Normal range vs optimal range

Where the lab’s “normal” range actually comes from

This surprises almost everyone. The range printed beside your result is not a medical standard handed down from a panel of experts. It’s a statistic the laboratory calculated.

A 2025 review in the Journal of the American Board of Family Medicine describes the method: “120 healthy individuals are tested… The lowest 2.5% and highest 2.5% of the values are excluded and the values comprising the central 95% are adopted as the reference range.”

Read that again, because the consequence is the part nobody explains. If you keep the middle 95% of healthy people, you have just defined 5% of healthy people as abnormal. The same paper says it plainly: “By definition, 5% of normal/healthy people will have an abnormal result.”

So a flagged result does not mean something is wrong, and an unflagged result does not mean everything is fine. The authors’ conclusion is worth keeping: “it is not generally appreciated that the normal values reported by the testing laboratory are not a standard but derived through a process with many assumptions.”

Why two labs give you two different ranges

Because each lab builds its own, from its own group of people, on its own equipment. The differences are not small. The same review found that between laboratories using the same methods and instruments, the difference in the upper and lower limits of acceptable results reached 35% for creatinine.

Age and sex matter too, and the paper gives a clean illustration: a hemoglobin of 12.0 g/dL is normal in an adult woman, while a newborn girl with that same number is considered anemic. One number, two meanings.

This is why every guide on this site tells you to compare your result to the range printed on your own report, and not to a number you found online. Including any number you find here.

Vitamin D: watch the line move

Vitamin D is the clearest case of all, because you can watch the recommended number change over time — and then watch the largest body in the field decline to give one at all.

So the line climbed for about fifteen years — and then, in June 2024, the body that had raised it in the first place stepped back. Its own summary says: “the optimal vitamin D intake and serum 25(OH)D concentrations for disease prevention remain uncertain.” One of the guideline’s companion papers is titled “Vitamin D Insufficiency and Epistemic Humility.”

That 2024 guideline did not come from a fringe committee. It was co-sponsored by the American Association of Clinical Endocrinology, the European Society of Endocrinology, the American Society for Bone and Mineral Research, the American Society for Nutrition and five other organizations.

Instead of naming a target, it recommends supplementation without testing for particular groups — including adults aged 75 and over, to lower mortality risk — and recommends against routine testing for everyone else.

What this means if you’re looking at your own number

Take a result of 25 ng/mL. Under the NIH figure it is adequate. Under the 2011 Endocrine Society thresholds it is insufficient. Under the 2024 guideline there is no threshold to apply. Same blood, same day, three different answers — and none of them is a mistake.

The choice of line also moves enormous numbers of people between categories. In one studied population, shifting the cutoff from 20 to 30 ng/mL raised the share classified as deficient to 70.6%.

Two limits are worth knowing at the top end, because “higher is better” is an easy assumption to make. NIH places a caution at above 50 ng/mL, calling such levels “too high and might cause health problems.” And in older adults specifically, research on falls and fractures describes a U-shaped pattern, where both too little and too much are associated with higher risk. Researchers who favor higher targets dispute where that upper limit sits. That disagreement is real, and it is not settled.

Which is the honest summary of this whole page: the experts do not agree, the guidance has moved before and may move again, and your result means something only in the context of you.

The case that the trials asked the wrong question

A group of researchers argues that the null results above don’t mean what they appear to mean — not because the trials were dishonest, but because they were built the way drug trials are built, and vitamin D isn’t a drug. It’s a nutrient, and nutrients behave differently. This argument deserves a hearing on its merits.

A drug trial makes two assumptions that don’t hold here

A conventional trial assumes the study is the only source of the substance being tested, and that more of it produces proportionally more effect. Neither is true of vitamin D. Your body makes it from sunlight, you eat some in fortified food, and participants can take their own supplements — so the “placebo” group is never actually at zero.

