03/20/2026
THIS IS A CONCEPT THAT A LOT OF PEOPLE DON'T THINK ABOUT. BUT IT'S ACTUALLY CRITICAL TO UNDERSTANDING THE MEANING OF YOUR LAB VALUES.
On a lab sheet you will see a column for reference range. But there are actually two different ways to derive a reference range and they are fundamentally different.
The first is a doctor recommended range. A good example of this is LDL cholesterol, where the recommended range is less than 100 mg/dl. Generally when you see recommended ranges they are nice round numbers because they are basically just a line in the sand to trigger conversation. For example, the reference range for LDL cholesterol is 100, for total cholesterol it is 200, and for triglycerides it is 150. When the lab value falls outside recommended range such as an LDL cholesterol of 105, the number turns from black to red on the lab sheet to get everyone's attention..
Because a recommended range does not change, the number of people who fall outside the range depends upon how unhealthy the population is. If you test a population of very healthy men you will have fewer red numbers than in an obese population, for instance. So the reference range is fixed and the percentage of red numbers is variable depending on the health of the population tested.
But the exact opposite applies to labs like testosterone.
We don't actually have a recommended value for free testosterone. So we derive the reference range by doing a population sample. These cutoffs are frequently fractions rather than whole numbers. These ranges are mathematically guaranteed to always encompass approximately 95% of men because by convention the range is 2 standard deviations from the mean in either direction. Every few years a new sample is taken and a new bell curve is generated. So as the population gets sicker the reference range will reset and the bell curve will move to the left. So unlike with recommended ranges, the reference range moves with the health of the population while the number of red results stays stable at approximately 5%. This means that a guy who falls within the reference range in 2026 may have fallen below the reference range in 1980!