Reference range

How ranges are set

A lab tests many people who are presumed healthy and records the spread of results. The reference range captures where most of those results fall. By design, some perfectly healthy people land just outside it, which is why MedlinePlus cautions that the word "normal" can be misleading.

Ranges are often split by age and sex, and sometimes by pregnancy, smoking or altitude. Hematocrit, IGF-1 and testosterone are good examples of tests where the right comparison group matters a lot.

Why your lab's range is the one to use

Different labs use different instruments and methods, so they publish different ranges for what looks like the same test. Compare your result only with the range printed on your own report. When you track a marker over months or years, sticking with one lab makes changes easier to interpret.

Reference ranges versus decision thresholds

Some markers are judged against cutoffs set by medical guidelines rather than the healthy-population range. A total testosterone below 300 ng/dL is the American Urological Association's reasonable cutoff for low testosterone. For vitamin D, the National Academies describe 20 ng/mL as adequate for most people, while the Endocrine Society's 2024 guideline declined to set a target level at all.

You may also see "optimal" ranges promoted by wellness companies. These are usually narrower and not always backed by outcome evidence, so ask what research supports them.

Things that can push a result out of range

• Food and drink, or not fasting when asked to.
• Hard exercise shortly before the draw.
• Medicines and supplements, including biotin.
• Your menstrual cycle phase or time of day.
• Recent illness, dehydration or poor sleep.

How to use the range wisely

A single flagged value is a prompt to look closer, not a diagnosis. Your clinician weighs the size of the deviation, the trend over time, your symptoms and other tests. At HealifyMD, your clinician reviews every result with you and explains which ones matter for your plan.

Bringing results to your visit

If you have results from earlier years or other labs, bring them. Your clinician can line them up, account for differences in method and units, and look for patterns that a single report cannot show. Note any fasting, medications, illness or hard training before each test so unusual values can be explained rather than chased with more testing.