The second assumption matters more. As one review of the argument puts it, responses to nutrients “follow a sigmoid curve, with little change in the intended effect… until a critical point is reached.” In plain terms: topping up someone who already has enough does very little, while correcting a real shortage can do a lot. The effect isn’t spread evenly across the dose.

There’s a practical consequence. Drugs can be tested across a thousandfold range of doses, so a trial can work in the steep part of the curve. Nutrients operate across a range perhaps ten or twenty times from bottom to top, which leaves far less room to find the part where the effect lives.

What the critics say a proper trial would do

• Measure everyone’s level before starting, rather than recruiting whoever volunteers.
• Enrol only people who are actually low. Giving vitamin D to the already-sufficient tests nothing.
• Give enough to move them into the range being tested.
• Measure the level they reached, and analyze the results by that achieved concentration rather than by the dose on the bottle.

That last point is the heart of it. If what matters is the level in your blood, then sorting people by how many units they were assigned — when they started at different places and responded by different amounts — blurs exactly the signal you are looking for.

This is not a fringe complaint. The authors of the mainstream reviews concede the substance of it: the large trials enrolled “mostly vitamin D replete adults,” and where researchers have looked specifically at people with the poorest status, small benefits have appeared.

A finding that survived anyway

Worth knowing, because it cuts against a tidy story in the other direction. The same VITAL trial that found nothing for cancer or heart disease did find something for autoimmune disease. Published in The BMJ in 2022, it reported 123 confirmed cases in the vitamin D group against 155 on placebo — a hazard ratio of 0.78, roughly a 22% reduction.

Handle it carefully: the p-value sat right at 0.05, the total number of cases was small, and the authors note their participants were older than the age at which many autoimmune conditions begin. It is a real result rather than a decisive one. But it shows the trials were not uniformly negative, and it is the kind of signal the critics say a better-designed study would find more of.

Where that leaves it

Both things are true at once, and this is the honest position: the large trials did not demonstrate broad benefit from supplementing people who already have enough vitamin D — and those trials were not designed in a way that could confidently detect benefit in people who don’t.

Which is why the 2024 guideline recommends supplementing certain groups without testing them, rather than picking a number to chase. And why, if you take away one thing from this page, it should be the question rather than any figure: who set this number, when, and from what evidence?

Why this gets harder with age

This matters more for readers of this site than for most people. NIH lists older adults among those most likely to have trouble getting enough, for a straightforward physiological reason: “As you age, your skin’s ability to make vitamin D when exposed to sunlight declines.” Skin tone matters too — the more melanin, the less vitamin D produced from the same exposure.

So the same half hour outdoors does not do the same work at 75 as it did at 25. That is part of why the 2024 Endocrine Society guideline recommends supplementation for adults aged 75 and over without testing them first — not because sun doesn’t count, but because it counts for less at that age.

NIH pairs its description of sunlight with a caution, and it belongs here too: “Ultraviolet radiation from sunshine can cause skin cancer, so it’s important to limit how much time you spend in the sun.” The same UVB that makes vitamin D is the UVB that burns. Where the sensible balance sits for you depends on your skin, your history and where you live, and that is a conversation for your doctor rather than a number this page can hand you.

Why this matters for what you pay

This is where a definition stops being academic. A stricter target doesn’t just relabel your result — it generates activity. More people fall short, so more people get retested, and retested again to see whether the number moved.

The same 2020 review is blunt about where that leads. It notes that “there is currently no evidence to demonstrate the benefits of vitamin D deficiency screening in the general population,” and that the growth in testing has produced “treatments and re-tests that are not always appropriate.”

Medicare’s lab benefit is built around diagnosis and monitoring, not around optimization. A test ordered because a condition is being investigated or managed is covered and costs you nothing. A test ordered to check on a wellness target, with no medical reason attached, is screening — and screening is where the signature forms and the surprise bills come from.

Both of those pages set out exactly when Medicare covers the test and when it doesn’t. If you’re handed a form to sign before a draw, it’s worth knowing what it is first. What to do before you go →