Age- and Sex-Specific Reference Intervals for Laboratory Parameters from Aging Trajectories in Healthy Beijing Adults (2010–2026)

Every time a doctor orders a blood test, the result is compared against a standard range to decide if a person is healthy or sick. These ranges, known as reference intervals, are supposed to represent the values found in 95 percent of healthy people. For decades, these standards have been applied broadly, often using the same numbers for a twenty-year-old and an eighty-year-old, regardless of how their bodies have changed over time. The underlying assumption has been that what counts as normal for a young adult is also normal for an older adult. However, human biology is not static; it shifts and evolves as we age. Organs function differently, hormone levels fluctuate, and metabolic processes slow down or speed up. When a single, unchanging standard is used to measure a body that is constantly changing, the result can be a misleading diagnosis. A value that is perfectly natural for an older person might be flagged as dangerous, while a value that signals a real problem in a younger person might be dismissed as normal.

A team of researchers at Peking University Third Hospital and the Peking University Health Science Center set out to fix this mismatch. They analyzed laboratory data from nearly 4,400 healthy adults in Beijing, spanning ages from 18 to 91, collected over a period of sixteen years. Instead of treating age as a minor detail, they treated it as the central story of the data. Their goal was to map exactly how blood markers change as people grow older and to identify the specific moments, or "inflection points," where the body's trajectory shifts significantly. By doing this, they aimed to create new, age-specific standards that reflect the reality of aging rather than an idealized, uniform snapshot of health.

The researchers began by carefully selecting their group of participants. They did not simply take anyone who walked into a clinic; they rigorously screened for people who were truly healthy, excluding those with known chronic diseases, long-term medication use, or recent major health events. To avoid a common trap in medical research—where new standards are just copies of old ones because the selection process relies on the old standards—they used a three-step filtering method. First, they ruled out people with specific infectious diseases. Second, they removed individuals with extreme values that clearly indicated a medical crisis, such as dangerously high blood sugar or kidney failure, using thresholds based on disease diagnosis rather than standard lab ranges. Finally, they used statistical tools to separate the healthy population from any remaining outliers. This ensured that the new standards were built on a foundation of genuine health, not just the absence of the most obvious sickness.

Once they had their clean dataset, the team looked for patterns in how 24 different blood markers changed with age. They found that aging is not a straight, slow line for everyone. Instead, they identified four key turning points in life where the body's chemistry shifts noticeably: around age 35, 50, 60, and 70. The most dramatic and consistent shift occurred around age 50, particularly in women, coinciding with the period of perimenopause when estrogen levels change. Before this point, many markers follow one path; after it, they follow another. For example, certain liver enzymes and cholesterol levels rise steadily until midlife and then begin to decline, while other markers, like those related to kidney function, remain stable for decades before accelerating their change in later years.

Using these turning points, the researchers divided the participants into five age groups and calculated new reference intervals for each group, separated by sex. The results showed that the old, one-size-fits-all standards were often wrong. In many cases, the old standards mislabeled healthy older adults as abnormal. A striking example was found in cholesterol levels for women between the ages of 50 and 59. Under the old uniform standard, nearly 60 percent of healthy women in this age group would have been flagged as having high cholesterol. With the new age-specific standards, these women were correctly recognized as having values that are normal for their age. Similarly, for older men, the standard upper limit for a liver enzyme called ALT was too high. The old standard allowed values up to 50 units, but the new data showed that for men over 70, a value above 45 units is actually abnormal, meaning the old standard was missing early signs of liver stress in the elderly.

The study also looked at how these blood markers have changed over the sixteen years the data was collected, a phenomenon known as secular trends. They found that some values have shifted significantly in the population over time. For instance, levels of a liver enzyme called GGT increased by 35 percent, and uric acid levels rose by 17 percent over the sixteen-year period. Interestingly, levels of LDL cholesterol, often called "bad" cholesterol, dropped by 18 percent in this specific group of healthy, medication-free adults. The researchers suggest this drop might reflect positive changes in diet and lifestyle among health-conscious individuals in Beijing, such as switching from animal fats to vegetable oils, rather than a change in the general population.

To test if their new standards actually worked better, the researchers applied them to a separate group of elderly participants from a 2024 survey. When they used the old uniform standards, nearly 15 percent of these healthy older adults were flagged as having abnormal results. When they used the new, age-specific standards, that number dropped to 8 percent, much closer to the expected 5 percent of healthy people who naturally fall outside the normal range. This confirmed that the new standards were better at distinguishing between a healthy aging body and actual disease.

The implications of this work are significant for how doctors interpret blood tests. The study suggests that labeling a post-menopausal woman with high cholesterol as "sick" when her levels are normal for her age could lead to unnecessary treatment and anxiety. Conversely, failing to notice that an older man's liver enzyme levels are creeping up because the standard is too loose could mean missing a treatable condition. The researchers argue that medical standards should not be static; they must evolve as our understanding of human aging deepens. By recognizing that the body changes at specific milestones and that these changes are different for men and women, doctors can make more accurate decisions. The study concludes that these new, age-stratified intervals offer a clearer, more honest picture of health, reducing the number of false alarms and ensuring that real problems are not overlooked in the noise of normal aging.