Interpreting Test Results Without Overinterpreting

What this page covers

Longevity and wellness culture often presents blood results as scores to optimize. This page explains, in general terms, how test results are built and why they are easy to over-read. It is information only. It will not tell you what your own number means, what range to aim for, or what to do about a result. Your clinician, who knows your history, is the right person for that. Evidence labels used: established, insufficient evidence and marketing claim.

What a reference range is

A reference range is the set of numbers that form the low and high ends of results considered normal, based on test results from large groups of healthy people. [1] MedlinePlus notes that because ranges vary, the term "normal" can be misleading. [1] Two consequences follow:

• A result outside the range may or may not signal a health problem, and it is common for healthy people to sometimes have results outside the reference range. [1]
• A result inside the range is not always a guarantee of good health. [1]

"Normal" versus "optimal"

Some wellness services say a result can be "normal" yet not "optimal" and offer narrower target ranges. In the sources read for this page, Proper evidence not found that shows narrower personal targets improve outcomes for healthy people. What we did find:

• On vitamin D, the Endocrine Society says more research is needed to determine optimal levels for specific health benefits. [2]
• On testosterone, two professional bodies use different reference approaches: one uses a range established in young men, the other uses age-standardized ranges. [3]
• Authors reviewing wellness testing programs questioned the need to "improve" biomarker levels. [4]

Label: calling a narrower range "optimal" is a marketing claim unless a guideline or trial supports the target. Where a source states a range, it is a laboratory or guideline convention, not a personal goal.

Why numbers differ between labs and days

• Labs often use different reference ranges and methods, so you cannot compare your result with ranges found elsewhere or with results from a different lab. [1]
• To follow a trend over time it helps to use the same lab. [1]
• Test results are measured in different units, so check the units on the report. [1]
• Results can be affected by medicines or supplements, hard exercise before the test, and whether preparation instructions such as fasting were followed. [1]

Healthy bodies also fluctuate: in a study of 60 people without diabetes or prediabetes, glucose sensors showed a median of 1.6% of time below 70 mg/dL. [5]

Accurate is not the same as useful

The FDA separates analytical validity (does the test measure what it claims) from clinical validity (does the measurement predict a state of health). [6] A third question is whether acting on the result improves outcomes. A change in a biomarker is not the same as a better health outcome; see Biomarker Changes vs Meaningful Health Outcomes.

False alarms and missed problems

No test is perfect. A false positive shows a condition you do not have; a false negative misses one you do. [1] Symptoms and results can also disagree: the NHS notes that some people have B12 deficiency symptoms with seemingly normal levels, and some have low levels without symptoms. [7] For these reasons clinicians combine results with a physical exam, history and other tests rather than reading one number alone. [1]

Single numbers and "biological age"

Panels and apps that combine many results into one "age" or score involve the same issues, with more moving parts. See Biomarkers and Biological Age Tests and Biological Age Clocks Compared.

What to do with an unexpected result

The sensible step is to share it with the clinician who knows your health, not to change supplements or medicines on your own. MedlinePlus advises talking with your provider about results, even if they are normal, and says at-home tests should never replace testing your provider orders. [1]

Questions to bring to a clinician

• Was this the right test for my situation, and does the result change my care?
• Is the result in range for my age, sex and the lab used?
• Could medicines, supplements, fasting or recent exercise explain it?
• Should the test be repeated, and when?
• What is the chance this is a false alarm?

Related guides

See Home Lab Testing and Longevity Clinic Panels, Hormone and Nutrient Testing in Aging and Standard Blood Tests and Healthy Aging.