A woman who went to her GP with a urinary tract infection (UTI) missed the chance to catch her cancer early because she relied on her son to translate.
Dr Shanti Karupiah said when the woman was examined she was found to have stage four cervical cancer.
A Senedd report recommends translators are provided to patients to help avoid health conditions being missed.
The Welsh government said it would consider the recommendations.
The report by the Equality and Social Justice Committee said mistakes and misdiagnoses occurred often due to the “wholly inappropriate use of family members as interpreters in medical settings, rather than trained professionals”.
It added that failing to provide adequate interpretation in a medical situation was a “potential breach of their human rights”.
Vice chair of policy and public affairs at the Royal College of GPs, Dr Karupiah, said sometimes a patient would not say symptoms to their partners or family members because of embarrassment, which could lead to a misdiagnosis.
“There are cases where the son says one thing, because mum is so embarrassed to say the thing that she wants to say, and he interprets, as best he can, and then there’s a missed opportunity, near misses and misdiagnosis.
“If you can’t speak the same language, it’s hard to get appropriate care.
“If a misdiagnosis is made, it can be the difference between life and death.”
‘Communication issue’
Dr Karupiah said she saw one woman in her 50s, who had been suffering with recurrent UTIs, visit the practice with a relative who was keen to translate for them.
“I suspected something wasn’t getting across so I suggested a perineal examination.
“That revealed she had cervical cancer, and when I referred her for a USC [urgent suspected cancer], it can back as stage four.”
Dr Karupiah said this was one example of something that with better translation support could have possibly been picked up earlier.
“Because of the communication issue, it was missed,” she added.
‘Inadequate language interpretation’
The committee has called on the Welsh government to end the reliance on family members for translation as a matter of urgency, as part of the Anti-Racist Wales Action Plan.
Jenny Rathbone MS, chair of the Equality and Social Justice Committee, said it showed there was more to be done before Wales can become an anti-racist nation.
“It was worrying to hear evidence regarding mistakes and misdiagnoses that can result from inadequate language interpretation.
“The Welsh government must recognise that failing to provide adequate interpretation to individuals with incomplete command of either English or Welsh in a medical situation is a potential breach of their human rights,” she said.
The report also showed healthcare was not the only public service where people from ethnic minority backgrounds got a worse service in Wales.
There were calls to address the issues in education and criminal justice.
“The Welsh government has set itself the aim of an anti-racist Wales by 2030, a mere six years hence, that requires us to be active, not passive to resist, rather than resign ourselves to racial discrimination.
“And to recognise that it is time for action, not words,” Ms Rathbone added.
The report makes 10 recommendations to the Welsh government to improve the Anti-Racist Wales Action Plan, which includes strengthening efforts to eliminate the use of family members as interpreters in medical settings.
It also calls on the Welsh government to work with the health and social care sector to provide a safe way to report racist behaviour and to take steps to improve awareness of the anti-racist Wales agenda in schools, with a nationwide teacher training day.
The Welsh government said: “We are committed on driving forward our Anti-Racist Wales Action Plan and working towards achieving an anti-racist Wales by 2030.
“We recognise the importance of reducing instances where family members are acting as interpreters.”
They added the Wales Interpretation and Translation Service already provided professional interpreters and translators for the public sector and it is working with Public Health Wales to explore the feasibility of a directory of interpreters.