Guide to Biological Age Testing

WRITTEN BY

The Levels Team

Updated: 05/22/2025|5 min read

Summary

Your biological age measures how old your body appears physiologically based on nine blood biomarkers—including glucose, creatinine, albumin, and inflammatory markers—compared to your chronological age. A lower biological age suggests slower aging and reduced disease risk, while a higher biological age may indicate accelerated aging processes.

What This Is Telling You

Your biological age, calculated using the PhenoAge method, shows how old your body appears physiologically based on key blood biomarkers, rather than how many years you've been alive (your chronological age).

When your biological age is less than your chronological age, your body may be aging more slowly than average, which researchers associate with a lower risk of age-related diseases and mortality (death). Conversely, a biological age that's higher than your chronological age may indicate accelerated aging processes, potentially signaling a higher risk for age-related health issues.

While the PhenoAge measure provides valuable directional insight, you should view it as one piece of your health puzzle rather than a definitive verdict. The calculation has inherent limitations and works best when interpreted alongside other health metrics and clinical evaluations.

How It's Calculated

The PhenoAge calculator uses nine specific blood biomarkers, plus your chronological age, to estimate your biological age:

• Albumin levels (liver function)
• Creatinine (kidney function)
• Glucose levels (metabolic health)
• C-reactive protein (inflammatory marker)
• Lymphocyte percentage (immune function)
• Mean red cell volume (blood health)
• Red cell distribution width (blood cell variability)
• Alkaline phosphatase (liver/bone health)
• White blood cell count (immune system activity)

Researchers selected these biomarkers because they collectively capture different physiological systems relevant to aging and mortality risk. The algorithm they created combines these values using a sophisticated statistical model developed by following large groups of people over many years (longitudinal studies), tracking both their biomarker levels and whether and when they died. This approach helped identify which biomarkers are most strongly associated with mortality risk, and how they could mathematically combine these biomarkers to create the best predictor of lifespan.

The Science Behind the Calculation

Unlike epigenetic clocks, which measure DNA methylation patterns (chemical modifications to your DNA that change with age), the PhenoAge calculator uses only standard blood biomarkers. This distinction matters---epigenetic clocks examine your specific genetic aging markers at a cellular level, while PhenoAge looks at circulating markers in your bloodstream that reflect your overall physiological state.

The term "phenotypic age" refers to an estimated age based on observable or measurable characteristics (phenotypes) rather than genetic information. It captures how various systems in your body function compared with population norms for different ages.

Researcher Morgan Levine, PhD, and colleagues at Yale University developed the PhenoAge method in 2018. Their approach involved a two-step process:

1. First, they analyzed data from the National Health and Nutrition Examination Survey (NHANES)---a massive nationwide pool of health data---to identify which combination of clinical biomarkers best predicted mortality risk. In this initial development phase, they used statistical methods to create a formula that combined these biomarkers to estimate what they called phenotypic age.
2. Second, they tested this formula in completely different datasets (independent population samples) that weren't used to create the original formula. This validation step confirmed that the PhenoAge formula successfully predicted various health outcomes beyond just mortality, including specific diseases, physical functioning, and comorbidities. This step was crucial to prove that their formula wasn't just picking up on statistical quirks in the original NHANES data but actually captured meaningful biological aging processes that applied universally.

Research demonstrates PhenoAge functions as a robust predictor of health outcomes. In the original study, each one-year increase in PhenoAge above chronological age was associated with a 9% increase in mortality risk. Independent research confirmed these findings, with one study showing that when predicting who would die within 10 years, PhenoAge correctly classified people about 9% more often than using chronological age alone. This significant improvement in predictive accuracy demonstrates why biological age offers more insight than simply knowing how many birthdays you've had.

PhenoAge also strongly predicted comorbidity burden (the total number of chronic conditions a person has simultaneously), with each five-year increase in biological age associated with approximately one additional chronic condition.

Limitations of Biological Age Calculations

While biological age calculations like PhenoAge provide valuable insights, they have several important limitations:

• Point-in-time measurement: Your biological age represents a snapshot of your health status at the time of testing. It can fluctuate based on recent lifestyle changes, illness, or even time of day when blood is drawn.
• Population-based, not personalized: The calculator relies on statistical relationships derived from large population studies, not your unique biology. It doesn't account for your specific genetic makeup, family history, or unique environmental exposures that influence how you personally age.
• Limited systems capture: While the nine biomarkers span multiple physiological systems, they don't capture all aspects of aging that predict longevity. Important dimensions like neurological health, muscle mass, cardiovascular fitness, or cellular senescence don't factor into the calculation, even though these critically determine lifespan and healthspan.
• Disease state detection gaps: The calculation may not fully capture current disease states unless they specifically affect the nine biomarkers measured. Many serious health conditions might not significantly impact these particular markers, especially in early stages.
• Relative, not definitive: You should interpret the difference between your biological and chronological age as a relative risk indicator rather than an absolute measure. For example, a biological age that's five years higher than your chronological age doesn't mean you've precisely "lost five years of life"; rather, the gap suggests you have aging-related changes comparable to someone who is five years older. Small differences (such as a three-year gap) may fall within normal variation.
• Not diagnostic: Biological age does not function as a diagnostic tool for specific conditions. An elevated biological age should prompt further investigation rather than conclusions about particular diseases.

For the most comprehensive understanding of your aging process, consider biological age alongside other health metrics, comprehensive medical evaluation, and lifestyle factors.

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