November 3, 2013

expert reaction to HPV screening versus cytology (smear test) screening for cervical cancer

A paper in the Lancet found screening for the Human Papilloma Virus (HPV) provides 60–70% greater protection against invasive cervical cancer than the ‘smear-test’ screening currently used in most countries.

Dr Amanda Herbert, Consultant Cytopathologist and Editor of Cytopathology, said:

“Women should not be given the impression that HPV testing is a complete replacement for cytology, which already reduces cervical cancer incidence by about 75%.  Most types of HPV test detect reversible as well as persistent infection with the virus; it is persistence that matters.  HPV tests will be positive in many women who do not have precancerous changes that need to be treated, particularly in the younger age groups in whom most of the precancerous changes are found.  Women with positive HPV tests will have to be ‘triaged’ by cytology to identify more accurately who needs to be investigated, so primary HPV testing will require accurate cytology as well. Also, although more sensitive than cytology, which has to be repeated 3-yearly, HPV tests are not 100% sensitive.  As with any test, false negatives cannot be avoided completely.”

Professor Margaret Stanley, Professor of Epithelial Biology, University of Cambridge, said:

“Each of the individual studies referred to in this paper have been previously published and form the basis of the changes in cervical cancer screening that are taking place in many countries including the UK.  Basically HPV testing (detecting the DNA of the HPV types that cause cancer) in cervical smears is a more sensitive predictor of the presence of precancers than conventional (cytology-based) Pap smears.  Why?  Pap smears are very specific but it is a fairly insensitive method of detection, there are sampling errors and the smear may only contain a very few cells that could be missed by the screeners.  This is why Pap smears have to be repeated every 3 years in the UK (every year in some countries) so that the risk of missing a precancer is reduced – the more you test the less likely you are to miss something.  Since HPV DNA testing is more sensitive than the Pap smear more women are directed to colposcopy and more precancers are detected and treated – treatment of these precancers prevents the development of invasive cancer.  The outcome therefore should be that with HPV testing more invasive cancers are prevented than with Pap smears only and this is what the current study shows. 

“So why is HPV testing not introduced for everyone?  HPV testing is being included as an adjunct to Pap smears in the UK programme now but the Pap smear remains at present the primary screen.  Until the current cohort of girls/women who have had the HPV vaccine come through then HPV infection will remain as a very common infection in the under 30s, most of whom will get rid of the infection so HPV detection doesn’t tell you in that group who is at risk and who is not.  HPV testing is of most value for the over 30s because if they are still infected with one of the cancer causing HPVs, particularly HPV 16 and 18, their risk is higher for the presence and/or development of precancers.   When the vaccinated cohorts reach screening age – 25 in England – then HPV testing will have to be the screening method since most of the dangerous precancers caused by HPV 16 and 18 will have been prevented and the Pap smear test will lose its specificity.”

‘Efficacy of HPV-based screening for prevention of invasive cervical cancer: follow-up of four European randomised controlled trials’ by Guglielmo Ronco et al. published in the Lancet on Sunday 3 November.