Where do "optimal" lab ranges come from?

A lab range comes from one of three places: a threshold set by a clinical guideline or public-health body, a reference interval measured by a laboratory in a healthy population, or a narrower target used in functional and integrative practice. Many "optimal range" charts mix all three without saying which edge came from where. You can usually work it out by asking what the number was based on, and whether anyone has tied it to an outcome.

I build Sera, a lab-review workspace for practitioners, and its range set is the source of the examples below. I've used it because every figure in it is already linked to the source it came from, so you can check my working.

The three kinds of source

Guideline and public-health thresholds. These are cut-offs a body such as the American Diabetes Association or the World Health Organization uses to diagnose a condition or decide on treatment. They are usually built from outcome data: the point where risk starts to rise, or where a test best separates people with a condition from people without it. They are the strongest kind of edge, and they still differ between countries. Fasting glucose is a good case: the ADA's line, NICE's line and Diabetes Canada's line are not the same number (fasting glucose sources).

Laboratory reference intervals. This is the range printed next to a result on the report. A laboratory measures a group of healthy people on its own equipment and takes the range that covers most of them. It describes what is common in that group. It does not say what is ideal, and it can shift with the assay, the population and the lab.

Practitioner and functional targets. These are narrower bands used in functional, integrative and longevity practice, usually the part of the reference interval a practitioner prefers to see a client in. Some rest on cohort data. Many rest on clinical experience and repetition, and practitioners don't all use the same ones. That's not a reason to throw them out. It is a reason to label them.

How to tell which one you're looking at

When you meet a range on a chart, a handout or a website, these questions sort it quickly:

1. Is a source named for each edge? Not for the range as a whole, for each edge. The low end and the high end often come from different places.
2. Is the source a guideline, a lab, or a person? A guideline names a condition or risk. A lab names an assay and a population. A person or a course names neither.
3. Does the number match the source? Open it. A chart can cite a guideline whose figure is not the one on the chart.
4. Is it the same unit as your result? A vitamin D range in ng/mL read against a result in nmol/L is wrong by a factor of about 2.5.

If a range can't answer the first question, treat it as an opinion until you find where it came from.

Worked example: vitamin D

Sera's vitamin D page uses an optimal band of 50 to 80 ng/mL (125 to 200 nmol/L) and a sufficient band of 30 to 100 ng/mL. Each edge has a different origin:

• 20 ng/mL is where the US National Academies consider most people sufficient, as the NIH Office of Dietary Supplements sets out. The same source notes possible adverse effects above 50 ng/mL.
• 30 ng/mL, Sera's lower sufficient edge, is where the older Endocrine Society guideline placed sufficiency. So a result of 20 to 29 is flagged in Sera even though the National Academies would call it adequate.
• 50 to 80 ng/mL, the optimal band, is a functional target. It sits above what mainstream bodies recommend. The 2024 Endocrine Society guideline found no clear evidence defining an optimal level, and Mayo Clinic Laboratories lists 51 to 80 ng/mL as a band with increased risk of hypercalciuria.
• 100 ng/mL is the top of Labcorp's reference range.

That is three kinds of source in four numbers. A chart that shows "optimal 50 to 80" with no note hides the fact that a major reference lab treats the same band as a caution zone.

Worked example: TSH

On the TSH page, the sufficient band of 0.5 to 4.5 mIU/L is the traditional reference range as the American Thyroid Association describes it. Laboratories print slightly different limits for their own assays: Mayo Clinic Laboratories uses 0.3 to 4.2, Labcorp 0.45 to 4.5, and a UK NHS laboratory 0.38 to 5.33.

The optimal band ends at 2.0. That's a functional target with no guideline behind it. The best outcome data point slightly the other way: in pooled data on more than 134,000 adults, cardiovascular and mortality risk was lowest at a TSH of roughly 1.9 to 2.9 mIU/L, not below 2. TSH also follows a daily rhythm, highest overnight and in the early morning, so the time of the draw matters when you compare results.

Worked example: ferritin

Ferritin has more guideline support than most markers, and its edges show it:

• 30 ng/mL, the lower sufficient edge, is where the American Society of Hematology's 2026 guideline diagnoses iron deficiency in adults. The WHO threshold of 15 is lower and more specific.
• 50 ng/mL, the lower optimal edge, is ASH's threshold for higher-risk groups, such as people with heavy menstrual bleeding.
• 200 in women and 300 in men, the upper sufficient edges, are where European liver guidelines prompt a check for iron overload when transferrin saturation is also raised.
• 150 ng/mL, the optimal ceiling, is a functional target with no outcome evidence behind it.

Ferritin also shows why one number is rarely enough. It rises with inflammation regardless of iron stores, so ASH considers iron deficiency possible up to 100 ng/mL when inflammation is present. A ferritin in the "optimal" band can still sit beside a low transferrin saturation.

How Sera grades its ranges

Every range Sera uses carries one of these evidence grades, with a note that says which edge came from where:

In the examples above, ferritin is graded as an established threshold, TSH as a published reference interval and vitamin D as a functional target. The grade describes the marker's bands as a whole. The note is where you see that ferritin's optimal ceiling, for one, is still a target.

What to do with your own targets

If you use your own ranges, write down the source for each edge, the date you chose it, and which kind of source it is. It makes it much easier to explain a target to a client, or to revisit it when a new guideline appears, as the ASH iron deficiency guideline did in 2026.

None of this tells you what a result means for a particular client. A range is a reference point. Reading it in context, and deciding when to refer, is the practitioner's job.

Where Sera fits

Sera publishes every range it uses, more than 280 biomarkers across 12 health areas, each with its sources and an evidence grade. A client's results are shown against those bands as optimal, sufficient or needs attention, over years and across labs. It keeps each result in the unit the lab reported. It doesn't diagnose or suggest treatment.

It's free for three clients, with no card. There's a 3-minute tour if you'd rather watch first.

Alina Radu is the founder of Sera and a software engineer. The idea started with her own Hashimoto's tests, scattered across years without the big picture.