What is an IGF-1 SDS and why is it used?

An IGF-1 standard deviation score (also called a z-score) expresses how far a measured IGF-1 lies from the age- and sex-matched population mean, in standard deviations. Because raw IGF-1 falls steeply with age and differs by sex, an SDS lets clinicians compare a result against the right peer group rather than a single fixed range.

How is the SDS computed from the reference data?

This tool uses the LMS method. Each age and sex has three parameters: L (skewness, Box-Cox power), M (median), and S (coefficient of variation). The score is ((value/M)^L − 1) / (L × S), which corrects for the right-skewed distribution of IGF-1. When L is near zero a logarithmic form is used instead.

Are the reference values assay-specific?

Yes. IGF-1 immunoassays differ substantially between manufacturers, so a result should ideally be scored against the reference panel that matches the assay used. The values here are illustrative LMS-style references and should be replaced with your laboratory's own validated panel before clinical use.

What SDS range is considered normal?

Most laboratories treat an IGF-1 SDS between −2 and +2 as within the reference interval, capturing roughly 95 percent of healthy individuals. A low SDS supports investigation for growth hormone deficiency, while a high SDS may suggest acromegaly or excess growth hormone, always interpreted with clinical context.

Can this replace a stimulation test?

No. IGF-1 SDS is a screening and monitoring aid, not a diagnostic substitute. Growth hormone deficiency and excess are confirmed with dynamic testing such as stimulation or suppression tests, alongside imaging and full clinical assessment.

IGF-1 Standard Deviation Score Calculator

IGF-1 (insulin-like growth factor 1) is the main downstream mediator of growth hormone, and its concentration changes dramatically with age and sex. A single fixed reference range cannot capture that, so endocrinologists convert a measured value into a standard deviation score (SDS) against the matched population. This calculator performs that conversion.

How it works

The tool uses the LMS method, the standard approach for skewed biological measurements. For a given age and sex, three reference parameters are looked up and interpolated: L (the Box-Cox power that corrects skewness), M (the median), and S (the coefficient of variation). The SDS is then:

if L ≠ 0:   SDS = ((value / M)^L − 1) / (L × S)
if L = 0:   SDS = ln(value / M) / S

This is mathematically equivalent to a z-score on the normalised, skewness- corrected scale. A result is interpolated between the two nearest tabulated ages so that intermediate ages are handled smoothly.

Example, notes, and limits

A 10-year-old girl with an IGF-1 of 350 ng/mL scored against a reference with M = 220, S = 0.30, L = 0.7 yields an SDS near +1.8 — high-normal. The reference panel built into this tool is illustrative; IGF-1 assays vary widely between manufacturers, so for clinical work you must substitute your laboratory’s own assay-specific LMS table. Treat an SDS outside the −2 to +2 band as a prompt for further evaluation, never as a standalone diagnosis.