The future of cervical screening is being carefully scrutinised. There is no doubt that cytology is one of the most valuable of all screening modalities, being able to detect pre-cancerous lesions while they are amenable to curative procedures that prevent more serious disease.
The profession and the public are analysing the role of all population screening strategies with the harms being objectively assessed as well as the benefits. There is a temptation to become caught up in the preventative fervour of prophylactic screening without looking at the downside implicit in all programmes. These negative aspects are derived from an over-reaction to minor deviations or difficult-to-interpret results, as well as the psychological and emotional fall-out generated by false positives. There is a spectrum of under-recognised harm from a pre-occupation with abnormal labels, through to the financial interests of business to grow the screening industry.
Health professionals exhort their patients to prevent disease and it is easy to slip into the simplistic mantra of early detection being the equivalent of prevention. Screening is no more preventative than insuring your home is preventative of its burning down. While reminding ourselves of the differences between screening and prevention, it is as well to remember the fundamentals of an effective screening test which should have the following characteristics (Clark Cancer Control 1995;2:485-92):
1. The disease sought should be an important health problem
2. A presymptomatic stage of the disease should exist
3. The natural history of the disease should be well understood
4. There should be an acceptable screening test available
5. Screening tests should be acceptable to the population being tested
6. Outcomes after presymptom diagnosis and treatment should be better than those after symptoms
7. Reduced morbidity/mortality should outweigh harms from false-positive tests
8. Benefits of the test should be achieved at acceptable risk
So does population based cervical cytology measure up to these ideals?
The massive reduction in deaths from cervical cancer in countries where programmes have been introduced does not preclude its re-evaluation as every intervention must be reconsidered in the present economic melt down. Fortunately cytology does hold up cost-effectively in developed countries like the United States where the burden of the disease has decreased by 75% but there are other strategies which need to be considered in developing countries where the costs of clinics, laboratories administration and personnel are prohibitive.
The role of HPV DNA testing in screening is starting to emerge. At present HPV tests are used to triage women with equivocal cytology who may or may not need colposcopy.
The next focus for HPV tests has been in women over the age of 30 years. These women are past the stage of self-limiting infections, and if they are HPV negative with normal cytology then they may constitute a group in whom fewer smears are necessary. Less frequent screening carries large financial implications.
Castle et al (Obstet Gynecol 2009;113:595-600) looked at the number of women who had oncogenic HPV positive tests in the general population of California and evaluated their cytology at the same time. Those between 30 and 34 years had 10% HPV oncogenic positive results but this dropped to around 5% in women older than 40 years. In the entire population the HPV positive rate was lower than anticipated thus not realising epidemiologists' fears of a sharply increased need for further investigation if widespread HPV screening is introduced. Conversely women with negative HPV tests plus negative cytology had a very low risk of incipient precancer and their screening can safely be extended beyond 3 years.
In some practices an “annual smear” has become traditional and women may be reluctant to give up their routine check-ups for fear of failing to detecting early disease. Cotesting with both cytology and HPV DNA may resolve this issue.
When to stop screening is an unsettled matter. There is no point in cytological screening in women who have had their cervix removed by hysterectomy for benign indications. Vault smears are not justified, but for older women with a cervix, when should screening end? Recommendations vary from country to country with 65 or 70 being the most frequently advised age on both sides of the Atlantic but this is in low-risk women who are asymptomatic. Certainly the latest data from Denmark (Rebolj et al (BMJ 2009; 338:b1354) indicates that negative smears in women in their fifties have the same predictive value as women in their thirties suggesting continued vigilance is a good idea.
Finally, Strander (BMJ 2009:338:b809) believes the story will unfold as the technology improves with computer generated risk factors guiding the frequency and duration of screening. Surely algorithms can be devised which include lifestyle considerations plus previous cytology and HPV results which would streamline services, save unnecessary retesting as well as indicating when to stop screening?
