containing not just blood but also uterine tissue, mucus, and other secretions. Its composition changes from person to person—and even throughout a single period.

“Blood is literally thicker than water,” said Samuelson Bannow. “And that matters when we’re testing products designed to hold it.”

The reliance on saline stems from research protocols developed in the 1980s, following the rise in toxic shock syndrome cases linked to tampons. These early tests set the foundation for absorbency ratings still in use today. But with saline being far easier, cheaper, and safer to use than blood, manufacturers stuck with it—despite the inaccuracy.

What the Study Found

In a controlled lab setting, the research team tested 21 menstrual products—including tampons, pads, discs, cups, and underwear—using actual blood.

The findings were startling:

• Period underwear absorbed the least: just 1–3 mL, depending on size.

• "Light" pads absorbed 3–4 mL.

• Tampons absorbed 20–34 mL, varying by brand and flow rating.

• "Heavy" pads, advertised as holding 10–20 mL, actually held up to 52 mL.

• Menstrual discs held the most: up to 80 mL, with an average around 61 mL.

  
  

These discrepancies matter. Currently, heavy menstrual bleeding is diagnosed based on visual assessments (the Pictorial Blood Loss Assessment Chart, or PBAC) and frequency of product changes. For example, using two 20-mL tampons within two hours is considered a sign of heavy flow.

But if a menstrual disc holds more than three times that amount and is worn for hours, heavy bleeding might go completely undiagnosed.

“We’re probably missing a lot of people with heavy menstrual bleeding,” Samuelson Bannow said.

Why This Matters

Heavy menstrual bleeding isn’t just inconvenient. It can signal serious underlying conditions—such as fibroids, endometriosis, bleeding disorders, and even some cancers. It’s also a major quality-of-life issue, leading to missed work or school, sleep disruptions, and emotional distress. Left untreated, it can cause chronic iron-deficiency anemia and debilitating fatigue.

Dr. Candace Tingen, of the Gynecologic Health and Disease Branch at the U.S. National Institute of Child Health and Development, calls the findings “a game-changer.”

“This isn’t just about the products—it’s about how we diagnose and care for people,” Tingen said. “It’s time we revisit our standards.”

Why Isn’t Blood Used More Often?

The answer lies in cost, safety, and logistics. Blood is considered a biohazard, requiring trained personnel, safety protocols, and high costs—up to $100 for a 10-mL vial of research-grade blood. Synthetic blood is also expensive. Meanwhile, saline is cheap and easy: about $45 for a full litre.

But relying on convenience over accuracy means failing millions of menstruators. The medical system continues to lag behind in research and development when it comes to women’s health, often relying on outdated assumptions and flawed tools.

The Bigger Picture

This study underscores a broader truth: menstruation remains under-researched, underfunded, and surrounded by stigma. Conversations about menstrual health are often siloed into the “feminine hygiene” aisle, instead of being treated as vital to public health and gender equity.

“If we want better care, better products, and better outcomes, we need to break the silence,” said Samuelson Bannow. “We need to start talking about periods—not just in our homes, but in our clinics, labs, and boardrooms.”

The Female Body is committed to spotlighting research like this that challenges outdated medical standards and pushes for change. As Tingen put it: “The science isn’t settled—and that means we still have work to do.”