Androgen Receptor Sensitivity (CAG Repeat)MALE

What Androgen Receptor Sensitivity (CAG Repeat) actually measures — the analyte, and the assay

This test counts repeats, and counting is the whole measurement. Exon 1 of the androgen receptor gene contains a polymorphic CAG trinucleotide repeat encoding a polyglutamine tract in the receptor's transactivation domain. The assay amplifies that region by PCR with a fluorescently labeled primer and sizes the product by capillary electrophoresis against an internal size standard Bharaj 1999 Boorman 2002.

Sizing a repeat is not the same as counting it. Fragment length is inferred from electrophoretic mobility, which depends on sequence composition as well as size, so an uncorrected fragment analysis can be offset by a repeat or two against sequencing. Automated methods were developed and published specifically to make the determination rapid and accurate, which is an admission that the naive version was neither Bharaj 1999 Boorman 2002.

PCR across a triplet repeat also produces stutter. Polymerase slippage generates minor peaks one repeat shorter than the true allele, and calling the wrong peak shifts the reported number.

The result differs by sex in a way that is easy to miss. The androgen receptor is X-linked, so a man has one allele and a woman has two — and in a woman the biologically relevant quantity is not the mean of her two alleles but the length weighted by X chromosome inactivation, which has to be measured separately Sugawa 2009.

Androgen Receptor Sensitivity (CAG Repeat): what changes the blood, and what only changes the reading

What changes the repeat length: nothing in an adult. The length is germline and stable across a lifetime. Small intergenerational changes occur through meiotic instability, which is why a father and son can differ by a repeat, but nothing a person does moves their own number.

What the repeat length changes in you:

1. Transcriptional efficiency of the receptor. A longer polyglutamine tract reduces transactivation in reporter assays, so the same testosterone concentration produces less androgen-dependent transcription. The relationship is continuous and shallow, not a switch.
2. Semen quality, in an ancestry-dependent way. Repeat length varies between ethnic groups and its association with semen parameters differed by group in a study of young men from Russia Osadchuk 2022, which is a warning against applying one population's cut-points to another.
3. The extreme end, which is a disease. Above roughly 38 to 40 repeats the phenotype is spinal and bulbar muscular atrophy — a neurodegenerative condition, not an androgen-sensitivity variant, and the only length with unambiguous clinical meaning.
4. In women, X inactivation, which decides which allele is expressed in which tissue and cannot be inferred from the genotype Sugawa 2009.

What changes only the reported number:

1. Fragment sizing versus sequencing, which can differ by one or two repeats Boorman 2002.
2. Stutter peaks, which shift a call downward.
3. Whether the laboratory counts the CAG repeat alone or the adjacent CAA-interrupted tract with it, a real source of between-laboratory disagreement in how the polyglutamine length is defined Bharaj 1999.
4. Reporting a woman's two alleles as an average, which is not the biologically weighted length Sugawa 2009.
5. Which population's reference distribution is printed, since mean repeat length differs substantially between ancestries Osadchuk 2022.

Reference interval or decision threshold — which kind of number Androgen Receptor Sensitivity (CAG Repeat) is

It is a population distribution, not a reference interval and not a threshold. Repeat length in men runs roughly from the high teens to the low thirties, with the mean differing between ancestral groups Osadchuk 2022. There is no cut-off separating sensitive from insensitive.

The one real threshold is pathological and far outside the normal distribution. The expansion associated with spinal and bulbar muscular atrophy sits above 38 to 40 repeats, and a result in the normal range says nothing about it.

The associations reported within the normal range are small, inconsistent between populations, and mostly cross-sectional. The same repeat length has been associated with opposite outcomes in different cohorts, which is the signature of a weak effect studied many times Osadchuk 2022.

No treatment decision uses this number. No androgen dose, target or regimen is adjusted by CAG repeat length in any guideline, and no trial has randomized anyone by it — which places it in the same category as the other genotype pages in this cohort Hickey 2013.

How you would know your Androgen Receptor Sensitivity (CAG Repeat) was wrong — and when to redraw

Never, for the length. A germline repeat does not change, so a repeat test is a check on the laboratory. The only reason to redo it is a result near the pathological expansion range, where sequencing rather than fragment sizing is the appropriate confirmation Boorman 2002.

What is worth measuring instead is androgen action. The receptor's sensitivity is a modifier; the ligand and its downstream effects are measurable and they move.

Conditions that must match for those downstream measurements: the same laboratory and method for any steroid, morning collection, and mass-spectrometry-derived reference intervals where available Kunz 2023.

What would have to change for the repeat length to mean something:

• Long repeat with symptoms of low androgen action? The measurements that matter are total testosterone and SHBG, because the ligand is the part that can be changed. A long repeat does not license treating a normal testosterone.
• Short repeat with acne, hair loss or prostate symptoms? DHT and PSA measure the androgen-dependent tissue responses directly, and they respond to intervention; the repeat length does not.
• Considering a testosterone dose based on the repeat? There is no dosing algorithm using it, and the numbers that guide dosing are testosterone, estradiol sensitive and the CBC hematocrit.
• Result near 38 repeats? That is the one situation requiring confirmation by a method that counts rather than sizes Bharaj 1999.

What Androgen Receptor Sensitivity (CAG Repeat) cannot tell you

It cannot predict how you will respond to testosterone. No prospective study has shown repeat length predicting symptomatic or biochemical response to androgen therapy well enough to guide dosing.

It cannot measure receptor density or tissue-level androgen action. Transactivation efficiency is one input among many, and coactivator availability and local steroid metabolism are others.

It cannot be interpreted in a woman without X inactivation data Sugawa 2009.

It cannot be compared between laboratories unless both define and size the repeat the same way Boorman 2002 Bharaj 1999.

The wrong inference readers actually draw is that a long repeat explains why a normal testosterone feels inadequate, and therefore justifies treating the number. The effect within the normal range is small and inconsistent across populations, no dosing algorithm uses it, and the honest use of the result is as context for a conversation rather than as a reason to move a dose Osadchuk 2022 Hickey 2013.

Sources read for these sections

• Boorman DW, et al. Automated fragment analysis method for determining androgen receptor CAG repeat length. Biotechniques 2002 · PMID 12139238
• Bharaj BS, et al. Rapid and accurate determination of (CAG)n repeats in the androgen receptor gene using polymerase chain reaction and automated fragment analysis. Clinical Biochemistry 1999 · PMID 10480446
• Sugawa F, et al. Premature ovarian failure and androgen receptor gene CAG repeat lengths weighted by X chromosome inactivation patterns. Fertility and Sterility 2009 · PMID 18281036
• Osadchuk L, Vasiliev G, Kleshchev M, Osadchuk A. Androgen Receptor Gene CAG Repeat Length Varies and Affects Semen Quality in an Ethnic-Specific Fashion in Young Men from Russia. International Journal of Molecular Sciences 2022;23(18):10594 · PMID 36142533
• Kunz S, et al. Age- and sex-adjusted reference intervals for steroid hormones measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) using a widely available kit. Endocrine Connections 2023 · PMID 37938144
• Hickey SE, et al. ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing. Genetics in Medicine 2013 · PMID 23288205