Doctors say there is now "encouraging" evidence that an annual blood test may cut ovarian cancer deaths by a fifth.

Ovarian tumours are often deadly as they are caught too late.

A 14-year study on 200,000 women, published in the Lancet, has been welcomed as a potentially landmark moment in cancer screening.

But the researchers and independent experts say it is still too soon to call for mass screening because of concerns about the analysis.

Ovarian cancer is difficult to pick up as symptoms, including abdominal pain, persistent bloating and difficulty eating, are common in other conditions.

Blood measure

The UK Collaborative Trial of Ovarian Cancer Screening is one of the biggest clinical trials ever conducted and is supposed to give the definitive verdict on screening.

It monitored levels of a chemical called CA125 in women's blood.

Doctors tracked changes in the levels of CA125, which is produced by ovarian tissue, over time and if levels became elevated then the women were sent for further tests and ultimately surgery.

The results are now in, but the interpretation is a bit messy and the researchers admit it is "controversial".

Ovarian cancer is the seventh most common cancer in women worldwide with around 240,000 cases each year. Photo: SPL

Their initial statistical analysis of the data showed no benefit to screening. But there was a benefit when they removed the data from any women who may have already started to develop ovarian tumours.

The researchers then performed a more forgiving statistical analysis, which also showed a benefit.

Trial leader Prof Usha Menon, from UCL, told the BBC News website: "Is there clear evidence? I would say no.

"We don't have clear evidence to go ahead with screening, but what we have are really encouraging estimates of around a 20% reduction, which we need to confirm."

Any benefit to screening seems to be delayed - kicking in towards the end of the trial.

The researchers are continuing to follow the patients for what is expected to be another three years to confirm whether there is a benefit.

Kevin McConway, a professor of applied statistics at the Open University, said: "Doing these extra analyses can be seen as an appropriate response to how the data turned out to look, which in some respects weren't as they originally expected.

"But equally it is also the case that the more analyses done, the more likely it is that one of the results will come out as positive.

"The results are promising, but perhaps not all that promising."

To screen or not to screen?

There is also the risk that screening can do harm and the test led to some women having unnecessary surgery to remove benign growths.

Dr Adam Shaw, the clinical lead for cancer genetics at Guy's & St Thomas' NHS Foundation Trust, said the findings were "very encouraging" but there was still more work to do.

"Nonetheless, this study is a landmark step in devising effective screening for ovarian cancer, which is often portrayed as the silent killer."

Dr Fiona Reddington, from Cancer Research UK, said: "It's uncertain whether or not screening can reduce ovarian cancer deaths overall.

"While this is an important step in ovarian cancer research, we would not recommend a national screening programme at this point."

The UK's National Screening Committee, which decides what diseases should be screened for, says it will have to make a "scientifically sound decision" and will review the findings.