A new method of carrying out cervical checks being rolled out across the NHS does not improve the reliability of results, scientists have argued.
10% of traditional smear tests were deemed unusable
A review in the Lancet of 56 studies found no evidence liquid-based cytology tests cut the rate of unusable tests or detected more serious abnormalities.
But the Sydney University team said there were other possible benefits such as cost and speed of diagnosis.
NHS experts said the method had cut unusable tests by 90% in England.
In liquid-based cytology (LBC) tests, samples are taken in the usual way but using a brush-like device rather than a spatula and the whole device is placed in a vial of fluid.
In a conventional smear test, cells are taken from the cervix, or neck of the womb, using the spatula and then spread onto a glass slide to be analysed.
LBC was adopted in England in 2003 after the National Institute of Clinical Excellence called for the traditional pap test, which had been used by the NHS since 1989, to be replaced.
By the middle of 2006, half of all cytology laboratories will have fully implemented or commenced implementation of LBC.
The NHS in Scotland and Wales has already adopted the new method.
Research had suggested one in 10 traditional smear tests taken had been classed as "inadequate", with women then being recalled for repeat tests.
This compared with a 1.6% failure rate for the new method.
Pilot studies also suggested the new LBC technique was better at picking up potential cancers.
But the research team, led by Dr Elizabeth Davey, said its review of 56 primary studies found no evidence that liquid-based cytology tests reduced the proportion of unsatisfactory slides.
Her team reported that most of the studies they looked at had "methodological deficiencies" and failed to follow up on patients.
The team also suggested if LBC testing was more sensitive and did pick up more high-grade abnormalities, it could also increase the number of low-grade abnormalities - leading to more false positive results.
Julietta Patnick, director of NHS Cancer Screening Programmes, said the new system had been introduced on the basis of strong research evidence.
"The latest statistics show that LBC has reduced the rate of inadequate results by up to 90% in the first three laboratories in England where it was introduced.
"As a result, fewer women are having to return for a repeat test because the first cannot be read.
"Once LBC is fully implemented, women will also receive their results faster, reducing anxiety and uncertainty."
Director of screening services in Wales, Dr Hilary Fielder, said LBC had reduced unsatisfactory sample rates from around 8% to around 2%.
"This means that compared with historical rates using the conventional (Pap) smear test, more than 10,000 women in Wales have not had to attend for a repeat test."
Dr Anne Szarewski, clinical consultant at Cancer Research UK, said the review was interesting but that its conclusions did not tally with experience of using the new tests.
According to large-scale pilot studies, cytology screening had significantly reduced the number of unnecessary, repeat tests.
"In addition, the time required to read the slides and the fact that other tests can be done from the same specimen, adds to the benefits."