Methodology

What "normative values" means here

Some users search for normative values or normative data. On LabNorms, that term refers to descriptive population distributions and percentiles drawn from published datasets, not healthy, ideal, or prescriptive ranges.

Unit of Output

Each page represents one analyte, one demographic group, and one data source. Percentiles are not pooled across sources unless explicitly stated. Where multiple sources exist for the same analyte, each is presented and attributed separately.

What We Do

For each analyte and demographic group, we identify a suitable published dataset, apply defined and documented inclusion and exclusion criteria, and compute percentile values (P5, P25, P50, P75, P95) from the participant-level microdata. The same computation and filtering approach is applied consistently across pages unless explicitly stated otherwise. The source, criteria, and sample size are stated on every data page. All values can be independently reproduced from the stated source, survey cycle, and exclusion criteria.

Source Selection

We only use sources that are:

• Peer-reviewed and publicly available
• Based on a defined population with documented sampling methodology
• Measured using standardized laboratory methods
• Of sufficient sample size for the demographic group
• Representative of a defined measurement context (for example, fasting vs. non-fasting, clinical vs. general population)
• Free from obvious conflicts of interest

Sources vary by analyte. Each is documented in full on its reference page. No assumption is made that values from different sources are directly comparable.

How Percentiles Are Computed

Percentiles are computed from participant-level microdata using the method stated for the source. For NHANES-based pages, we use survey-weighted quantile estimation and apply the appropriate MEC or fasting-subsample weights so the resulting distributions reflect the reference population defined by the survey design. If a future source uses a different sampling structure or a published methodology that does not call for survey weighting, that will be stated explicitly on the relevant pages.

We report P5, P25, P50 (median), P75, and P95 for each demographic group and survey cycle.

Reference Populations

The reference population varies by analyte. Some pages use the full survey population for a demographic group; others apply additional filters (for example, excluding pregnant participants or restricting to fasting samples). Criteria are chosen to match the measurement protocol of the source dataset. There is no universal reference definition applied across all analytes. Exclusion criteria are listed on each data page.

What a Percentile Means

A value at P50 is the median: half the reference population measured above it, half below. A value at P5 means 95% of the reference population measured higher.

Percentiles reflect measured values, not biological absolutes. A result is influenced by pre-analytic conditions (fasting status, sample handling, time of day), assay method, and calibration. The distributions here reflect those real-world conditions as documented in the source study.

Percentiles describe distributions. They do not define what is healthy, normal, or optimal. The same value can carry different clinical significance depending on context, symptoms, and other factors only a clinician can assess.

Thyroid Panel Data Source

Thyroid hormone analytes (TSH, Total T4, Free T4, Total T3) use a different source than other analytes on this site. NHANES collected thyroid data only in 2007-2008, 2009-2010, and 2011-2012 as part of the Thyroid Profile component, measured on a one-third random subsample of participants (Subsample A). This component was discontinued after 2012 and is not available in more recent NHANES cycles including the 2017-March 2020 files used for most other analytes.

LabNorms pools the 2009-2010 (THYROD_F) and 2011-2012 (THYROD_G) cycles after confirming distributional equivalence between cycles (medians within 10%, no meaningful directional shift). The Subsample A two-year weight WTSA2YR is divided by 2 when pooling to maintain correct population-level estimates. Because this is a one-third subsample, per-slice sample sizes are lower than for full MEC analytes. See the NHANES 2009-2012 Thyroid reference page for full details.

Nutrition Data Sources

Nutrition analytes use several NHANES sources. 25-hydroxyvitamin D, vitamin A (retinol), vitamin E (alpha-tocopherol), and vitamin C come from the NHANES 2017-2018 single cycle (J-cycle files VID_J, VITAEC_J, VIC_J) using the cycle's published two-year MEC weight WTMEC2YR applied directly. Serum and red blood cell folate come from the NHANES 2017-March 2020 pre-pandemic combined cycles (P_FOLFMS and P_FOLATE) using the pre-pandemic folate subsample weight WTFOLPRP applied directly.

Vitamin B12 and methylmalonic acid use NHANES 2013-2014 (VITB12_H and MMA_H) with WTMEC2YR, because those analytes were not published in the 2017-2018 or 2017-March 2020 public lab files. Serum zinc, serum copper, and serum selenium use the NHANES 2015-2016 serum trace-elements file (CUSEZN_I) with the published WTSA2YR one-third subsample weight.

Single-cycle and older-cycle estimates are not pooled across sources. Tail values (P5 and P95) carry wider effective intervals than analytes pooled across cycles, especially for one-third subsample trace-element analytes. Each page states its source cycle, weight, and unweighted slice n.

No supplementation-based exclusions are applied. NHANES does not capture supplementation reliably enough to permit consistent supplementation exclusions across analytes, and the site does not apply supplementation exclusions for any other analyte.

25-Hydroxyvitamin D values are stored in ng/mL (US-clinical convention) and converted from NHANES nmol/L by dividing by 2.496. The unit toggle exposes the SI alternate.

Limitations

• Survey design matters. Percentiles depend on the source design and weighting approach. For NHANES pages we apply survey weights, but results still describe the surveyed reference population and should not be overgeneralized beyond it.
• Population scope. Much of the current data is drawn from US surveys and may not generalize to other countries or underrepresented populations.
• Cross-sectional. Survey data is a snapshot. Values from older cycles may not reflect current distributions.
• Measured, not biological. Distributions reflect what was measured under source study conditions. Pre-analytic variation, assay differences, and calibration shifts are not corrected for.
• Assay variability. Methods and calibrations differ between studies. Values from different sources are not always directly comparable.
• Small subgroups. Narrow demographic cuts may rest on small samples, reducing the stability of P5 and P95.
• No clinical validation. These are descriptive statistics, not validated diagnostic thresholds.
• Source heterogeneity. Sources vary in population, design, and measurement standards. Each is evaluated individually; no claim is made that values across sources are directly comparable.

What This Site Does Not Do

LabNorms does not diagnose, interpret results clinically, define healthy or optimal ranges, personalize results, or give advice of any kind. Individual lab results should always be interpreted by a qualified clinician.