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.

What is the IGF-1 Standard Deviation Score Calculator?

Calculates IGF-1 SDS from a measured value, patient age, and sex using lambda-mu-sigma (LMS) reference data to standardise results across assay platforms. Built for paediatric and adult endocrinologists assessing growth hormone status. It runs free in your browser on Gera Tools, with nothing uploaded.

IGF-1 Standard Deviation Score Calculator

Name: IGF-1 Standard Deviation Score Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Why IGF-1 needs age- and sex-specific scoring

Raw IGF-1 in ng/mL is almost meaningless without context. A result of 300 ng/mL could be very high-normal for a 5-year-old, mid-range for a 14-year-old at peak puberty, and elevated-to-suspect-acromegaly for a 50-year-old. Concentrations typically:

Rise steeply through childhood
Peak in mid-puberty, often reaching their highest lifetime values
Decline gradually but substantially through adulthood and into older age
Differ between males and females particularly during the pubertal window

The SDS expresses where a result sits within the expected distribution for that exact age and sex, making it the only meaningful way to compare one patient’s result to another or track a patient’s trajectory over time.

Clinical context: what the SDS means

SDS range	Interpretation
Below −2.0	Low; consistent with possible growth hormone deficiency or other cause of low IGF-1
−2.0 to −1.0	Low-normal; worth monitoring, especially in short children
−1.0 to +1.0	Mid-normal range
+1.0 to +2.0	High-normal
Above +2.0	Elevated; may warrant investigation for excess growth hormone

A low SDS is used as a screening marker for growth hormone deficiency in children with poor growth, but it is not diagnostic alone — stimulation tests are required to confirm GHD. A markedly elevated SDS in an adult raises concern for acromegaly and typically leads to an oral glucose suppression test. IGF-1 SDS also guides dosing during growth hormone replacement therapy: the goal during treatment is usually to maintain SDS between −1 and +1, avoiding prolonged elevation above +2.

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.