Five-second answer

Claim we investigated

None of the three ways I-638 lets doctors verify sex is accurate.

False. Anatomy and chromosomes match a student's sex in all but rare cases, and adult testosterone ranges don't overlap. A 2006 international pediatric endocrine consensus puts genital anomalies at 1 in 4,500 births, NIH genetics references put chromosome-body mismatches at roughly 1 in 20,000 to 1 in 80,000, and an Endocrine Society review finds adult male and female testosterone ranges do not overlap.

Quote or cite this fact-check

FactHub rates the claim “None of the three ways I-638 lets doctors verify sex is accurate” False (high confidence): Anatomy and chromosomes match a student's sex in all but rare cases, and adult testosterone ranges don't overlap. Reviewed October 28, 2026.

One-minute summary

What the evidence establishes

  • Established. The text lets a provider rely on any one of three bases: reproductive anatomy, genetic makeup, or normal endogenously produced testosterone levels.
  • Established. Anatomy: genital anomalies occur in about 1 in 4,500 births, per a 2006 international medical consensus, so a child's anatomy shows her sex in nearly every case.
  • Established. Chromosomes: girls typically have XX and boys XY. Conditions where chromosomes and body don't match are rare, such as complete androgen insensitivity (2 to 5 in 100,000 newborn girls) and Swyer syndrome (1 in 80,000).
  • Established. Testosterone: measured by mass spectrometry, healthy adult men range from 7.7 to 29.4 nmol/L and premenopausal women from 0 to 1.7 nmol/L, with no overlap.
  • Established. Before puberty, boys' and girls' testosterone levels are the same, so a testosterone test can't verify a younger student's sex; anatomy and genetics can.
  • Established. Puberty blockers fully suppress the body's own testosterone, so a testosterone result alone wouldn't identify a transgender girl whose male puberty was blocked; anatomy or genetics would.
  • Not shown. The text doesn't say which basis governs in the rare cases where they disagree, or what counts as a 'normal' testosterone level.
Important context: Initiative IL26-638 is on Washington's November 3, 2026 ballot. Section 3(2) requires a student entering girls' school competition to provide a statement from her personal health care provider verifying her biological sex, relying only on one or more of reproductive anatomy, genetic makeup, or normal endogenously produced testosterone levels. Critics argue that none of these bases is accurate. This check rates that claim; it does not judge whether verification is wise, and FactHub takes no position on I-638.

The evidence

The conclusion rests on the combined record. Expand each item to see its role and limitations.

