Her 2003 report, "The Quality of Health Care Delivered to Adults in the United States,” was first to empirically measure the disparity between care expected and care received in the U.S. The 10-year study polled 13,000 adults in 12 metro-areas, and considered 439 quality-criteria for 30 acute and chronic conditions. Conclusion: American adults receive 55% of recommended care for illness like asthma, cancers and heart failure. Her subsequent studies indicate the ratio is consistent, regardless of geography, socioeconomic status, race, gender, or insurance status. A 2007 report shows children fare even worse, receiving 47% of recommended care. Now called the Community Quality Index Study, McGlynn's work influences current research into geographic disparities of healthcare spending and evidence-based medicine. She is also the force behind RANDCompare, a performance measurement engine, based on her research.

"People have a hard time agreeing on what quality means because it's multi-factorial; it's in the eye of the beholder. But that 55% figure was a clear signal that things didn’t look very good in the U.S. The report's major contribution was allowing us to get on with the conversation. There was a shift from 'do we have a problem or not?' to 'we have a problem and lets get on with it.'"