IGF-1 Standard Deviation Score (IGF-1 SDS)

The IGF-1 standard deviation score is a normalisation, not a new measurement. It is the observed concentration minus the mean of a healthy reference group of the same age and sex, divided by that group's standard deviation. The score exists because raw IGF-1 is almost uninterpretable on its own: concentrations peak in adolescence and then fall steadily through adult life, so a value that is unremarkable at 20 can be clearly high at 60. Expressed as an SDS, results from different ages become comparable, and roughly 95 percent of a healthy population falls between minus 2 and plus 2.

The catch is that the score is only as good as the reference set behind it. IGF-1 immunoassays differ in how they strip the binding proteins away, and different platforms have historically returned different concentrations for the same serum, which is why an SDS should never be recalculated across assays. International standardisation to a common recombinant preparation has narrowed the spread, and modern reference intervals are built from large cohorts on a single platform, but the number remains method-bound.

Clinically the SDS is what turns IGF-1 into a decision variable. In acromegaly, normalising IGF-1 SDS is the accepted biochemical target of treatment and tracks outcomes better than a single growth hormone value. In suspected adult growth hormone deficiency a low SDS supports the diagnosis but a normal one does not exclude it, so stimulation testing is still required in most cases.

Two misuses recur. The first is comparing SDS values from different laboratories as though they were a shared scale, which manufactures apparent changes that are purely analytical. The second is treating a high-normal SDS as a fitness metric worth pushing upward, when the entire clinical apparatus around this number exists to bring elevated values down.