Independence note: This check uses 5 methods across 6 institutions and 5 countries or international groupings.
Evidence map
Method Institution / country Year(s) What it shows Key limitation Primary links
Primary legal text review Washington Secretary of State United States 2025 Section 3(2) of the filed text requires a provider-signed statement verifying biological sex, relying only on one or more of reproductive anatomy, genetic makeup, or normal endogenously produced testosterone levels. Any one basis can suffice. The text does not define a 'normal' testosterone level, say which basis governs if they disagree, or name a test method. Implementing guidance has not been written. Washington Secretary of State
International clinical consensus statement on disorders of sex development Lawson Wilkins Pediatric Endocrine Society, European Society for Paediatric Endocrinology United States, United Kingdom 2006 The 2006 consensus statement on the management of intersex disorders, published in Archives of Disease in Childhood, estimates that genital anomalies occur in 1 in 4,500 births. For all other births, external anatomy is typical of one sex. The figure is an estimate from 2006 and counts all genital anomalies, not only those that leave sex unclear. It describes births, not school-age girls entering sports. Archives of Disease in Childhood (Hughes, Houk, Ahmed, Lee; LWPES/ESPE Consensus Group)
Review of National Library of Medicine genetics reference pages on sex chromosomes and differences of sex development U.S. National Library of Medicine (MedlinePlus Genetics) United States 2026 MedlinePlus Genetics says girls and women typically have 46,XX and boys and men 46,XY. Conditions in which chromosomes and body disagree are rare: complete androgen insensitivity syndrome (XY with a female body) affects 2 to 5 in 100,000 newborn females; Swyer syndrome (XY with female reproductive structures) about 1 in 80,000 people; and 46,XX testicular difference of sex development about 1 in 20,000 people with a male appearance. These are summary reference figures, not a single population study, and prevalence estimates for differences of sex development vary with definitions. Partial androgen insensitivity is thought to be at least as common as the complete form. U.S. National Library of Medicine (MedlinePlus Genetics) U.S. National Library of Medicine (MedlinePlus Genetics) U.S. National Library of Medicine (MedlinePlus Genetics)
Peer-reviewed review of circulating testosterone by sex and age, measured by liquid chromatography-mass spectrometry Endocrine Society (Endocrine Reviews) Australia, Sweden, Monaco 2018 Using mass spectrometry, the 95% reference ranges are 7.7 to 29.4 nmol/L in healthy men and 0 to 1.7 nmol/L in healthy premenopausal women, with complete separation. Before puberty there is no sex difference in circulating testosterone. Reference ranges are for healthy adults; women with polycystic ovary syndrome can run higher, and some differences of sex development produce male-range levels in a female body. The review was written to inform athletics eligibility rules, and one author has worked with World Athletics. Handelsman DJ, Hirschberg AL, Bermon S. Endocrine Reviews
Review of method-comparison studies of routine testosterone immunoassays Australasian Association for Clinical Biochemistry and Laboratory Medicine (Clinical Biochemist Reviews) Australia 2005 A 2005 review found that studies advised against using routine immunoassays to measure testosterone in the normal female or children's ranges, while finding some adequate to detect levels above 3.7 nmol/L in women and children. Its concern was measuring low levels precisely, not separating men from women. The studies reviewed tested immunoassays in use around 2003-2004; they say nothing about mass spectrometry or about telling male from female levels. Clinical Biochemist Reviews (Australasian Association for Clinical Biochemistry and Laboratory Medicine)
1. The text allows any one of three bases Primary legal text review

Section 3(2) of the filed text requires a provider-signed statement verifying biological sex, relying only on one or more of reproductive anatomy, genetic makeup, or normal endogenously produced testosterone levels. Any one basis can suffice.

Limit: The text does not define a 'normal' testosterone level, say which basis governs if they disagree, or name a test method. Implementing guidance has not been written.

Verify: Initiative Measure No. IL26-638, final filed text (filed June 2, 2025)

2. Genital anomalies occur in about 1 in 4,500 births International clinical consensus statement on disorders of sex development

The 2006 consensus statement on the management of intersex disorders, published in Archives of Disease in Childhood, estimates that genital anomalies occur in 1 in 4,500 births. For all other births, external anatomy is typical of one sex.

Limit: The figure is an estimate from 2006 and counts all genital anomalies, not only those that leave sex unclear. It describes births, not school-age girls entering sports.

Verify: Consensus statement on management of intersex disorders (Arch Dis Child 2006;91(7):554-563)

Related claim: Pediatric checkups already document sex, though many Washington girls lack a regular provider. — Pediatric checkups already document sex, though a quarter of Washington children lack a personal provider to sign.

3. Chromosome-body mismatches are rare Review of National Library of Medicine genetics reference pages on sex chromosomes and differences of sex development

MedlinePlus Genetics says girls and women typically have 46,XX and boys and men 46,XY. Conditions in which chromosomes and body disagree are rare: complete androgen insensitivity syndrome (XY with a female body) affects 2 to 5 in 100,000 newborn females; Swyer syndrome (XY with female reproductive structures) about 1 in 80,000 people; and 46,XX testicular difference of sex development about 1 in 20,000 people with a male appearance.

Limit: These are summary reference figures, not a single population study, and prevalence estimates for differences of sex development vary with definitions. Partial androgen insensitivity is thought to be at least as common as the complete form.

Verify: Swyer syndrome Androgen insensitivity syndrome 46,XX testicular difference of sex development

4. Adult testosterone ranges don't overlap; children's are the same Peer-reviewed review of circulating testosterone by sex and age, measured by liquid chromatography-mass spectrometry

Using mass spectrometry, the 95% reference ranges are 7.7 to 29.4 nmol/L in healthy men and 0 to 1.7 nmol/L in healthy premenopausal women, with complete separation. Before puberty there is no sex difference in circulating testosterone.

