Cervical smear cells under microscope

The research, published in the Journal of Clinical Pathology, may be a statistical quirk, say the scientists involved - but they have called for larger scale studies to confirm or rule out a problem.

A cervical smear comprises a sample of cells taken using a spatula from the neck of the womb.

The cells are examined under the microscope to see any signs either of cancerous cells, or abnormalities which may show that cancer is likely.

Cervical screening has been credited for a 7% drop in deaths from cervical cancer each year in the UK.

Repeated smears

However, some smears contain too few cells, or are unreliable because more than half the cells are obscured by blood, "inflammatory cells" or bacteria.

Women who provide these are recalled for a new smear test to be carried out.

However, the researchers, from Manor Hospital in Walsall, West Midlands, looked at more than 25,000 smears taken between April 1995 and March 1996.

Just under 2,000 women had a "poor quality" smear during that period.

When their medical records for the following five years were checked, it emerged that these women appeared to be at twice the usual risk of a further smear test revealed "high grade" abnormal cell changes.

Smears obscured by inflammatory cells led to the biggest increase in risk.

Statistically weak

However, the researchers conceded that the relatively small number of women in the study meant that the results did not prove a link.

On the basis that screening saves lives, we encourage women to accept their invitation for a smear test, but in the end it is not our choice, it's up to the individual woman Julietta Patnick, NHS Cancer Screening Programme

They suggested that a bigger study might reveal stronger evidence for or against the phenomenon.

Julietta Patnick, the director of the NHS Cancer Screening Programme, told BBC News Online: "This study is interesting because it associates inadequate smears with an increased risk of subsequent cancer.

"In fact, the most common reasons for an inadequate smear are an infection, the cells on the slide may be hidden by blood or mucus, there may not have been enough cervical cells on the slide to give an accurate assessment, the sample may not have been properly prepared.

"There is also the remote possibility of cervical cancer, which is why our guidelines state that when a smear from a woman is deemed inadequate three times in a row further investigation is recommended. Inadequate smears are always repeated."

"We know that screening prevents cervical cancer and that the NHS Cervical Screening Programme is directly responsible for a 42% drop in incidence in cervical cancer between 1988 and 1997.