Doctors in Wales say waiting times for diagnostic tests here are amongst the worst in Europe and could be making the difference between life and death.
Official figures today show numbers are falling but Plaid Cymru claim Welsh patients are still waiting up to 20 times longer than those in the rest of the UK.
They say, on average, 41.2% of patients in Wales wait more than six weeks for such tests - ten times more than in Scotland and 20 times more than in England.
The Welsh Government said they "expect waiting times for diagnostic tests to come down" after NHS statistics showed Wales was the worst-performing country in the UK in that area.
Elin Jones, Plaid Cymru health spokeswoman, said the figures showing that Wales has high waiting list times for life-saving tests highlighted "the stark reality of Labour's mismanagement of the NHS".
NHS statistics which show that Wales has the worst waiting times for life-saving tests in the UK are "the most disturbing" health figures seen "in many years", a doctor said.
Carmarthen-based doctor Dewi Evans, who has been working in the health service since 1971, said early diagnostic tests were important because they could be a matter of life and death.
The checks - such as MRI scans and cystoscopies - can be used by medics to check whether a person has cancer.
Wales has the worst waiting times record for life-saving tests in the UK, according to new figures.
Around 42% of people in Wales waiting for diagnostic tests had to wait more than six weeks before they were finally seen, according to government statistics.
This compares with 1.8% in England and 3.8% in Scotland.
And the statistics also show 16.6% of patients on the Welsh diagnostic waiting list wait longer than 12 weeks.
In Northern Ireland, 15.5% on the list had to wait more than nine weeks.
The Welsh Government said there have been "unprecedented emergency care pressures" in recent months, which have hurt the NHS in Wales' ability to meet referral-to-treatment targets.
The Welsh Conservatives have pointed out that waiting time targets from referral to treatment are "consistently" being missed, and that the figures published this morning show 389,019 people - around one in eight in Wales - are waiting for treatment of some kind on the NHS in Wales.
The NHS in Wales has slipped further away from hitting its targets for the time it takes for patients to be treated in hospital, after being referred.
- It aims to have 95% of patients starting treatment within 26 weeks of referral - but, for those who began treatment in December, that only reached 82.9%
- It aims to have 100% of patients starting treatment within 36 weeks - but, for those who began treatment in December, that only reached 96.3%
- By the end of December, there were more than 36,000 patients still waiting, after more than 26 weeks, to start treatment - a larger number than at any time over the last two years
- There were more than 5,000 still waiting, after more than 36 weeks, to start treatment - the most since January 2012