Limit: Reference ranges are for healthy adults; women with polycystic ovary syndrome can run higher, and some differences of sex development produce male-range levels in a female body. The review was written to inform athletics eligibility rules, and one author has worked with World Athletics.

Verify: Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance (2018)

5. Older routine assays were imprecise in the female range Review of method-comparison studies of routine testosterone immunoassays

A 2005 review found that studies advised against using routine immunoassays to measure testosterone in the normal female or children's ranges, while finding some adequate to detect levels above 3.7 nmol/L in women and children. Its concern was measuring low levels precisely, not separating men from women.

Limit: The studies reviewed tested immunoassays in use around 2003-2004; they say nothing about mass spectrometry or about telling male from female levels.

Verify: Are Routine Testosterone Assays Good Enough? (Sydney S. Sacks; Clin Biochem Rev 2005;26(1):43-45)

Common questions

“Testosterone tests are inaccurate.”

Short answer: Not after puberty

Older routine immunoassays were too imprecise for measuring testosterone within the normal female range, as a 2005 review found, though some could detect levels above 3.7 nmol/L. That concerns precision at low levels, not telling men from women. Measured by mass spectrometry, adult male and female ranges show complete separation. The real limit is age: before puberty the levels are the same.

“Intersex conditions make sex verification unreliable.”

Short answer: Rare exceptions

Differences of sex development are real, and they are where the three bases can disagree. They are rare: genital anomalies occur in about 1 in 4,500 births, complete androgen insensitivity in 2 to 5 in 100,000 newborn girls, Swyer syndrome in about 1 in 80,000 people, and 46,XX testicular difference of sex development in about 1 in 20,000 people with a male appearance. The text does not say how a provider should resolve a conflict, which is a gap implementing guidance would need to fill.

“A testosterone test would not detect a transgender girl on puberty blockers.”

Short answer: True, on its own

Correct. A GnRH analog fully suppresses the body's own testosterone, so a girl whose male puberty was blocked could have a level in the female range. The text lets a provider rely on 'one or more' bases, so a provider relying only on testosterone could sign. Anatomy or a chromosome test would show male sex. This is a limit of the testosterone route, not of verification as a whole.

“No test can verify a young student's sex.”

Short answer: Two of three can

Before puberty, testosterone levels do not differ by sex, so that basis fails for younger students. Anatomy and chromosomes do not depend on puberty: genital anomalies occur in about 1 in 4,500 births, and chromosome-body mismatches are rarer still.

Conclusion

The claim that none of I-638's three verification bases is accurate is contradicted by the medical evidence for each. Anatomy: the 2006 consensus statement of the U.S. and European pediatric endocrine societies estimates genital anomalies at 1 in 4,500 births, so for nearly every child, external anatomy shows her sex. Genetics: girls typically have XX chromosomes and boys XY, and the conditions in which chromosomes and body disagree are rare. Complete androgen insensitivity affects 2 to 5 in 100,000 newborn girls, Swyer syndrome about 1 in 80,000 people, and 46,XX testicular difference of sex development about 1 in 20,000 people with a male appearance. Testosterone: measured by mass spectrometry, healthy adult men range from 7.7 to 29.4 nmol/L and premenopausal women from 0 to 1.7 nmol/L, with complete separation between them. Each basis has a real limit. Testosterone cannot verify sex before puberty, when boys' and girls' levels are the same, so for younger students the bases that work are anatomy and genetics. Older routine immunoassays were too imprecise to measure testosterone within the normal female range, a problem a 2005 review documented; the separation above comes from mass spectrometry. A GnRH analog, the drug class used as a puberty blocker, fully suppresses the body's own testosterone, so a testosterone result alone would not identify a transgender girl whose male puberty was blocked; anatomy or genetics would. The text lets a provider rely on 'one or more' bases, and it neither says which governs when they disagree nor defines a 'normal' level. Partial androgen insensitivity is thought to be at least as common as the complete form, so cases where the bases conflict, while rare, are somewhat more common than the complete-form figure alone suggests. Confidence is High because each basis rests on long-established clinical findings from independent bodies in the United States, Europe and Australia, and the claim is absolute: showing that the bases are accurate for nearly all students contradicts it. What the evidence does not settle is how providers would handle the rare conflicting case, which depends on implementing guidance that has not been written. This check rates whether the bases are accurate, not whether verification is wise or what it costs. FactHub's companion checks cover whether the measure requires a genital exam, what each route costs, and whether pediatric records already document sex.

