Reference Ranges vs "Optimal" Ranges

What a reference range is

When a lab report shows a range next to your result, that range is usually a reference interval. It is a selection of results, usually the middle 95%, measured in a well-defined reference population. [1] Laboratories ideally set their own ranges by testing at least 120 people without discernible disease for each relevant age, sex and other group. [2]

Two consequences follow:

• By construction, about 5% of healthy people fall outside the range, 2.5% above and 2.5% below. [1][2]


• A result outside the range does not necessarily indicate disease. [3]

Reference interval vs decision limit

Not every number on a report works the same way. The IFCC committee on reference intervals distinguishes two ideas: reference intervals describe the typical results in a healthy reference population, while clinical decision limits are associated with a significantly higher risk of adverse outcomes or are diagnostic for a specific disease. [3] Cholesterol, glucose and hemoglobin A1c are examples interpreted using decision limits. [3] A doctor can explain which kind applies to your test.

Why the same number can look different

Ranges depend on testing method, age, sex, ethnicity, season and random variation, and laboratories differ. [2] Reference populations also matter: a reference group of mostly white adults aged 25 to 60 would be unsuitable for a population with many ethnicities and a wider age range. [1] Standard ranges may also accidentally include people with undetected disease. [1] For repeated testing, the review suggests using the same laboratory because different methods add imprecision. [2] See Reference Ranges and Repeatability.

"Optimal" ranges

Wellness, longevity and functional-medicine reports often add a narrower "optimal" band inside the lab's range and flag results outside it. Label: marketing claim, with no standard definition that we found. We searched PubMed through Europe PMC and the professional sources above and found no accepted definition of an "optimal range" for healthy people to set against reference intervals or decision limits. Proper evidence not found.

What we can say from the sources:

• A decision limit is tied to outcomes; a reference interval is tied to the distribution in healthy people. [3] An "optimal" band is neither unless the seller shows outcome data.


• Redefining a threshold can create large numbers of new patients. Lowering the cholesterol threshold from 240 to 200 mg/dl was described in a review as creating 42 million new cases eligible for treatment in the US overnight. [4] Narrower bands for the wellness market can widen "abnormal" in the same way.


• A number moving toward a target is not the same as better health; see Biomarker Changes vs Meaningful Health Outcomes.


• Where studies do link a level to outcomes, the result is specific to the test, the population and the outcome, and we do not turn it into a personal target.

Questions to ask a report

1. Is this a reference interval or a decision limit?
2. Who was the reference group, and is it like me?
3. What outcome evidence supports the "optimal" band?
4. What action does the report want me to take, and has a clinician confirmed it matters?

We do not interpret uploaded or personal results. For lab-testing context see Biomarker Tracking and At-Home Tests and Home Lab Testing and Longevity Clinic Panels.