NHS patients are being put at risk of “avoidable harm”and may be missing vital cancer screening tests because sex and gender are not recorded separately, a study warns.

The NHS number, a lifelong identifier given at birth, codes people by biological sex with odd numbers for male and even numbers for female.

This drives sex specific automated screening invitations for diseases such as cervical or breast cancer and other which primarily affect men including aortic aneurysm.

Many trans or non-binary people opt to change their NHS number and no medical intervention is required to do this.

Research involving the University of St Andrews found that this effectively renders the NHS number a marker of gender instead of sex and risks patients missing health services that are determined by biological sex. 

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The Royal College of General Practitioners already recommends that sex and gender are recorded separately in medical records and the study calls for this to be replicated across the NHS.

The World Health Organisation defines gender as the “characteristics of women and men that are socially constructed” while sex refers to those that are “biologically determined” such as the gametes, chromosomes, hormones and reproductive organs.

The study, which also involved the universities of Cambridge and Dundee  found that contemporary medical research and clinical practice “often erroneously use sex and gender interchangeably” which it said was leading to “avoidable harm”.

It points to a recent report which concluded that sex and gender are both powerful risk factors for virtually every disease and affect every organ.

Differences in drug metabolism are increasingly well recognised,while gender can significantly affect how individuals experience healthcare and engage with treatment.

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The study concludes that “untangling” the difference between sex and gender is crucial for safe, dignified,and effective healthcare of all groups.

Margaret McCarthy, one of the study authors, said: “The NHS asks whether you are male or female. 

“However this can be changed on request, so the opposite is put on the records.

"No medical intervention is needed in order to change the sex marker on NHS records. 

“The NHS will then use that changed marker for screening, lab tests, etc. 

“The NHS, in my view, needs to record sex - because that drives so much medical care, and not just gender, which it has then become.”

Researchers said missing data on patients on sex and gender had meant that there was no accurate data on whether gender-diverse people were more or less affected by Covid-19 or whether public health advice around the virus should be different for trans people who are taking cross-sex hormones.

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Nicola Sturgeon has said that trans people are “some of the most prejudiced against, discriminated against and stigmatised groups” in the country.

The SNP has plans to reform the gender recognition law proposing a change to allow  transgender people to self-declare their gender identity rather than needing a medical diagnosis of gender dysphoria.

The policy reform has been met with a number of concerns from various sectors of society including the SNP’s Women’s Pledge group, which has said it puts women’s sex-based rights at risk.

The 2021 census in England and Wales will now following a legal challenge ask individuals their sex according to their birth or gender recognition certificate, followed by questions on gender identify.

In Scotland, the national statistician is currently consulting on the wording on sex and gender for next year’s census.

The paper ‘Sex, gender and medical data’ is published in the BMJ .