How we checked

  1. Defined the claim: whether any of the three bases Section 3(2) allows (reproductive anatomy, genetic makeup, normal endogenous testosterone) accurately identifies a student's sex, as opposed to whether verification is wise or affordable.
  2. Read Section 3(2) of the filed initiative text.
  3. For anatomy, used the 2006 international consensus statement of the U.S. and European pediatric endocrine societies, read on the journal's own page.
  4. For genetics, read the National Library of Medicine's MedlinePlus Genetics pages on Swyer syndrome, androgen insensitivity syndrome and 46,XX testicular difference of sex development for typical karyotypes and prevalence.
  5. For testosterone, read a 2018 Endocrine Reviews review (open copy on PubMed Central) for reference ranges, prepubertal levels and the effect of GnRH analogs, and a 2005 review of routine immunoassays, which a critic cited.
  6. Looked for evidence that each basis fails, and recorded each real limit: prepubertal levels, older assays, puberty blockers, and rare differences of sex development.
  7. Tried to read the Endocrine Society's 2017 guideline on puberty suppression; the publisher's site blocked automated access, so the article relies on the 2018 review for the effect of GnRH analogs.

Editorial note: Scope: this article rates the claim that none of the three verification bases in Section 3(2) of Initiative 638 is accurate. It does not evaluate the measure's merits, its cost to families, or how implementing guidance will treat conflicting results; companion FactHub checks cover cost and the genital-exam claim. Prevalence figures for differences of sex development vary with definitions; we use the figures published by the sources cited. FactHub takes no position on I-638. No external specialist review has been recorded.

Expert-review status

No external specialist review has been recorded.

Funding and conflicts

Funding: No external funding for this piece.

Ownership: Published by the FactHub Research Team.

Conflicts: None known.

Sources

Sources are labeled by kind, with what this fact check takes from each, so readers can check the evidence independently.

  1. Government Washington Secretary of State. Initiative Measure No. IL26-638, final filed text (filed June 2, 2025)

    The measure's full text; Section 3(2) lists the three verification bases.

    Reliability: The official filed text posted by the Secretary of State. It is the measure itself; the sponsor wrote it.

    • A provider may rely on any one of anatomy, genetic makeup or normal endogenous testosterone; the text names no test and defines no 'normal' level.

      What the source says
      relying only on one or more of the following: The student's reproductive anatomy, genetic makeup, or normal endogenously produced testosterone levels.
      Sec. 3(2)
  2. Peer-reviewed study Archives of Disease in Childhood (Hughes, Houk, Ahmed, Lee; LWPES/ESPE Consensus Group). Consensus statement on management of intersex disorders (Arch Dis Child 2006;91(7):554-563) DOI: 10.1136/adc.2006.098319

    The international consensus statement that introduced the term 'disorders of sex development' and set out their management.

    Reliability: Convened by the Lawson Wilkins Pediatric Endocrine Society and the European Society for Paediatric Endocrinology; it predates I-638 and does not address sports.

    • Genital anomalies occur in about 1 in 4,500 births, so for nearly every child external anatomy is typical of one sex.

      What the source says
      It is estimated that genital anomalies occur in 1 in 4500 births.
      Management of intersex disorders, opening paragraph
  3. Government U.S. National Library of Medicine (MedlinePlus Genetics). Swyer syndrome

    The National Library of Medicine's genetics reference page on Swyer syndrome (46,XY complete gonadal dysgenesis).

    Reliability: A consumer genetics reference maintained by the National Library of Medicine, with cited references; it does not address sports or I-638.

    • Girls typically have XX chromosomes and boys XY, which a chromosome test detects.

      What the source says
      Girls and women typically have two X chromosomes (46,XX karyotype), while boys and men typically have one X chromosome and one Y chromosome (46,XY karyotype).
      Description

    • Qualifies: Swyer syndrome, XY chromosomes with female reproductive structures, occurs in about 1 in 80,000 people.

      What the source says
      Swyer syndrome occurs in approximately 1 in 80,000 people.
      Frequency
  4. Government U.S. National Library of Medicine (MedlinePlus Genetics). Androgen insensitivity syndrome

    The National Library of Medicine's genetics reference page on androgen insensitivity syndrome.

    Reliability: A consumer genetics reference maintained by the National Library of Medicine, with cited references; it does not address sports or I-638.

    • Qualifies: Complete androgen insensitivity, XY chromosomes with a female body, affects 2 to 5 in 100,000 newborn girls.

      What the source says
      Complete androgen insensitivity syndrome affects 2 to 5 in 100,000 newborn females.
      Frequency

    • Qualifies: Partial androgen insensitivity is thought to be at least as common as the complete form.

      What the source says
      Partial androgen insensitivity is thought to be at least as common as complete androgen insensitivity.
      Frequency
  5. Government U.S. National Library of Medicine (MedlinePlus Genetics). 46,XX testicular difference of sex development

    The National Library of Medicine's genetics reference page on 46,XX testicular difference of sex development.

    Reliability: A consumer genetics reference maintained by the National Library of Medicine, with cited references; it does not address sports or I-638.

    • Qualifies: About 1 in 20,000 people with a male appearance have XX chromosomes.

      What the source says
      Approximately 1 in 20,000 individuals with a male appearance have 46,XX testicular difference of sex development.
      Frequency
  6. Peer-reviewed study Handelsman DJ, Hirschberg AL, Bermon S. Endocrine Reviews. Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance (2018) Free copy DOI: 10.1210/er.2018-00020

    Peer-reviewed review of testosterone levels in males and females by age and how they are measured.

    Reliability: Published by the Endocrine Society. One author (Bermon) has worked with World Athletics on eligibility rules; the findings cited here, on reference ranges, prepubertal levels and GnRH suppression, are standard endocrinology.

    • Measured by mass spectrometry, healthy adult men range from 7.7 to 29.4 nmol/L and premenopausal women from 0 to 1.7 nmol/L.

      What the source says
      with 95% references ranges of 7.7 to 29.4 nmol/L in healthy men and 0 to 1.7 nmol/L in healthy premenopausal women
      Essential Points

    • Adult male and female testosterone levels do not overlap when measured by mass spectrometry.

      What the source says
      Using LC-MS measurement, circulating testosterone in adults has a strikingly nonoverlapping bimodal distribution with wide and complete separation between men and women.
      Paragraph beginning 'Prior to puberty, levels of circulating testosterone as determined by LC-MS'

    • Qualifies: Before puberty boys and girls have the same circulating testosterone, so the test cannot verify a younger student's sex.

      What the source says
      Prior to puberty, there is no sex difference in circulating testosterone concentrations or athletic performance
      Abstract

    • Qualifies: A GnRH analog, the drug class used as a puberty blocker, fully suppresses the body's own testosterone.

      What the source says
      Both sets of studies used a common design of fully suppressing all endogenous testosterone (to castrate levels) for the full duration of the experiment by administering a GnRH analog.
      Paragraph beginning 'Both sets of studies used a common design'
  7. Academic Clinical Biochemist Reviews (Australasian Association for Clinical Biochemistry and Laboratory Medicine). Are Routine Testosterone Assays Good Enough? (Sydney S. Sacks; Clin Biochem Rev 2005;26(1):43-45) Identifier: PMID 16278777

    A short review of 2003-2004 studies comparing routine testosterone immunoassays with reference methods.

    Reliability: A review in the journal of the Australasian clinical biochemistry association, summarizing studies of assays then in routine use. It predates I-638 and does not address sex verification.

    • Qualifies: Studies of routine assays in use around 2004 found them too imprecise to measure testosterone within the normal female or children's ranges.

      What the source says
      The authors conclude that none of the assays, including the RIAs, should be used to measure testosterone in the normal female or paediatric ranges.
      Paragraph on Wang et al.

    • The same review found some routine assays adequate for detecting testosterone above 3.7 nmol/L in women and children.

      What the source says
      The DPC RIA, the Elecsys and Vitros are probably, however, adequate for detecting abnormal elevations in testosterone in women and children (i.e. levels above 3.7 nmol/L) by virtue of their relative lack of bias and lesser differences from the reference results.
      Paragraph on Wang et al.

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